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Formation mechanism and microstructural and mechanical properties of in-situ Ti–Ni-based composite coatings by laser metal deposition
Formation mechanism and microstructural and mechanical properties of in-situ Ti–Ni-based composite coatings by laser metal deposition Chenglong Ma, Dongdong Gu, Chen Hong, Beibei He, Kun Chang, Qimin Shi PII: DOI: Reference:
S0257-8972(16)30075-5 doi: 10.1016/j.surfcoat.2016.02.013 SCT 20920
To appear in:
Surface & Coatings Technology
Received date: Revised date: Accepted date:
20 December 2015 26 January 2016 5 February 2016
Please cite this article as: Chenglong Ma, Dongdong Gu, Chen Hong, Beibei He, Kun Chang, Qimin Shi, Formation mechanism and microstructural and mechanical properties of in-situ Ti–Ni-based composite coatings by laser metal deposition, Surface & Coatings Technology (2016), doi: 10.1016/j.surfcoat.2016.02.013
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ACCEPTED MANUSCRIPT Formation Mechanism and Microstructural and Mechanical Properties of In-situ Ti-Ni-Based Composite Coatings by Laser Metal Deposition
College of Materials Science and Technology, Nanjing University of Aeronautics and
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Astronautics, Yudao Street 29, Nanjing 210016, PR China
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Chenglong Ma 1,2, Dongdong Gu 1,2,*, Chen Hong3, Beibei He 1,2, Kun Chang 1,2 , Qimin Shi1,2
Institute of Additive Manufacturing (3D Printing), Nanjing University of Aeronautics and
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Astronautics, Yudao Street 29, Nanjing 210016, PR China
Fraunhofer Institute for Laser Technology ILT/Chair for Laser Technology LLT, RWTH
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Aachen, Steinbachstraße 15, D-52074 Aachen, Germany
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* Corresponding author. Tel./fax: +86 25 52112626. E-mail: [email protected]
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ACCEPTED MANUSCRIPT Abstract The in-situ TiO2 reinforced Ti-Ni composite coating on carbon steel was successfully
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prepared by laser metal deposition (LMD) using Ti-Ni as-mixed powder with an atomic ratio
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of 60:40. With the aim of in-situ reaction design during LMD processing, a trace of oxygen
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mixed with the shielding gas was introduced. Different “laser energy input per unit length” (E) by changing the laser power was set to investigate the influence on the deposition quality and
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attendant microstructure and mechanical property of the LMD-processed layer. TiO2 particles with unique flower-like structure formed when the applied laser energy E≤96 kJ/m, while the
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apparent oxidation of grain boundaries was observed as E increased to 120 kJ/m. The
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formation mechanism of in-situ TiO2 particles with a flower-like structure was present. At the
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optimized process parameter of 96 kJ/m, the LMD-processed layer showed the highest densification degree free of any pores and cracks. The corresponding mechanical properties
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were measured, showing the relatively high average microhardness of HV0.2 790 and significantly improved tribological property containing lower coefficient of friction of 0.4 and
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more smooth worn surface.
Key words: laser metal deposition (LMD); Ti-Ni alloy; coating; composite
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ACCEPTED MANUSCRIPT 1. Introduction Ferrous alloys, as the typical structural material, have got wide applications in
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engineering filed due to their excellent comprehensive properties and low cost. Normally, the
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components made of ferrous alloys are exposed to various kinds of external environments,
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which may lead to an early failure because of some reasons such as corrosion and abrasion. Two main approaches are generally used to improve the properties of ferrous alloys, namely
has been paid more attentions into [1-3].
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alloying and surface modification. Taking into account of the cost, the surface modification
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Laser metal deposition (LMD), as one of the most prevailing additive manufacturing
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(AM) technologies, has got rapid development in recent years. Due to its highly versatile
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process capability, LMD has been widely applied to manufacture new components, to repair and rebuild worn or damaged components, and to prepare wear- and corrosion-resistant
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coatings [4-6]. When used for material surface property modification, LMD is also known as laser cladding or laser depositing. Based on the processing features of AM technology, namely
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layer-to-layer shaping and consolidation of feedstock using a computer-controlled laser as the energy source, LMD possesses the nature of highly non-equilibrium, rapid melting and solidification [7-9]. Consequently, uniform distribution of chemical components and novel microstructures, which are hard to be achieved by other conventional methods, can be introduced by LMD. Relative investigations on different types of laser-deposited coatings or layers on the surface of ferrous alloys have been reported. For example, Qunshuang Ma et al prepared the Ni60/WC composite coatings on the surface of Q550 steel. A special cored-eutectic structure was obtained, which had advantages of well-distribution and tight binding with the matrix [10]. Dariusz Bartkowski et al successfully achieved laser cladding 3
ACCEPTED MANUSCRIPT Stellite-6/WC composite coatings on the surface of low-carbon steel. This study indicated that the microhardness enhanced with the volume fraction of WC increasing while the corrosion
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resistance weakened [11].
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Titanium and its alloys, due to the excellent specific strength, fracture resistance
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characteristics and high corrosion resistance, have won extensive applications in engineering filed [12, 13]. However, the poor tribological properties including high friction coefficient,
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low adhesive and fretting wear resistance restricted the application of Titanium alloys as coatings to a large extent [14, 15]. Ti-Ni alloys have been proven to be an alternative coating
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material, owing to the significant intensification of tribological properties by the formation of
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Ti-Ni intermetallic. The correlative research works have been carried out. M．Salehi et al
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successfully prepared Ti-Ni intermetallic coatings on the surface of carbon tool steel by a duplex process. During the duplex process, the Ni coating with the thickness of 20 μm was
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firstly achieved by electroplating, and then Ni-coated specimens were packed in the Ti-rich powder mixtures in order to develop Ti-Ni intermetallic coatings. Besides, diffusion annealing
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was carried out to reduce compositional gradient profile from surface to core and the final phases were composed of TiNi, Ti2Ni, TiNi3 and FeNi [16]. F. T. Cheng et al systematically investigated laser cladding of AISI 316 stainless steel using respectively preplaced NiTi wire, NiTi strips and NiTi powder. In above three conditions, a good coating with high microhardness and tough interface, free of any pores and cracks, was all obtained. As a result, the surface cavitation erosion resistance of AISI 316 stainless steel achieved a significant improvement [17-19]. Further improvement has been obtained by addition of the third alloying element or reinforcement particles, such as N, Al and ZrO2. Cunshan Wang et al reasonably designed the Ti70.3Ni22.2Al7.5 alloy, optimized from a basic binary eutectic 4
ACCEPTED MANUSCRIPT Ti76Ni24 alloyed with different amounts of Al, and prepared the corresponding coating on AZ91HP magnesium alloy by laser cladding. The coating mainly consists of β-Ti solid
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solution and Ti2Ni intermetallic compound resulting in high hardness, good wear resistance
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and corrosion resistance [20]. Babatunde A. Obadele et al studied the influence of laser scan
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speed and additive ZrO2 particle on microstructure, microhardness, wear resistance and corrosion resistance for laser deposited Ti-Ni coating on the surface of AISI 316 stainless steel.
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When the laser scan speed reached 0.0067 m/s, the Ti-Ni coating showed the most excellent corrosion resistance and the addition of ZrO2 was able to enhance dramaticlly the corrosion
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resistance [16].
According to the above investigations, the addition of the third component in Ti-Ni alloy
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can introduce prominent “gain effect” and efficiently strengthen the mechanical or chemical property of Ti-Ni alloy. However, the investigations on the Ti-Ni composite coating reinforced
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by particles are still lacking, especially on non-equiatomic Ti-Ni composite coating as well as for the application on surface modification of ferrous alloy. In this study, in-situ TiO2
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reinforced Ti-Ni composite coating on carbon steel was successfully prepared by LMD using Ti-Ni as-mixed powder. By the introduction of little oxygen mixed with the shielding gas, in-situ TiO2 particles with different structures were observed to form within the matrix during the LMD processing with different process parameters, which had an important effect on the deposition quality and attendant mechanical property of the laser-deposited layer. TiO2 particle, as a reinforcing phase, has been applied for some other metal matrixes, such as A356 aluminum alloy [21], but few applications for Ti-Ni alloy matrix have been reported. Furthermore, this result will be helpful to prepare in-situ oxide particles reinforced Ti-Ni composite coating by the introduction of appropriate oxygen carrier during laser processing. 5
ACCEPTED MANUSCRIPT Hence, in this present paper, different laser processing parameters were set in order to investigate the influence on the deposition quality and attendant microstructure and
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2. Experiment procedure
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TiO2 particles with a flower-like structure was present.
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mechanical property of the deposited layer. Meanwhile, the formation mechanism of in-situ
2.1 Materials
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The gas-atomized, spherical Titanium powder (99.5% purity) with a mean particle size of 30 μm and the spherical-shaped Nickel powder (99.5% purity) in an average particle size of
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10 μm were used as feedstock powders in this study. Titanium powder and Nickel powder in
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an atomic ratio of 60:40 were uniformly mixed in a high-energy Pulverisette 6 planetary
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mono-mill (Fritsch GmbH, Germany) using a ball-to-powder weight ratio of 5:1, a ration speed of the main disc of 200 r/min, and a milling time of 4 h. The as-mixed powder still
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demonstrated original spherical morphology, as shown in Fig. 1b. Besides, carbon steel was utilized as the substrate material, which was sandblasted and ultrasonically cleaned
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successively in acetone and deionized water to remove surface contaminants before LMD. The detailed chemical compositions of raw powders and substrate were present in Table 1. Table 1 The chemical compositions of raw powders and substrate Composition in wt. % Carbon steel Ni-Ti mixed powder
Fe
C
Si
Mn
Cr
Ni
Cu
Ti
Balance
0.88
0.17
0.6
0.12
0.1
0.25
0
0
0
0
0
0
55.03
0
44.97
2.2 Laser metal deposition The schematic of the experimental procedure for laser depositing Ti-Ni alloy layer on the 6
ACCEPTED MANUSCRIPT surface of carbon steel substrate was present in Fig. 1a. Laser metal deposition (LMD) processing was carried out with a 5-axis CNC system, a Trumpf Nd:YAG laser system with a
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maximum output power of 3 kW and a focused spot diameter of 0.6 mm, integrated with a
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powder feeder system, and a coaxial powder nozzle. The premixed Ti-Ni powder was injected
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into the melted pool through the coaxial nozzle with a powder feeding rate of 2.4 g/min. At the same time, with the aim of in-situ reaction design, Argon mixed with a trace of oxygen
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acted as carrier gas during laser deposition. The multiple tracks were cladded for the layer with the dimension of 5 mm × 28 mm. LMD fabrication was based on the line-by-line laser
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cladding, using a constant laser scan speed of 500 mm/min and various laser powers of 400W,
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600 W, 800 W, 1000W and 1200W. An integrated parameter “laser energy input per unit
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length” (E), which was defined by E=P/ν, as shown in table 2, was used to estimate the laser energy input to the deposited layer. Although five samples with different processing
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parameters were prepared in this study, the microstructure features of sample N1 with 48 KJ/m were similar as sample N2 with 72 KJ/m and the microstructure features of sample N4
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with 120 KJ/m were similar as sample N5 with 144 KJ/m. Therefore, three samples (N2, N3, and N4), as the typical ones, were chosen for the microstructure study. The macrophotograph of final LMD-fabricated layers was present in Fig. 1c. Table 2 The LMD process parameter Sample
Laser power (W)
Scan speed (mm/min)
Energy density (kJ/m)
N1 N2 N3 N4 N5
400 600 800 1000 1200
500 500 500 500 500
48 72 96 120 144
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ACCEPTED MANUSCRIPT 2.3 Characterization of microstructure, chemical composition and phase The phases of the deposited layer were identified by a Bruker D8 Advance X-ray
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diffractometer (XRD) with Cu Kα radiation (λ = 0.154 18 nm) at 40 kV and 40 mA using a
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continuous scan mode. Three Specimen N1, N2, and N3 for metallographic examinations
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were prepared according to the standard procedures and etched with a solution composing HF, HNO3, and distilled water with a volume ratio of 1:6:7 for 3 s. A PMG3 optical microscope
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(Olympus Corporation, Japan) was used to observe the low-magnification three-dimension morphology and cross-sectional microstructures of LMD-processed layer on the carbon steel.
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High-resolution studies of the microstructural features of the deposited layer were
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characterized using an S-4800 field emission scanning electron microscope (FE-SEM)
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(Hitachi, Japan) at an accelerating voltage of 5.0 kV. Chemical compositions were determined by an EDAX energy dispersive x-ray spectroscope (EDX) (EDAX, Inc., USA).
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2.4 Microhardness and dry sliding wear test The Vickers hardness was measured using a MicroMet 5101 microhardness tester
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(Buehler GmbH, Germany) at a load of 0.2 kg and an indentation time of 15 s. The tribological property of the specimen was estimated by the dry sliding wear tests conducted in a HT-500 ball-on-disk tribometer (Lanzhou ZhongKe KaiHua Sci. &Technol. Co., Ltd., China) in air at room temperature. The counter material was GCr15 bearing steel ball with a diameter of 3 mm and a mean hardness of HRC 60, using a test load of 220 g was applied. The friction unit was rotated at a speed of 560 rpm for 15 min, with the rotation radius of 2 mm. The coefficient of friction (COF) of the specimens was recorded during wear tests.
3. Results and Discussion 3.1 The quality of deposition and element distribution 8
ACCEPTED MANUSCRIPT Figs. 2a-4a demonstrate the three-dimension (3D) morphology of laser deposited layer at laser energy input per unit length (E) of 72, 96 and 120 kJ/m, respectively. Furthermore, to
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better reflect the quality of deposition, the corresponding composition profiles along the depth
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of the deposited layer as determined by EDX are also shown in Figs. 2b-4b. The parabolic
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shape bond line between the deposited layer and the substrate, characteristic microstructure feature of laser processing, was clearly observed from the cross-section of each sample (Figs.
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2-4a). Although adhesion of the deposited layer and the substrate was good at each parameter E, the overall quality of the deposited layer varied. When the applied E was 72 kJ/m, some
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cracks propagating in the overlap zone between two adjacent tracks could be obviously seen
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(Fig. 2a). Besides, some agminated pores were found at the beginning position of the crack
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lying in X-direction cross section. Then the EDX detection indicated that the concentrations of Fe, Ti, and Ni element along the depth of the deposited layer presented an apparent,
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near-invariable gradient (Fig. 2b). As E further increased to 96 kJ/m, a nearly full dense layer was achieved, free of any cracks and pores (Fig. 3a). In this condition, a transfer zone with an
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approximately 300 μm depth showed up and the concentrations of all elements tended to be invariable when the distance from the interface was over 300 μm (Fig. 3b). However, cracks running through the deposited layer were observed at the Y-direction cross section when the applied E reached 120 kJ/m (Fig. 4a). In this case a more homogenous element distribution with high Fe content was obtained (Fig. 4b). During LMD processing, both part of substrate material and coaxial feeding powder were melted, forming a molten pool with a continuous liquid front. The different laser processing parameter determined different amount of the liquid formed in the molten pool and resultant quality of deposition. When the E was relatively low, the amount of the liquid within 9
ACCEPTED MANUSCRIPT the molten pool was limited due to insufficient laser energy input, which made the liquid hardly spread out smoothly. As a result, the densification behavior within the molten pool
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weakened along with the emergence of pores and meanwhile the homogenization rate of all
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elements was restricted dramaticlly. Moreover, the fierce shrinkage induced by the melt
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solidifying rapidly along the horizontal and vertical direction of the molten pool led to the formation of tensile stress in the deposited layer. Specially, the overlap zone usually
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experienced the remelting and solidification process repeatedly, thus causing a higher stress concentration [22]. Then the combined effect of pores and tensile stress led to the initiation
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and propagation of cracks. On the other hand, when an excessive E was applied, more laser
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energy was absorbed by the substrate material and powder and then the temperature within
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the molten pool rose rapidly. Consequently, the formed melt was in a state of overheating, thus leading to the accumulation of thermal stress during subsequent solidification processing.
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On the maximum stress accumulated in the solidified layer exceeded the strength of the deposited alloy, cracks occurred immediately with the release of the accumulated stress. In
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this case, the excessive heat input accelerated vastly the diffusion of elements, thus achieving a more homogenous distribution state of elements. Besides, the height of the deposited layers shows an apparent fluctuation in Figs.2-4. This fluctuation is mainly attributed to the combined effect of surface tension and powder vaporization [23]. Due to the introduction of oxygen in this study, the surface tension gradient may show positive value, thus leading to formation of the Marangoni flow from the edge of melt pool to the center of melt pool and an increase of height of melt pool. With the laser power increasing, surface tension gradient enhances significantly and therefore the increasing effect of height of melt pool is more apparent. However, when excessive laser energy is input, 10
ACCEPTED MANUSCRIPT the powder vaporization effect gets intensified remarkably and consequently decreases the height of melt pool.
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3.2 Microstructure evolution along the depth of the deposited layer
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According to the above analysis, at an optimized E of 96 kJ/m, the optical micrograph of
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the cross-section microstructure of the deposited layer, locating at different regions from the surface, is shown in Fig. 5. In fact, the sample N2 has the similar cross-sectional
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microstructure features as the sample N3. Grain morphology transformations of the deposited layer from the top to the bottom are relatively coarsening steering dendrites, directional
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equiaxed grains, fine dendrites and planar crystals. The previous investigations have proved
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that the temperature gradients (G) and the solidification rates (R) during solidification can
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determine microstructure developments [24, 25]. The temperature gradient G and the solidification rate R can be calculated in the following equations [25]:
2 K (T T0 ) 2 P
(1) (2)
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R Vs cos
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G
where K is the thermal conductivity of the material (W m−1 K−1), T is the liquidus temperature of the alloy (K), T0 is the initial temperature of the substrate (K), η is the laser absorption coefficient, P is the laser power (W), Vs is the laser scanning speed (m s-1) and θ is the angle between Vs and R. In general, the value of G/R was used to evaluate the influence on the morphology of the liquid/solid interface or microstructure developments [25]. According to the above equations, G/R could be estimated by:
G/R
2 K (T T0 ) 2 PVs cos
(3)
At the bottom of the molten pool (~50 μm from the interface), the solidification rate R was 11
ACCEPTED MANUSCRIPT nearly perpendicular to the laser scanning speed Vs and the temperature gradient G reached the maximum value due to the contacting the substrate, resultantly G/R getting the maximum
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value. Thus the melt was in a state of overheating and consequently the solidification
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microstructure was planar crystal. With the solidification front of the melt advancing (~300
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μm from the interface), the previously solidified layer acted as the substrate and consequently the initial temperature T0 increased, thus leading to a decrease of temperature gradient G
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located in the solidification front of the melt. At the same time, the angle θ between Vs and R became lower, causing the increase of solidification rate R. In this condition, the value of G/R
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decreased significantly and as a result constitutional undercooling occurred. In this case,
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dendrites growth pattern took over a leading position. When the liquid/solid interface
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continued to move, both the bottom and the top of the molten pool were being solidified due to the contacting the substrate and the air, which led to the change of G direction and
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subsequently the formation of steering dendrites at the top of the molten pool (~50 μm from the surface). Then the solidification rate from the bottom to the top was faster than that from
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the top to the bottom, leading to two similar temperature gradients in the upper part of the deposited layer (~200 μm from the surface) and resultant directional equiaxed grains [24]. 3.3 Microstructure evolution and phase identification as E increasing Also, to depict the influence of E by changing laser power on the cross-section microstructures of the deposited layer, the corresponding FE-SEM characterization is performed at the same distance (~300 μm) from the interface, as shown in Figs. 6-8. At a relatively low E of 72 kJ/m, the main microstructure demonstrates typical dendrite and at the same time some dispersive particles with submicron scale are observed (Fig. 6a and b). Highmagnification FE-SEM micrograph shows the detailed microstructure of these dispersive 12
ACCEPTED MANUSCRIPT particles, a cluster structure by several finer particles getting together (Fig. 6c). The XRD experiment identifies Fe2Ti, FeNi3 and TiO2 as the main constituent of the microstructure (Fig.
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6d). Besides, a few Ti-Ni intermetallic phases, such as Ni4Ti3 and Ni3Ti, were also detected.
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The elevated E (reaching 96 kJ/m) results in the transformation from typical dendrite to
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cellular dendrite (Fig. 7a and b). Interestingly, particles with unique flower-like structure are found to distribute within the intercellular regions homogenously (Fig. 7c). The detected main
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phases are still Fe2Ti, FeNi3 and TiO2 and meanwhile Ni4Ti3 phase are also identified clearly, according to the XRD results presented in Fig. 7d. As an excessive E is applied (120 kJ/m),
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the solidification microstructure is completely different, showing relatively loose
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microstructure (Fig. 8a). In this case, the diffraction intensity of Ti-Ni intermetallic phase is
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weak, as shown in Fig. 8b. Besides, the intergranular region was observed to get apparent oxidation reaction, as shown in Fig. 8c, which was proved by the further EDX
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characterization (Fig. 8d).
Based on the above transformation feature of grain morphology with E increasing, the
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typical dendrites development in sample N2 happens prior to that in N3 sample. The previous analysis has depicted the importance of G and R on the grain morphology. Moreover, the solution composition also influences the grain morphology [27]. The critical temperature gradient Gc was defined as [28]: Gc
ml C S (1 k ) kRDl
(4)
where ml was the slop of the liquidus line, Cl was the liquid composition in the liquid /solid interface, Cs=k Cl , the solid composition in the liquid /solid interface and Dl represented the diffusion coefficient. Constitutional undercooling occurred based on the premise of G < Gc 13
ACCEPTED MANUSCRIPT meeting constitutional undercooling [29], thus leading to the development of cellular or dendritic grains (the lower constitutional undercooling tends to induce cellular grains while
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the higher constitutional undercooling tends to induce dendritic grains). Due to the
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insufficient laser energy input in sample N2, the maximum temperature within the molten
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pool and attendant Marangoni convention are limited, which causes the decrease of diffusion coefficient. Hence, the critical temperature gradient Gc is improved and consequently the
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constitutional undercooling region gets extended in N2 sample, which can account for the difference in the microstructures of N2 and N3 samples. However, it makes a difference when
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E reaches 120 KJ/m. As an excessive energy input is applied, a large amount of substrate
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material is melt and the melt are in a state of overheating, which accelerates the oxidation
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behavior within the molten pool. A number of oxygen atoms aggregate in the grain boundaries to cause the oxidation of grain boundaries, which may be believed to account for the
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formation of the observed solidification microstructure. 3.4 Formation mechanism of dispersive TiO2 submicronflower
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With the aim of in-situ reaction design during the LMD processing, a trace of oxygen by controlling the airflow velocity of shielding gas was introduced. EDX detection found that a number of particles observed in Figs. 6-8 belonged to an oxide phase TiO2 (Fig. 6e). The corresponding XRD characterization had indicated that the oxide phase was rutile TiO2 with relative strong diffraction peaks (110) and (211) (Fig. 6d, Fig. 7d and Fig. 8b). Specially, TiO2 particles with the flower-like structure could be observed when the applied laser energy E≤96 kJ/m (Fig. 6c and Fig. 7c). The flower-like structure became invisible when an excessive E value of 120 KJ/m was applied. Fig. 9a shows the crystal structure of Rutile, a tetragon with Ti4+ surrounded by six O2- at the corners of a slightly distorted octahedron and each O214
ACCEPTED MANUSCRIPT surrounded by three Ti4+ lying in a plane at the corners of an equilateral triangle [29]. As the molten pool formed by interaction between the laser and the powder, dissociative Ti atoms
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Ti O TiO
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formation processing of TiO2 might experience two stages:
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tended to unite with oxygen atoms, taking into account of the higher activity of Ti atom. The
TiO O TiO2
(5) (6)
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TiO2, as a high melting point phase, preferentially precipitated in the melt pool to form nanocrystalline. The observed crystal morphology schematic of rutile is shown in Fig. 9b [30].
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Due to the fact that the (110) crystal plane is the plane with the least energy, the growth rate of the (110) crystal plane along the <001> direction is higher than that of other planes [31].
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Hence, the zone size of the (110) crystal plane was larger than that of other planes in Fig. 9b. In XRD detection, the main peak of TiO2 corresponding to the (110) crystal plane
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demonstrated the strongest intensity, thereby proving the above point. In order to reduce the number of crystal planes with high energy, a cluster structure formed as more Ti and O atoms
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continued to deposit on the (110) crystal planes, thus improving the stability of precipitated TiO2 nanocrystalline. When the applied laser energy was elevated, the growth of the (110) crystal plane along the <001> direction got enhanced due to the sufficient heat input during the LMD processing, thereby leading to the formation of submicroflower structure. The corresponding schematic diagram is shown in Fig. 9c. However, when excessive laser energy is applied, a mass of heat input leads to the introduction of excessive Fe component and attendant improvement of activity of Fe atom. As a result, the combination of Fe and O atom is enhanced, which accounts for the formation of iron oxide in the intergranular region. Besides, the Marangoni convention [32] induced by the surface tension gradient within the 15
ACCEPTED MANUSCRIPT molten pool was also intensified significantly, thus disturbing the direction of heat flow and hindering the formation of TiO2 submicronflower.
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3.5 Mechanical property characterization
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Fig. 10 depicts the microhardness distributions along the depth of LMD-processed layers.
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For all laser-deposited samples, the average microhardness achieved significant improvement compared with the substrate material showing a very low average value of 280 HV0.2. The
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formation of Fe-Ti and Fe-Ni intermetallic as well as dispersive oxide particles were the main factors for the improvement. The maximum microhardness in N2 sample was measured on
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the near surface, reaching a level as high as 840.8 HV0.2, which was higher than that in other
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samples, implying that more Ti-Ni intermetallic phases might form in the near surface of N2
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sample in view of the XRD results. For the chosen typical processing parameters, the N4 sample had the lowest average microhardness in comparison with N2 and N3 samples. Due to
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the significantly intensified oxidation behavior and resultant loose solidification microstructure induced by an excessive laser energy input, the capacity of plastic deformation
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resistance gets weakened.
Fig. 11 shows the coefficients of friction (COFs) as well as the corresponding worn morphologies of the deposited layers and the substrate material. For the substrate material, the worn surface showed a very high roughness with severe material delamination and spallation. As a result, relatively high COF of 0.8 was obtained. An apparent improvement in tribological property was achieved when Ti-Ni alloy coating was prepared on the surface of substrate material. At the initial running-in stage of the wear test, the COF curves all exhibited fluctuation to some degree, which was attributed to the relatively rough surface and the existence of oxide film [33]. With the sliding time increasing, the fluctuation of COF tended 16
ACCEPTED MANUSCRIPT to be steady because of the decreased surface roughness and the stripping of oxide film. The worn surface morphologies of samples for typical processing parameters are shown
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in Fig 11. Nevertheless, the differences in COF values and worn morphologies indicated that
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the tribological property of the deposited layer was influenced significantly by the laser
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processing parameter. At the relatively low E of 72 kJ/m, the average COF value reached a high level of 0.58 and a number of wear debris composed of ultrafine particles produced on
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the worn surface. As the E increased to 96 kJ/m, the average COF value significantly decreased to 0.4, and the much smoother worn surface with shallow grooves was obtained,
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nearly free of any wear fragments. When the applied E increased to 120 kJ/m, the average
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COF value increased to the higher level of 0.65 and more wear debris as well as crumbling
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were found on the worn surface.
The tribological property of the deposited layer is closely related to its densification level,
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microstructures and microhardness [33]. When the E value is low, densification behavior of the LMD-processed layer is restricted due to the formation and expanding of
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stress/pore-induced cracks. This leads to a remarkable increase of the surface roughness, thus enlarging the amount of abrasive fragments. As a result, more hard and brittle fine particles produce as the surface levels off and the COF value shows an apparent fluctuation, showing a typical abrasive wear. Taking into account of the highest surface microhardness, further wear of the deposited layer is limited after the surface becomes relatively smooth. As the proper E is applied, sufficient laser energy guarantees the formation of good densification and homogenous microstructures, therefore leading to the enhanced wear resistance. At an excessive laser energy input, densification level decreases again due to the existence of cracks. Besides, the loose solidification microstructure with oxidized grain boundaries as well as the 17
ACCEPTED MANUSCRIPT lowest microhardness can be responsible for the relatively poor worn surface. However, the homogenously dispersed oxide particles with submicrometer scale in N2 and N3 samples are
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believed to play a role not to be ignored in wear test. These oxide particles can improve
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efficiently the strength of the deposited layer and then enhance the capability of plastic
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deformation resistance.
4 Conclusion
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The in-situ TiO2 reinforced Ti-Ni composite coating on carbon steel was successfully prepared by LMD using Ti-Ni as-mixed powder with an atomic ratio of 60:40. Different
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“laser energy input per unit length” (E) by changing the laser power was set to investigate the
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influence on the deposition quality and attendant microstructure and mechanical property of
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the LMD-processed layer. The main results can be summarized as: (1) The highest densification level and homogenous element distribution were obtained
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when a proper E of 96 kJ/m was applied, while the application of relatively lower or higher E resulted in the formation of cracks. Besides, the diffusion effect of Fe as well as oxidation
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behavior in the deposited layer got enhanced as an excessive laser energy input was applied. (2) At the proper E, the cross-sectional microstructure along the depth of deposited layer demonstrated variant transformations of grain morphology due to the change of temperature gradient and solidification rate. At the same distance from the interface, the transformation of grain morphology delayed with the applied E increasing, owing to the decrease of Gc induced by the accelerated diffusion coefficient within the molten pool. (3) Submicron TiO2 particles with unique flower-like structure were obtained when the applied laser energy E≤96 kJ/m. The formation mechanism of TiO2 submicronflower was present, mainly owing to the preferred growth of TiO2 along the <001> direction. 18
ACCEPTED MANUSCRIPT (4) As a proper laser energy E was set, the deposited layer achieved a relatively high microhardness with an average of HV0.2 790 while the substrate material showed a low
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average value of 280 HV0.2. Furthermore, the much smoother worn surface with shallow
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grooves as well as a considerably lower COF of 0.4 was obtained, in comparison to the
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substrate material. The formation of Fe-Ti and Fe-Ni intermetallic as well as dispersive oxide particles were the main factors for the above improvement.
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ACKNOWLEDGMENTS
The authors gratefully acknowledge the financial support from the National Natural
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Science Foundation of China (Nos. 51322509 and 51575267), the Top-Notch Young Talents
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Program of China, the Outstanding Youth Foundation of Jiangsu Province of China (No.
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BK20130035), the Program for New Century Excellent Talents in University (No. NCET-13-0854), the Science and Technology Support Program (The Industrial Part), Jiangsu
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Provincial Department of Science and Technology of China (No. BE2014009-2), the 333 Project (No. BRA2015368), Science and Technology Foundation for Selected Overseas
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Chinese Scholar, Ministry of Human Resources and Social Security of China, the Aeronautical Science Foundation of China (No. 2015ZE52051), the Shanghai Aerospace Science and Technology Innovation Fund (No. SAST2015053), the Fundamental Research Funds for the Central Universities (Nos. NE2013103 and NP2015206), and the Priority Academic Program Development of Jiangsu Higher Education Institutions, the Foundation of Graduate Innovation Center in NUAA and the Fundamental Research Funds for the Central Universities (No. kfjj20150605).
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[2] Hongxi Liu,Chuanqi Wang,Xiaowei Zhang,Yehua Jiang,Chuanxiong Cai,Shujun Tang, Improving the corrosion resistance and mechanical property of 45 steel surface by laser cladding with Ni60CuMoW alloy powder, Surf. Coat. Technol. 228(2013) S296-S300.
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ACCEPTED MANUSCRIPT JÜrgen Eckert, Selective laser melting of in situ titanium-titanium boride composites: Processing, microstructure and mechanical properties, Acta Mater. 76(2014) 13-22.
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[14] Babatunde A.Obadele, Masego L.Lepule, Anthony Andrews, Peter A.Olubambi, Tribocorrosion characteristics of laser deposited Ti-Ni-ZrO2 composite coatings on AISI316 stainless steel, Tribol. Int. 78(2014) 160-167.
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[15]R. Song, J. Li, J.Z. Shao, L.L. Bai, J.L. Chen, C.C. Qu, Microstructural evolution and wear behaviors of laser claddingTi2Ni/α(Ti) dual-phase coating reinforced by TiB and TiC, Appl. Surf. Sci. 355 (2015) 298-309.
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[16] M. Salehi, F. Karimzadeh, A. Tahvilian, Formation of Ti-Ni intermetallic coatings on carbon tool steel by a duplex process, Surf. Coat. Technol. 148(2001) 55-60.
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[20] Cunshan Wang, Ting Li, Biao Yao, Rui Wang, Chuang Dong, Laser cladding of eutectic-based Ti-Ni-Al alloy coating on magnesium surface, Surf. Coat. Technol. 205(2010) 189-194.
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[21] M Karbalaei Akbari, S Rajabi, K Shirvanimoghaddam and HR Baharvandi, Wear and friction behavior of nanosized TiB2 and TiO2 particle-reinforced casting A356 aluminum nanocomposites: A comparative study focusing on particle capture in matrix, J. Compos. Mater. 49(2015) 3665-3681. [22] Xiaoming Zhao, Xin Lin, Jing Chen, Lei Xue and Weidong Huang, The effect of hot isostatic pressing on crack healing, microstructure, mechanical properties of Rene88DT superalloy prepared by laser solid forming, Mater. Sci. Eng., A, 504(2009) 129-134. [23] Santanu Paul, Ishank Gupta, Ramesh K. Singh, Characterization and Modeling of Microscale Preplaced Powder Cladding Via Fiber Laser, J. Manuf. Sci. Eng., 137(2015) 031019/1-031019/13. [24] Kaibin Li, Dong Li, Dongyu Liu, Guangyu Pei and Lei Sun, Microstructure evolution and mechanical properties of multiple-layer laser cladding coating of 308L stainless steel, Appl. Surf. Sci. 340(2015) 143-150. [25] Y.J. Huang, X.Y. Zeng, Q.W. Hu, S.F. Zhou, Microstructure and interface interaction in 21
ACCEPTED MANUSCRIPT laser induction hybrid cladding of Ni-based coating, Appl. Surf. Sci. 255(2009) 3940-3945.
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[26] Kun Liu, Yajiang Li, JuanWang, Qunshuang Ma, Effect of high dilution on the in situ synthesis of Ni-Zr/Zr-Si (B, C) reinforced composite coating on zirconium alloy substrate by laser cladding, Mater. Den. 87(2015) 66-74.
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[27] Merton C. Flemings, Solidification processing, McGraw-Hill Inc., New York, 1974, pp. 42-98.
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[28] W. Kurz, D. J. Fisher, Foundmentals of solidification, Trans Tech Publications Inc., Adamstown, 1990, pp. 63-85.
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[29] Ulrike Diebold, The surface science of titanium dioxide, Surf. Sci. Rep. 48(2003) 53-229.
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[30] Peter M. Oliver, Graeme W. Watson, E. Toby Kelsey and Stephen C. Parker, Atomistic simulation of the surface structure of the TiO2 polymorphs rutile and anatase, J. Mater. Chem. 7(1997) 563-568.
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[31] Abbas Sadeghzadeh Attar, Zahra Hassani, Fabrication and Growth Mechanism of Single-crystalline Rutile TiO2 Nanowires by Liquid-phase Deposition Process in a Porous Alumina Template, J. Mater. Sci. Technol. 31(2015) 828-833.
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[32] J.P. Kruth, G. Levy, F. Klocke and T.H.C. Childs, Consolidation phenomena in laser and powder-bed based layered manufacturing, CIRP Ann. Manuf. Technol.56 (2007) 730-759.
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[33] Dongdong Gu, Yves-Christian Hagedorn, Wilhelm Meiners, Guangbin Meng, Rui João Santos Batista, Konrad Wissenbach, Reinhart Poprawe, Densification behavior, microstructure evolution, and wear performance of selective laser melting processed commercially pure titanium, Acta Mater. 60 (2012) 3849-3860.
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Table 1
The chemical compositions of raw powders and substrate Composition in wt. % 0.88
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Mn
Cr
Ni
Cu
Ti
0.17
0.6
0.12
0.1
0.25
0
0
0
55.03
0
44.97
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Si
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Table 2
C
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Carbon steel Ni-Ti mixed powder
Fe
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The LMD process parameter
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Sample N1 N2 N3 N4 N5
Laser power (W)
Scan speed (mm/min)
Energy density (kJ/m)
400 600 800 1000 1200
500 500 500 500 500
48 72 96 120 144
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Captions:
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Fig.1 (a) The schematic diagram of LMD processing Ti-Ni layer; (b) the raw as-mixed Ti/Ni
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powder with an atomic ratio of 60:40; (c) the final LMD-processed Ti-Ni layers. Fig.2 (a) 3-D optical metallograph (OM image) composite showing the deposition quality of
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Ti-Ni layer at the relatively low E of 72 kJ/m; (b) the corresponding composition profiles along the depth of the deposited layer.
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Fig.3 (a) 3-D optical metallograph (OM image) composite showing the deposition quality of Ti-Ni layer at the proper E of 96 kJ/m; (b) the corresponding composition profiles along the depth of the deposited layer. Fig.4 (a) 3-D optical metallograph (OM image) composite showing the deposition quality of Ti-Ni layer at an excessive E of 120 kJ/m; (b) the corresponding composition profiles along the depth of the deposited layer. Fig.5 The optical micrograph of the cross-section microstructure of the deposited layer at (a) the near-surface region, (b) 1000 μm distance from the surface, (c) 500 μm distance from the surface, and (d) the bottom of the deposited layer under the proper E of 96 kJ/m. 24
ACCEPTED MANUSCRIPT Fig.6 (a) The cross-sectional microstructure of the deposited layer at ~300 μm distance from the interface, showing the typical dendritic morphology when a relatively low E of 72 kJ/m
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was applied; (b) the high-magnification FE-SEM micrograph of (a); (c) particle morphology
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with a cluster structure; (d) XRD patterns obtained over a wide range of 2θ (20-90º); (e) EDX
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characterization of point 1 located in (c).
Fig.7 (a) The cross-sectional microstructure of the deposited layer at ~300 μm distance from
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the interface, showing the cellular dendrite morphology when a proper E of 96 kJ/m was applied; (b) the high-magnification FE-SEM micrograph of (a); (c) particle morphology with
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a flower-like structure; (d) XRD patterns obtained over a wide range of 2θ (20-90º).
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Fig.8 (a) The cross-sectional microstructure of the deposited layer at ~300 μm distance from
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the interface, showing the iron filing shaped morphology when an excessive E of 120 kJ/m was applied; (b) the high-magnification SEM image of (a); (c) EDX characterization of point
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2 located in (b); (d) XRD patterns obtained over a wide range of 2θ (20-90º). Fig.9 (a) The crystal structure of Rutile; (b) the observed crystal morphology schematic of
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rutile; (c) the corresponding schematic diagram of particle morphology evolution. Fig.10 The microhardness distributions along the depth of LMD-processed layer, respectively corresponding to N1, N2, N3, N4 and N5 sample. Fig.11 The coefficients of friction (COFs) as well as the partially corresponding worn surface morphologies of the substrate and the deposited layer obtained at different Es of 44 kJ/m, 72 kJ/m, 96 kJ/m, 120 kJ/m and 144 kJ/m during the wear test. Table 1 The chemical compositions of raw powders and substrate. Table 2 The LMD process parameter.
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Graphical abstract
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ACCEPTED MANUSCRIPT Highlights (1) The in-situ TiO2 reinforced Ti-Ni composite coating was successfully prepared by LMD; (2) The microstructure evolution along the depth of the deposited layer as well as with the
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variant of E was analyzed; (3) The formation mechanism of in-situ TiO2 submicronflower was present; (4) The LMD-processed layer obtained at 96 kJ/m showed high average microhardness and improved tribological property.
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S0257-8972(16)30075-5 doi: 10.1016/j.surfcoat.2016.02.013 SCT 20920
To appear in:
Surface & Coatings Technology
Received date: Revised date: Accepted date:
20 December 2015 26 January 2016 5 February 2016
Please cite this article as: Chenglong Ma, Dongdong Gu, Chen Hong, Beibei He, Kun Chang, Qimin Shi, Formation mechanism and microstructural and mechanical properties of in-situ Ti–Ni-based composite coatings by laser metal deposition, Surface & Coatings Technology (2016), doi: 10.1016/j.surfcoat.2016.02.013
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ACCEPTED MANUSCRIPT Formation Mechanism and Microstructural and Mechanical Properties of In-situ Ti-Ni-Based Composite Coatings by Laser Metal Deposition
College of Materials Science and Technology, Nanjing University of Aeronautics and
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Astronautics, Yudao Street 29, Nanjing 210016, PR China
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Chenglong Ma 1,2, Dongdong Gu 1,2,*, Chen Hong3, Beibei He 1,2, Kun Chang 1,2 , Qimin Shi1,2
Institute of Additive Manufacturing (3D Printing), Nanjing University of Aeronautics and
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Astronautics, Yudao Street 29, Nanjing 210016, PR China
Fraunhofer Institute for Laser Technology ILT/Chair for Laser Technology LLT, RWTH
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Aachen, Steinbachstraße 15, D-52074 Aachen, Germany
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* Corresponding author. Tel./fax: +86 25 52112626. E-mail: [email protected]
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ACCEPTED MANUSCRIPT Abstract The in-situ TiO2 reinforced Ti-Ni composite coating on carbon steel was successfully
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prepared by laser metal deposition (LMD) using Ti-Ni as-mixed powder with an atomic ratio
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of 60:40. With the aim of in-situ reaction design during LMD processing, a trace of oxygen
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mixed with the shielding gas was introduced. Different “laser energy input per unit length” (E) by changing the laser power was set to investigate the influence on the deposition quality and
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attendant microstructure and mechanical property of the LMD-processed layer. TiO2 particles with unique flower-like structure formed when the applied laser energy E≤96 kJ/m, while the
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apparent oxidation of grain boundaries was observed as E increased to 120 kJ/m. The
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formation mechanism of in-situ TiO2 particles with a flower-like structure was present. At the
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optimized process parameter of 96 kJ/m, the LMD-processed layer showed the highest densification degree free of any pores and cracks. The corresponding mechanical properties
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were measured, showing the relatively high average microhardness of HV0.2 790 and significantly improved tribological property containing lower coefficient of friction of 0.4 and
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more smooth worn surface.
Key words: laser metal deposition (LMD); Ti-Ni alloy; coating; composite
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ACCEPTED MANUSCRIPT 1. Introduction Ferrous alloys, as the typical structural material, have got wide applications in
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engineering filed due to their excellent comprehensive properties and low cost. Normally, the
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components made of ferrous alloys are exposed to various kinds of external environments,
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which may lead to an early failure because of some reasons such as corrosion and abrasion. Two main approaches are generally used to improve the properties of ferrous alloys, namely
has been paid more attentions into [1-3].
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alloying and surface modification. Taking into account of the cost, the surface modification
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Laser metal deposition (LMD), as one of the most prevailing additive manufacturing
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(AM) technologies, has got rapid development in recent years. Due to its highly versatile
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process capability, LMD has been widely applied to manufacture new components, to repair and rebuild worn or damaged components, and to prepare wear- and corrosion-resistant
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coatings [4-6]. When used for material surface property modification, LMD is also known as laser cladding or laser depositing. Based on the processing features of AM technology, namely
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layer-to-layer shaping and consolidation of feedstock using a computer-controlled laser as the energy source, LMD possesses the nature of highly non-equilibrium, rapid melting and solidification [7-9]. Consequently, uniform distribution of chemical components and novel microstructures, which are hard to be achieved by other conventional methods, can be introduced by LMD. Relative investigations on different types of laser-deposited coatings or layers on the surface of ferrous alloys have been reported. For example, Qunshuang Ma et al prepared the Ni60/WC composite coatings on the surface of Q550 steel. A special cored-eutectic structure was obtained, which had advantages of well-distribution and tight binding with the matrix [10]. Dariusz Bartkowski et al successfully achieved laser cladding 3
ACCEPTED MANUSCRIPT Stellite-6/WC composite coatings on the surface of low-carbon steel. This study indicated that the microhardness enhanced with the volume fraction of WC increasing while the corrosion
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resistance weakened [11].
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Titanium and its alloys, due to the excellent specific strength, fracture resistance
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characteristics and high corrosion resistance, have won extensive applications in engineering filed [12, 13]. However, the poor tribological properties including high friction coefficient,
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low adhesive and fretting wear resistance restricted the application of Titanium alloys as coatings to a large extent [14, 15]. Ti-Ni alloys have been proven to be an alternative coating
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material, owing to the significant intensification of tribological properties by the formation of
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Ti-Ni intermetallic. The correlative research works have been carried out. M．Salehi et al
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successfully prepared Ti-Ni intermetallic coatings on the surface of carbon tool steel by a duplex process. During the duplex process, the Ni coating with the thickness of 20 μm was
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firstly achieved by electroplating, and then Ni-coated specimens were packed in the Ti-rich powder mixtures in order to develop Ti-Ni intermetallic coatings. Besides, diffusion annealing
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was carried out to reduce compositional gradient profile from surface to core and the final phases were composed of TiNi, Ti2Ni, TiNi3 and FeNi [16]. F. T. Cheng et al systematically investigated laser cladding of AISI 316 stainless steel using respectively preplaced NiTi wire, NiTi strips and NiTi powder. In above three conditions, a good coating with high microhardness and tough interface, free of any pores and cracks, was all obtained. As a result, the surface cavitation erosion resistance of AISI 316 stainless steel achieved a significant improvement [17-19]. Further improvement has been obtained by addition of the third alloying element or reinforcement particles, such as N, Al and ZrO2. Cunshan Wang et al reasonably designed the Ti70.3Ni22.2Al7.5 alloy, optimized from a basic binary eutectic 4
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solution and Ti2Ni intermetallic compound resulting in high hardness, good wear resistance
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and corrosion resistance [20]. Babatunde A. Obadele et al studied the influence of laser scan
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speed and additive ZrO2 particle on microstructure, microhardness, wear resistance and corrosion resistance for laser deposited Ti-Ni coating on the surface of AISI 316 stainless steel.
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When the laser scan speed reached 0.0067 m/s, the Ti-Ni coating showed the most excellent corrosion resistance and the addition of ZrO2 was able to enhance dramaticlly the corrosion
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resistance [16].
According to the above investigations, the addition of the third component in Ti-Ni alloy
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can introduce prominent “gain effect” and efficiently strengthen the mechanical or chemical property of Ti-Ni alloy. However, the investigations on the Ti-Ni composite coating reinforced
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by particles are still lacking, especially on non-equiatomic Ti-Ni composite coating as well as for the application on surface modification of ferrous alloy. In this study, in-situ TiO2
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reinforced Ti-Ni composite coating on carbon steel was successfully prepared by LMD using Ti-Ni as-mixed powder. By the introduction of little oxygen mixed with the shielding gas, in-situ TiO2 particles with different structures were observed to form within the matrix during the LMD processing with different process parameters, which had an important effect on the deposition quality and attendant mechanical property of the laser-deposited layer. TiO2 particle, as a reinforcing phase, has been applied for some other metal matrixes, such as A356 aluminum alloy [21], but few applications for Ti-Ni alloy matrix have been reported. Furthermore, this result will be helpful to prepare in-situ oxide particles reinforced Ti-Ni composite coating by the introduction of appropriate oxygen carrier during laser processing. 5
ACCEPTED MANUSCRIPT Hence, in this present paper, different laser processing parameters were set in order to investigate the influence on the deposition quality and attendant microstructure and
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2. Experiment procedure
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TiO2 particles with a flower-like structure was present.
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mechanical property of the deposited layer. Meanwhile, the formation mechanism of in-situ
2.1 Materials
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The gas-atomized, spherical Titanium powder (99.5% purity) with a mean particle size of 30 μm and the spherical-shaped Nickel powder (99.5% purity) in an average particle size of
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10 μm were used as feedstock powders in this study. Titanium powder and Nickel powder in
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an atomic ratio of 60:40 were uniformly mixed in a high-energy Pulverisette 6 planetary
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mono-mill (Fritsch GmbH, Germany) using a ball-to-powder weight ratio of 5:1, a ration speed of the main disc of 200 r/min, and a milling time of 4 h. The as-mixed powder still
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demonstrated original spherical morphology, as shown in Fig. 1b. Besides, carbon steel was utilized as the substrate material, which was sandblasted and ultrasonically cleaned
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successively in acetone and deionized water to remove surface contaminants before LMD. The detailed chemical compositions of raw powders and substrate were present in Table 1. Table 1 The chemical compositions of raw powders and substrate Composition in wt. % Carbon steel Ni-Ti mixed powder
Fe
C
Si
Mn
Cr
Ni
Cu
Ti
Balance
0.88
0.17
0.6
0.12
0.1
0.25
0
0
0
0
0
0
55.03
0
44.97
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ACCEPTED MANUSCRIPT surface of carbon steel substrate was present in Fig. 1a. Laser metal deposition (LMD) processing was carried out with a 5-axis CNC system, a Trumpf Nd:YAG laser system with a
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maximum output power of 3 kW and a focused spot diameter of 0.6 mm, integrated with a
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powder feeder system, and a coaxial powder nozzle. The premixed Ti-Ni powder was injected
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into the melted pool through the coaxial nozzle with a powder feeding rate of 2.4 g/min. At the same time, with the aim of in-situ reaction design, Argon mixed with a trace of oxygen
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acted as carrier gas during laser deposition. The multiple tracks were cladded for the layer with the dimension of 5 mm × 28 mm. LMD fabrication was based on the line-by-line laser
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cladding, using a constant laser scan speed of 500 mm/min and various laser powers of 400W,
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600 W, 800 W, 1000W and 1200W. An integrated parameter “laser energy input per unit
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length” (E), which was defined by E=P/ν, as shown in table 2, was used to estimate the laser energy input to the deposited layer. Although five samples with different processing
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parameters were prepared in this study, the microstructure features of sample N1 with 48 KJ/m were similar as sample N2 with 72 KJ/m and the microstructure features of sample N4
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with 120 KJ/m were similar as sample N5 with 144 KJ/m. Therefore, three samples (N2, N3, and N4), as the typical ones, were chosen for the microstructure study. The macrophotograph of final LMD-fabricated layers was present in Fig. 1c. Table 2 The LMD process parameter Sample
Laser power (W)
Scan speed (mm/min)
Energy density (kJ/m)
N1 N2 N3 N4 N5
400 600 800 1000 1200
500 500 500 500 500
48 72 96 120 144
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diffractometer (XRD) with Cu Kα radiation (λ = 0.154 18 nm) at 40 kV and 40 mA using a
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continuous scan mode. Three Specimen N1, N2, and N3 for metallographic examinations
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were prepared according to the standard procedures and etched with a solution composing HF, HNO3, and distilled water with a volume ratio of 1:6:7 for 3 s. A PMG3 optical microscope
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(Olympus Corporation, Japan) was used to observe the low-magnification three-dimension morphology and cross-sectional microstructures of LMD-processed layer on the carbon steel.
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High-resolution studies of the microstructural features of the deposited layer were
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characterized using an S-4800 field emission scanning electron microscope (FE-SEM)
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(Hitachi, Japan) at an accelerating voltage of 5.0 kV. Chemical compositions were determined by an EDAX energy dispersive x-ray spectroscope (EDX) (EDAX, Inc., USA).
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2.4 Microhardness and dry sliding wear test The Vickers hardness was measured using a MicroMet 5101 microhardness tester
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(Buehler GmbH, Germany) at a load of 0.2 kg and an indentation time of 15 s. The tribological property of the specimen was estimated by the dry sliding wear tests conducted in a HT-500 ball-on-disk tribometer (Lanzhou ZhongKe KaiHua Sci. &Technol. Co., Ltd., China) in air at room temperature. The counter material was GCr15 bearing steel ball with a diameter of 3 mm and a mean hardness of HRC 60, using a test load of 220 g was applied. The friction unit was rotated at a speed of 560 rpm for 15 min, with the rotation radius of 2 mm. The coefficient of friction (COF) of the specimens was recorded during wear tests.
3. Results and Discussion 3.1 The quality of deposition and element distribution 8
ACCEPTED MANUSCRIPT Figs. 2a-4a demonstrate the three-dimension (3D) morphology of laser deposited layer at laser energy input per unit length (E) of 72, 96 and 120 kJ/m, respectively. Furthermore, to
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better reflect the quality of deposition, the corresponding composition profiles along the depth
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of the deposited layer as determined by EDX are also shown in Figs. 2b-4b. The parabolic
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shape bond line between the deposited layer and the substrate, characteristic microstructure feature of laser processing, was clearly observed from the cross-section of each sample (Figs.
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2-4a). Although adhesion of the deposited layer and the substrate was good at each parameter E, the overall quality of the deposited layer varied. When the applied E was 72 kJ/m, some
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cracks propagating in the overlap zone between two adjacent tracks could be obviously seen
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(Fig. 2a). Besides, some agminated pores were found at the beginning position of the crack
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lying in X-direction cross section. Then the EDX detection indicated that the concentrations of Fe, Ti, and Ni element along the depth of the deposited layer presented an apparent,
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near-invariable gradient (Fig. 2b). As E further increased to 96 kJ/m, a nearly full dense layer was achieved, free of any cracks and pores (Fig. 3a). In this condition, a transfer zone with an
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approximately 300 μm depth showed up and the concentrations of all elements tended to be invariable when the distance from the interface was over 300 μm (Fig. 3b). However, cracks running through the deposited layer were observed at the Y-direction cross section when the applied E reached 120 kJ/m (Fig. 4a). In this case a more homogenous element distribution with high Fe content was obtained (Fig. 4b). During LMD processing, both part of substrate material and coaxial feeding powder were melted, forming a molten pool with a continuous liquid front. The different laser processing parameter determined different amount of the liquid formed in the molten pool and resultant quality of deposition. When the E was relatively low, the amount of the liquid within 9
ACCEPTED MANUSCRIPT the molten pool was limited due to insufficient laser energy input, which made the liquid hardly spread out smoothly. As a result, the densification behavior within the molten pool
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weakened along with the emergence of pores and meanwhile the homogenization rate of all
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elements was restricted dramaticlly. Moreover, the fierce shrinkage induced by the melt
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solidifying rapidly along the horizontal and vertical direction of the molten pool led to the formation of tensile stress in the deposited layer. Specially, the overlap zone usually
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experienced the remelting and solidification process repeatedly, thus causing a higher stress concentration [22]. Then the combined effect of pores and tensile stress led to the initiation
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and propagation of cracks. On the other hand, when an excessive E was applied, more laser
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energy was absorbed by the substrate material and powder and then the temperature within
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the molten pool rose rapidly. Consequently, the formed melt was in a state of overheating, thus leading to the accumulation of thermal stress during subsequent solidification processing.
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On the maximum stress accumulated in the solidified layer exceeded the strength of the deposited alloy, cracks occurred immediately with the release of the accumulated stress. In
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this case, the excessive heat input accelerated vastly the diffusion of elements, thus achieving a more homogenous distribution state of elements. Besides, the height of the deposited layers shows an apparent fluctuation in Figs.2-4. This fluctuation is mainly attributed to the combined effect of surface tension and powder vaporization [23]. Due to the introduction of oxygen in this study, the surface tension gradient may show positive value, thus leading to formation of the Marangoni flow from the edge of melt pool to the center of melt pool and an increase of height of melt pool. With the laser power increasing, surface tension gradient enhances significantly and therefore the increasing effect of height of melt pool is more apparent. However, when excessive laser energy is input, 10
ACCEPTED MANUSCRIPT the powder vaporization effect gets intensified remarkably and consequently decreases the height of melt pool.
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3.2 Microstructure evolution along the depth of the deposited layer
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According to the above analysis, at an optimized E of 96 kJ/m, the optical micrograph of
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the cross-section microstructure of the deposited layer, locating at different regions from the surface, is shown in Fig. 5. In fact, the sample N2 has the similar cross-sectional
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microstructure features as the sample N3. Grain morphology transformations of the deposited layer from the top to the bottom are relatively coarsening steering dendrites, directional
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equiaxed grains, fine dendrites and planar crystals. The previous investigations have proved
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that the temperature gradients (G) and the solidification rates (R) during solidification can
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determine microstructure developments [24, 25]. The temperature gradient G and the solidification rate R can be calculated in the following equations [25]:
2 K (T T0 ) 2 P
(1) (2)
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R Vs cos
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G
where K is the thermal conductivity of the material (W m−1 K−1), T is the liquidus temperature of the alloy (K), T0 is the initial temperature of the substrate (K), η is the laser absorption coefficient, P is the laser power (W), Vs is the laser scanning speed (m s-1) and θ is the angle between Vs and R. In general, the value of G/R was used to evaluate the influence on the morphology of the liquid/solid interface or microstructure developments [25]. According to the above equations, G/R could be estimated by:
G/R
2 K (T T0 ) 2 PVs cos
(3)
At the bottom of the molten pool (~50 μm from the interface), the solidification rate R was 11
ACCEPTED MANUSCRIPT nearly perpendicular to the laser scanning speed Vs and the temperature gradient G reached the maximum value due to the contacting the substrate, resultantly G/R getting the maximum
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value. Thus the melt was in a state of overheating and consequently the solidification
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microstructure was planar crystal. With the solidification front of the melt advancing (~300
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μm from the interface), the previously solidified layer acted as the substrate and consequently the initial temperature T0 increased, thus leading to a decrease of temperature gradient G
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located in the solidification front of the melt. At the same time, the angle θ between Vs and R became lower, causing the increase of solidification rate R. In this condition, the value of G/R
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decreased significantly and as a result constitutional undercooling occurred. In this case,
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dendrites growth pattern took over a leading position. When the liquid/solid interface
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continued to move, both the bottom and the top of the molten pool were being solidified due to the contacting the substrate and the air, which led to the change of G direction and
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subsequently the formation of steering dendrites at the top of the molten pool (~50 μm from the surface). Then the solidification rate from the bottom to the top was faster than that from
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the top to the bottom, leading to two similar temperature gradients in the upper part of the deposited layer (~200 μm from the surface) and resultant directional equiaxed grains [24]. 3.3 Microstructure evolution and phase identification as E increasing Also, to depict the influence of E by changing laser power on the cross-section microstructures of the deposited layer, the corresponding FE-SEM characterization is performed at the same distance (~300 μm) from the interface, as shown in Figs. 6-8. At a relatively low E of 72 kJ/m, the main microstructure demonstrates typical dendrite and at the same time some dispersive particles with submicron scale are observed (Fig. 6a and b). Highmagnification FE-SEM micrograph shows the detailed microstructure of these dispersive 12
ACCEPTED MANUSCRIPT particles, a cluster structure by several finer particles getting together (Fig. 6c). The XRD experiment identifies Fe2Ti, FeNi3 and TiO2 as the main constituent of the microstructure (Fig.
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6d). Besides, a few Ti-Ni intermetallic phases, such as Ni4Ti3 and Ni3Ti, were also detected.
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The elevated E (reaching 96 kJ/m) results in the transformation from typical dendrite to
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cellular dendrite (Fig. 7a and b). Interestingly, particles with unique flower-like structure are found to distribute within the intercellular regions homogenously (Fig. 7c). The detected main
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phases are still Fe2Ti, FeNi3 and TiO2 and meanwhile Ni4Ti3 phase are also identified clearly, according to the XRD results presented in Fig. 7d. As an excessive E is applied (120 kJ/m),
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the solidification microstructure is completely different, showing relatively loose
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microstructure (Fig. 8a). In this case, the diffraction intensity of Ti-Ni intermetallic phase is
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weak, as shown in Fig. 8b. Besides, the intergranular region was observed to get apparent oxidation reaction, as shown in Fig. 8c, which was proved by the further EDX
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characterization (Fig. 8d).
Based on the above transformation feature of grain morphology with E increasing, the
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typical dendrites development in sample N2 happens prior to that in N3 sample. The previous analysis has depicted the importance of G and R on the grain morphology. Moreover, the solution composition also influences the grain morphology [27]. The critical temperature gradient Gc was defined as [28]: Gc
ml C S (1 k ) kRDl
(4)
where ml was the slop of the liquidus line, Cl was the liquid composition in the liquid /solid interface, Cs=k Cl , the solid composition in the liquid /solid interface and Dl represented the diffusion coefficient. Constitutional undercooling occurred based on the premise of G < Gc 13
ACCEPTED MANUSCRIPT meeting constitutional undercooling [29], thus leading to the development of cellular or dendritic grains (the lower constitutional undercooling tends to induce cellular grains while
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the higher constitutional undercooling tends to induce dendritic grains). Due to the
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insufficient laser energy input in sample N2, the maximum temperature within the molten
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pool and attendant Marangoni convention are limited, which causes the decrease of diffusion coefficient. Hence, the critical temperature gradient Gc is improved and consequently the
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constitutional undercooling region gets extended in N2 sample, which can account for the difference in the microstructures of N2 and N3 samples. However, it makes a difference when
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E reaches 120 KJ/m. As an excessive energy input is applied, a large amount of substrate
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material is melt and the melt are in a state of overheating, which accelerates the oxidation
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behavior within the molten pool. A number of oxygen atoms aggregate in the grain boundaries to cause the oxidation of grain boundaries, which may be believed to account for the
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formation of the observed solidification microstructure. 3.4 Formation mechanism of dispersive TiO2 submicronflower
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With the aim of in-situ reaction design during the LMD processing, a trace of oxygen by controlling the airflow velocity of shielding gas was introduced. EDX detection found that a number of particles observed in Figs. 6-8 belonged to an oxide phase TiO2 (Fig. 6e). The corresponding XRD characterization had indicated that the oxide phase was rutile TiO2 with relative strong diffraction peaks (110) and (211) (Fig. 6d, Fig. 7d and Fig. 8b). Specially, TiO2 particles with the flower-like structure could be observed when the applied laser energy E≤96 kJ/m (Fig. 6c and Fig. 7c). The flower-like structure became invisible when an excessive E value of 120 KJ/m was applied. Fig. 9a shows the crystal structure of Rutile, a tetragon with Ti4+ surrounded by six O2- at the corners of a slightly distorted octahedron and each O214
ACCEPTED MANUSCRIPT surrounded by three Ti4+ lying in a plane at the corners of an equilateral triangle [29]. As the molten pool formed by interaction between the laser and the powder, dissociative Ti atoms
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Ti O TiO
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formation processing of TiO2 might experience two stages:
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tended to unite with oxygen atoms, taking into account of the higher activity of Ti atom. The
TiO O TiO2
(5) (6)
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TiO2, as a high melting point phase, preferentially precipitated in the melt pool to form nanocrystalline. The observed crystal morphology schematic of rutile is shown in Fig. 9b [30].
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Due to the fact that the (110) crystal plane is the plane with the least energy, the growth rate of the (110) crystal plane along the <001> direction is higher than that of other planes [31].
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Hence, the zone size of the (110) crystal plane was larger than that of other planes in Fig. 9b. In XRD detection, the main peak of TiO2 corresponding to the (110) crystal plane
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demonstrated the strongest intensity, thereby proving the above point. In order to reduce the number of crystal planes with high energy, a cluster structure formed as more Ti and O atoms
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continued to deposit on the (110) crystal planes, thus improving the stability of precipitated TiO2 nanocrystalline. When the applied laser energy was elevated, the growth of the (110) crystal plane along the <001> direction got enhanced due to the sufficient heat input during the LMD processing, thereby leading to the formation of submicroflower structure. The corresponding schematic diagram is shown in Fig. 9c. However, when excessive laser energy is applied, a mass of heat input leads to the introduction of excessive Fe component and attendant improvement of activity of Fe atom. As a result, the combination of Fe and O atom is enhanced, which accounts for the formation of iron oxide in the intergranular region. Besides, the Marangoni convention [32] induced by the surface tension gradient within the 15
ACCEPTED MANUSCRIPT molten pool was also intensified significantly, thus disturbing the direction of heat flow and hindering the formation of TiO2 submicronflower.
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3.5 Mechanical property characterization
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Fig. 10 depicts the microhardness distributions along the depth of LMD-processed layers.
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For all laser-deposited samples, the average microhardness achieved significant improvement compared with the substrate material showing a very low average value of 280 HV0.2. The
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formation of Fe-Ti and Fe-Ni intermetallic as well as dispersive oxide particles were the main factors for the improvement. The maximum microhardness in N2 sample was measured on
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the near surface, reaching a level as high as 840.8 HV0.2, which was higher than that in other
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samples, implying that more Ti-Ni intermetallic phases might form in the near surface of N2
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sample in view of the XRD results. For the chosen typical processing parameters, the N4 sample had the lowest average microhardness in comparison with N2 and N3 samples. Due to
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the significantly intensified oxidation behavior and resultant loose solidification microstructure induced by an excessive laser energy input, the capacity of plastic deformation
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resistance gets weakened.
Fig. 11 shows the coefficients of friction (COFs) as well as the corresponding worn morphologies of the deposited layers and the substrate material. For the substrate material, the worn surface showed a very high roughness with severe material delamination and spallation. As a result, relatively high COF of 0.8 was obtained. An apparent improvement in tribological property was achieved when Ti-Ni alloy coating was prepared on the surface of substrate material. At the initial running-in stage of the wear test, the COF curves all exhibited fluctuation to some degree, which was attributed to the relatively rough surface and the existence of oxide film [33]. With the sliding time increasing, the fluctuation of COF tended 16
ACCEPTED MANUSCRIPT to be steady because of the decreased surface roughness and the stripping of oxide film. The worn surface morphologies of samples for typical processing parameters are shown
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in Fig 11. Nevertheless, the differences in COF values and worn morphologies indicated that
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the tribological property of the deposited layer was influenced significantly by the laser
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processing parameter. At the relatively low E of 72 kJ/m, the average COF value reached a high level of 0.58 and a number of wear debris composed of ultrafine particles produced on
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the worn surface. As the E increased to 96 kJ/m, the average COF value significantly decreased to 0.4, and the much smoother worn surface with shallow grooves was obtained,
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nearly free of any wear fragments. When the applied E increased to 120 kJ/m, the average
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COF value increased to the higher level of 0.65 and more wear debris as well as crumbling
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were found on the worn surface.
The tribological property of the deposited layer is closely related to its densification level,
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microstructures and microhardness [33]. When the E value is low, densification behavior of the LMD-processed layer is restricted due to the formation and expanding of
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stress/pore-induced cracks. This leads to a remarkable increase of the surface roughness, thus enlarging the amount of abrasive fragments. As a result, more hard and brittle fine particles produce as the surface levels off and the COF value shows an apparent fluctuation, showing a typical abrasive wear. Taking into account of the highest surface microhardness, further wear of the deposited layer is limited after the surface becomes relatively smooth. As the proper E is applied, sufficient laser energy guarantees the formation of good densification and homogenous microstructures, therefore leading to the enhanced wear resistance. At an excessive laser energy input, densification level decreases again due to the existence of cracks. Besides, the loose solidification microstructure with oxidized grain boundaries as well as the 17
ACCEPTED MANUSCRIPT lowest microhardness can be responsible for the relatively poor worn surface. However, the homogenously dispersed oxide particles with submicrometer scale in N2 and N3 samples are
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believed to play a role not to be ignored in wear test. These oxide particles can improve
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efficiently the strength of the deposited layer and then enhance the capability of plastic
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deformation resistance.
4 Conclusion
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The in-situ TiO2 reinforced Ti-Ni composite coating on carbon steel was successfully prepared by LMD using Ti-Ni as-mixed powder with an atomic ratio of 60:40. Different
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“laser energy input per unit length” (E) by changing the laser power was set to investigate the
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influence on the deposition quality and attendant microstructure and mechanical property of
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the LMD-processed layer. The main results can be summarized as: (1) The highest densification level and homogenous element distribution were obtained
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when a proper E of 96 kJ/m was applied, while the application of relatively lower or higher E resulted in the formation of cracks. Besides, the diffusion effect of Fe as well as oxidation
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behavior in the deposited layer got enhanced as an excessive laser energy input was applied. (2) At the proper E, the cross-sectional microstructure along the depth of deposited layer demonstrated variant transformations of grain morphology due to the change of temperature gradient and solidification rate. At the same distance from the interface, the transformation of grain morphology delayed with the applied E increasing, owing to the decrease of Gc induced by the accelerated diffusion coefficient within the molten pool. (3) Submicron TiO2 particles with unique flower-like structure were obtained when the applied laser energy E≤96 kJ/m. The formation mechanism of TiO2 submicronflower was present, mainly owing to the preferred growth of TiO2 along the <001> direction. 18
ACCEPTED MANUSCRIPT (4) As a proper laser energy E was set, the deposited layer achieved a relatively high microhardness with an average of HV0.2 790 while the substrate material showed a low
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average value of 280 HV0.2. Furthermore, the much smoother worn surface with shallow
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grooves as well as a considerably lower COF of 0.4 was obtained, in comparison to the
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substrate material. The formation of Fe-Ti and Fe-Ni intermetallic as well as dispersive oxide particles were the main factors for the above improvement.
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ACKNOWLEDGMENTS
The authors gratefully acknowledge the financial support from the National Natural
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Science Foundation of China (Nos. 51322509 and 51575267), the Top-Notch Young Talents
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Program of China, the Outstanding Youth Foundation of Jiangsu Province of China (No.
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BK20130035), the Program for New Century Excellent Talents in University (No. NCET-13-0854), the Science and Technology Support Program (The Industrial Part), Jiangsu
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Provincial Department of Science and Technology of China (No. BE2014009-2), the 333 Project (No. BRA2015368), Science and Technology Foundation for Selected Overseas
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Chinese Scholar, Ministry of Human Resources and Social Security of China, the Aeronautical Science Foundation of China (No. 2015ZE52051), the Shanghai Aerospace Science and Technology Innovation Fund (No. SAST2015053), the Fundamental Research Funds for the Central Universities (Nos. NE2013103 and NP2015206), and the Priority Academic Program Development of Jiangsu Higher Education Institutions, the Foundation of Graduate Innovation Center in NUAA and the Fundamental Research Funds for the Central Universities (No. kfjj20150605).
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Table 1
The chemical compositions of raw powders and substrate Composition in wt. % 0.88
0
Mn
Cr
Ni
Cu
Ti
0.17
0.6
0.12
0.1
0.25
0
0
0
55.03
0
44.97
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Balance
Si
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Table 2
C
0
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Carbon steel Ni-Ti mixed powder
Fe
0
The LMD process parameter
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Sample N1 N2 N3 N4 N5
Laser power (W)
Scan speed (mm/min)
Energy density (kJ/m)
400 600 800 1000 1200
500 500 500 500 500
48 72 96 120 144
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Captions:
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Fig.1 (a) The schematic diagram of LMD processing Ti-Ni layer; (b) the raw as-mixed Ti/Ni
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powder with an atomic ratio of 60:40; (c) the final LMD-processed Ti-Ni layers. Fig.2 (a) 3-D optical metallograph (OM image) composite showing the deposition quality of
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Ti-Ni layer at the relatively low E of 72 kJ/m; (b) the corresponding composition profiles along the depth of the deposited layer.
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Fig.3 (a) 3-D optical metallograph (OM image) composite showing the deposition quality of Ti-Ni layer at the proper E of 96 kJ/m; (b) the corresponding composition profiles along the depth of the deposited layer. Fig.4 (a) 3-D optical metallograph (OM image) composite showing the deposition quality of Ti-Ni layer at an excessive E of 120 kJ/m; (b) the corresponding composition profiles along the depth of the deposited layer. Fig.5 The optical micrograph of the cross-section microstructure of the deposited layer at (a) the near-surface region, (b) 1000 μm distance from the surface, (c) 500 μm distance from the surface, and (d) the bottom of the deposited layer under the proper E of 96 kJ/m. 24
ACCEPTED MANUSCRIPT Fig.6 (a) The cross-sectional microstructure of the deposited layer at ~300 μm distance from the interface, showing the typical dendritic morphology when a relatively low E of 72 kJ/m
T
was applied; (b) the high-magnification FE-SEM micrograph of (a); (c) particle morphology
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with a cluster structure; (d) XRD patterns obtained over a wide range of 2θ (20-90º); (e) EDX
SC R
characterization of point 1 located in (c).
Fig.7 (a) The cross-sectional microstructure of the deposited layer at ~300 μm distance from
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the interface, showing the cellular dendrite morphology when a proper E of 96 kJ/m was applied; (b) the high-magnification FE-SEM micrograph of (a); (c) particle morphology with
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a flower-like structure; (d) XRD patterns obtained over a wide range of 2θ (20-90º).
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Fig.8 (a) The cross-sectional microstructure of the deposited layer at ~300 μm distance from
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the interface, showing the iron filing shaped morphology when an excessive E of 120 kJ/m was applied; (b) the high-magnification SEM image of (a); (c) EDX characterization of point
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2 located in (b); (d) XRD patterns obtained over a wide range of 2θ (20-90º). Fig.9 (a) The crystal structure of Rutile; (b) the observed crystal morphology schematic of
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rutile; (c) the corresponding schematic diagram of particle morphology evolution. Fig.10 The microhardness distributions along the depth of LMD-processed layer, respectively corresponding to N1, N2, N3, N4 and N5 sample. Fig.11 The coefficients of friction (COFs) as well as the partially corresponding worn surface morphologies of the substrate and the deposited layer obtained at different Es of 44 kJ/m, 72 kJ/m, 96 kJ/m, 120 kJ/m and 144 kJ/m during the wear test. Table 1 The chemical compositions of raw powders and substrate. Table 2 The LMD process parameter.
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(b)
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Fig. 1
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CE P
Fig.2
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Fig.3
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(b)
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Fig.4
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ACCEPTED MANUSCRIPT ~200μm from the surface
(a)
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Top: ~50μm from the surface
(b)
Bottom: ~50μm from the interface
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~300μm from the interface
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CE P
Fig.5
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(a)
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(d)
(e) Fig.6
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(c)
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(d)
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Fig.7
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(b)
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(d)
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(c) Fig.8
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(b)
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(c)
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Fig.9
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Fig.10
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Fig.11
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Graphical abstract
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ACCEPTED MANUSCRIPT Highlights (1) The in-situ TiO2 reinforced Ti-Ni composite coating was successfully prepared by LMD; (2) The microstructure evolution along the depth of the deposited layer as well as with the
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CE P
TE
D
MA
NU
SC R
IP
T
variant of E was analyzed; (3) The formation mechanism of in-situ TiO2 submicronflower was present; (4) The LMD-processed layer obtained at 96 kJ/m showed high average microhardness and improved tribological property.
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