- Email: [email protected]
Microstructure evolution in Ti6Al4V alloy laser cladded with Premix Ti+TiB2 powders
Available online at www.sciencedirect.com
ScienceDirect Materials Today: Proceedings 4 (2017) 763–773
www.materialstoday.com/proceedings
5th International Conference of Materials Processing and Characterization (ICMPC 2016)
Microstructure evolution in Ti6Al4V alloy laser cladded with Premix Ti+TiB2 powders 1 2
M.O.H. Amuda1, 2*, E. T. Akinlabi1 and M. Moolla1
Modern and Advanced Manufacturing Systems Research Group, Department of Mechanical Engineering Science, University of Johannesburg, 2006, South Africa Materials Development and Processing Research Group, Department of Metallurgical and Materials Engineering, University of Lagos, Lagos, Nigeria 101017
Abstract:
Microstructure evolution in relation to the geometrical properties of Ti6Al4V treated with laser deposited premix Ti/TiB2 powders is presented. The microstructural characterisation is reinforced with X-ray diffractometry and scanning electron microscopy to identify the predominant phases responsible for the change inmicrohardness values of the cladded alloy. Geometrical analysis indicates that the aspect ratio is outside the range specified for the production of defect free claddings while the degree of dilution of XYZ ensured a thin interfacial layer which assists in avoiding crack in the cladded layer. A maximum microhardness value of WXY is achieved in the cladded layer which is about five folds greater than that of the substrate; and this is attributed to the presence of hard undissolved TiB2 particles in the cladded alloy. ©2017 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of Conference Committee Members of 5th International Conference of Materials Processing and Characterization (ICMPC 2016).
Keywords: Aspect ratio, bead geometry, dilution, microhardness, microstructure evolution, pre-mix powders, scan speed.
1.0 Introduction Titanium and its alloys are exotic materials that find application in major industrial fields such as aerospace, defense and armoury, marine and power, petrochemical, automobile and even in the medical field [1-3]. The wide acceptability of the material in the various fields is facilitated by a combination of attractive properties which are not readily obtainable in close competitor-material. These properties include lightweight, good corrosion and oxidation resistances in most common environments, good mechanical properties and excellent biocompatibility [4, 5]. In spite of these commendable properties, titanium alloys do perform poorly in service conditions requiring good tribological properties because of their low hardness, low galling resistance and high co-efficient of friction [6]. Additionally, unexpected early failures have been reported in medical implants produced from titanium materials essential due to corrosion-fatigue in the body. These E-mail address:[email protected] 2214-7853©2017 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of Conference Committee Members of 5th International Conference of Materials Processing and Characterization (ICMPC 2016).
M.O.H. Amuda et al./ Materials Today: Proceedings 4 (2017) 763–773
764
failures are further aggravated by the neurotoxicity and cytotoxity of aluminum and vanadium ions from the titanium alloys which harm the human body after long term immersion in body system [7]. These broaddeficiencies in titanium materials have sort of limited their application deficiencies in titanium materials have sort of limited their application to mild service conditions whereas they offer good potentials for extreme service conditions if these shortcomings could be addressed. Incidentally, these problems are surface phenomena based which do not affect the bulk properties of the material. Therefore, research efforts have focused on the means of manipulating the surface or near surface regions of titanium materials to improve their surface properties without altering the bulk features [2, 5, 6]. Conventional heat treatment methods cannot provide the necessary surface modification without affecting the bulk properties of the material. However, thermochemical treatments involving the introduction of gaseous alloying elements such as nitriding provided means for modifying both the composition and structure of the surface layer through the formation of hard TiN layer without altering the characteristics of the bulk material [8]. The depth of hardness achieved from the process is a function of temperature, time and nitrogen pressure; but the process requires high processing temperatures and long processing hours. This is the precursor to all known surface modification techniques in titanium and its alloys. Many other techniques have since then been applied to titanium materials for surface modification. These techniques include carburization, oxidation, physical vapour deposition, ion implantation, and laser surface modification (LSM). Among these techniques, LSM is widely applied for surface modification of titanium alloys because it is very flexible and offers minimum distortion of the substrate. Arising from these process attributes of LSM, many researchers have dedicated their investigations to the study of laser cladding of titanium materials to obtain defect-free coatings with improved surface properties particularly, excellent wear and oxidation resistances [5]. Progress is specifically being made in the cladding of titanium materials with ceramics to further improve the surface properties beyond what is obtainable with laser surface alloying through the formation of complex metal matrix ceramic composite coating [9]. In furtherance of the various works on laser cladding of titanium alloys with ceramics, the present work laser cladded aerospace grade titanium alloy with premix CpTi + TiB2 powders under different laser powers and characterized the resulting microstructure in relation to the microhardness of the substrate modified layer. The focus is to relate the degree of dilution and existence of defect to laser power in identifying powder range for preventing laser deposition defects. 2.0 Experimental Procedures Aerospace grade titanium alloy corresponding to Ti6Al4V commercial specification whose nominal composition is given in Table 1was used as the substrate material. The alloy was supplied by Titanium Metal Supply Inc. California in the annealed condition in 100 mm x 100 mm x 7.5 mm size dimensions. The working surface of the substrate was sand blasted with silica sand of grit size ~35 mesh size to brake-up oxide scales followed by vibratory rinsing in ethanol and acetone to dissolve-off descaled oxides and other dirt. The pre-deposition roughening was also used to improve absorptivity of the substrate. Commercially pure titanium (CpTi) and titanium diboride (TiB2) powders supplied by TLS Technik GmbH were premixed in the ratio 50/50 in a mechanical ball miller for 24 hours. The CpTi powder had a particle size ranging from 45 – 90 μm while that of the TiB2 powder was 40-120 μm. The premixed powder after mechanical ball milling is shown in Figure 1. Table 1: Compositional analysis of Ti6Al4V substrate material (%wt.) Elemental Composition
Substrate Ti6Al4V
Hardness(Hv)
C
H2
Fe
O
Al
V
Trace
Ti
0.030
0.006
0.120
0.170
5.98
3.70
0.4
Balance
380 ±2.75
M.O.H. Amudaet al./ Materials Today: Proceedings 4 (2017) 763–773
765
Laser surface cladding conducted using a continuous wave Rofin Sinar DY044, Nd: YAG laser with maximum power output of 4.0kW mounted on a KUKA articulated arm robot wasfacilitated by a 600 μm optical fibre which guided the raw laser beam to the cladding head. The powder nozzle and the laser beam were mounted 12 mm above the substrate and synchronized such that the powder stream coincided with the laser beam at the interaction zone. The pre-mixed powders were fed into the melt pool by means of an argon gas carrier which also acted as a shield against oxidation of the melt pool. A GTV powder feeder was used to feed the powder at a rate of 2.4 g/min while shielding was done at 10 l/min.The specimen clamped to the stage was traversed with respect to the laser beam in a single pass at a constant laser power but three different scan speeds using a XYZ translation stage. The energy density delivered to the cladding regions from the laser beam was estimated using Equation (1) [11];
Figure 1.Premixed Ti/TiB2 powders after mechanical ball milling with particle size averaging 60 μm.
P E = νD
(1)
where E is the energy density (J/mm2), P is the beam power (W), ν is the scan speed (m/min) and D is the beam diameter (mm). The schematic illustration of the laser cladding process is shown in Figure 2 while the process parameter details are presented in Table 2. After laser cladding, the samples were cross-sectioned in direction traverse to the laser track for microstructural examination. The samples hot mounted in resin were mechanically ground, polished and etched in Kroll’s reagent (100 ml H2O, 5 ml HNO3, and 2 ml HF) by swabbing for 15 seconds. The microstructures and geometric parameters were studied using Olympus Optical microscope (BX51M; Olympus) and TESCAN Scanning Electron Microscope (SEM) equipped with energy dispersive spectroscopy (EDS). The microhardness profiling of the clads was taken on the cross section of polished samples using MH-3 Vickers Microhardness indenter at a test load of 500 g for a dwell time of 15 seconds at indentation spacing of 100 μm. The phase identification in the laser cladded spots was conducted using CuK α radiation X-ray diffractometer model D8 Advance from Bruker Corporation supported with appropriate ICDD data file.
M.O.H. Amuda et al./ Materials Today: Proceedings 4 (2017) 763–773
766
Cladding Stage Figure 2. Schematic illustration of the laser cladding process (adapted from Cárcel et al. [10])
Table 2. Laser processing parameters for the cladding of premix CpTi/TiB2 powders on Ti6Al4V
Sample
Laser Power (kW)
Laser Spot Size (mm)
LC1 LC2 LC3
2.50
2
Scan Speed (m/min)
Energy Density (J/mm2)
0.50
150
1.00
75
1.50
50
Powder Flow Rate (g/min)
Gas Flow Rate (l/min)
2.40
1.50
3.0 Results and Discussion 3.1 Macroprofile and geometrical properties of clad The macrostructure of the cross section profile of a representative single track clad is shown in Figure 3 which depicts four distinct regions in the clad consisting of the resolidified zone as the fusion zone (FZ), the laser affected zone (LAZ), the heat affected zone (HAZ) and the unaffected substrate. The figure reveal good metallurgical bonding between the substrate and the deposited premixed powders but the presence of unmelted powder is also apparent in the figure but no cracks. The macroprofile describes the geometrical feature of the weld bead which provides a general reference for the cladding process in relation to the laser power and the scan speed. This profile is characterized in terms of the width and depth of the clad; and the area of the clad in relation to the area of the substrate melted. These characterizations are illustrated in Figure 4 where H (mm) is the clad height, W is the clad width, Ac is the area of
M.O.H. Amudaet al./ Materials Today: Proceedings 4 (2017) 763–773
767
5 μm Figure 3. Cross-section profile of a representative clad
Figure 4. Schematic illustration of main geometrical characteristics of substrate-clad profile [13]
the clad and Am is the area of the substrate melted. The areas of the clad and that of the melted substrate were estimated using the expression provided by Erinosho et al. [12]. The ratio of the clad width to clad depth is referred to as the aspect ratio (AR) as presented in Equation (2) and that of the area of clad region to the sum of area of the clad and area of the melted substrate as stated in Equation (3) is known as the degreeof dilution or dilution ratio (D). The AR represents a parameter that provides a range of quantitative value (3-5) for preventing porosity in laser cladding process particularly in overlapping tracks [13].The parameter D estimates the relative amount of the substrate that was molten during the laser irradiation and mixed with the clad material resulting in the formation of an interfacial bond which firmly bond the clad to the substrate.
W AR = H
(2)
Am D= Ac + Am
(3)
768
M.O.H. Amuda et al./ Materials Today: Proceedings 4 (2017) 763–773
The AR and parameter D of the three different clads are shown in Figure 5. The figure shows that the AR in the three clads range 12-18. This range is outside the window suggested for the avoidance of porosity stated earlier implying that under the process parameters considered for the laser cladding, the likelihood of incidence of porosity is very high and this is apparent in Figure 3. The figure equally indicates that the AR is a function of the energy density with the parameter increasing with increasing energy density. For a constant laser power, the energy density increases with decreasing scanning speed as shown in Equation (1). This is because a higher traverse speed reduced the beam interaction time with the melt ensuring that the powder fed was reduced and this translated to shorter deposit build. And the implication of this is that the cooling rate is equally high thus preventing the escape of entrapped shielding or in –process generated gas. This resulted in the presence of pores (Figure 3). In a single pass cladding, a high dilution ratio greater than 0.5 suggests that there was almost full depth melting of the substrate which mixed with the clad material producing a very wide interface region. This region has different thermochemical property relative to both the clad and the substrate; and thus represents a weak link for the initiation of cracks. In the present work, the interface region is quite thin and the dilution ratio range between 0.28 and 0.34 which is less than 0.5; hence the absence of cracks in the clad.
Figure 5. Aspect ratio and degree of dilution in cladded Ti6Al4V alloy under different energy densities
3.2 Microstructural Analysis
The microstructure of the uncoated substrate alloy and that cladded at specific energy density of 50 J/mm2 (P=2500W, ν=1.5m/min) is shown in Figure 6. The microstructure of the uncladded substrate (Figure 6a) consists of α and β composite phases with a refined grain structure elongated in the direction of rolling. This microstructure after laser cladding evolved to dendritic and columnar structure in the FZ and LAZ, respectively with equiaxed grains in the HAZ (Figure 6b). The columnar grains grown epitaxially from the interface which acted as the nucleation site towards the top surface. The figure revealed that there is a good adhesion of the coating to the substrate with no crack but isolated porosity. The growth direction of the grains suggests that solidification initiated from the substrate end towards the melt deposit. Fine interdendritic grain was observed within the columnar structure with isolated porosity. The region
M.O.H. Amudaet al./ Materials Today: Proceedings 4 (2017) 763–773
769
immediately outside the cladding zone is the LAZ which essentially consist of windmanstatten structure suggesting that melting and solidification also occurred in the zone except that it was not intense unlike the spot directly exposed to the laser beam. A magnified view of spot B in Figure 6b is shown in Figure 6c elucidating clearly the structural texture in the various zones of the cladded sample and the presence of isolated spores. The distribution of the dendritic structure is not-uniform indicating that the thermal flow across the cross-section of the samples is not uniform with different regions heated to different peak temperatures for different temporal times. Ravnikar et al. [14] reported similar structural condition in the LAZ of aluminum alloy laser coated with TiC/TiB2 powders attributing it to the existence of a temperature high enough to cause complete melting of the substrate under the coating. The dendritic arm spacings are narrow suggesting a very high cooling rate which is typical of laser melting process.
B
(b)
(a)
(c) Figure 6. Optical microstructure of: (a) uncladded Ti6Al4V substrate, (b) premix Ti/TiB2deposit on Ti6Al4V and (c) magnified view of circled spot B. spotat energy density of PQR (laser power=2500W, scan speed: 1.5m/min)
770
M.O.H. Amuda et al./ Materials Today: Proceedings 4 (2017) 763–773
The microstructural state observed in Figure 6 equally extends to samples laser treated with energy densities 50 J/mm2 except that the grain morphology in terms of the dendritic arm spacing differs owing to the difference in the cooling rate. Scanning electron microscopy image of the clad together with EDS established the presence of Ti, B, Al, and V (see Figure 7) which suggests the presence of Al-Ti intermetallics and undissolved TiB2. Ravnikar et al. [14] reported that no chemical reaction occurred between the molten substrate and the ceramic during laser remelting process. This implied that no dissolution of the ceramic component took place during the process. Chong et al. [15] and Anandkumar et al. [16] reported similar non-dissolution of TiC and TiB2 particles during the laser surface fabrication of metal matrix composite coating on the surface ofaluminum alloy, respectively. In the present case, it could be safely considered that there was no dissolution of the TiB2 powder during the laser cladding process. The TiB2 particles are however unevenly distributed in the resulting clad.
(a)
(b)
Figure 7. SEM micrograph and the corresponding elemental distribution: (a) image and (b)EDS spectrum
3.3 XRD Phase Identification The diffraction results were compared with the International Center Diffraction Data (ICDD) database and the measured values were close to the d values published in the database suggesting that the shift in the peak positions from the standard reference is not significant; and as such, the peaks can safely be considered as representing the identified phases. Therefore, the refined XRD spectra shown in Figure 8 indicate that the cladding surface consists of TiAl3, TiAl, Ti3Al, Ti and TiB2 phases. The intensity of the peaks show that the predominant phase in the cladding is TiB2 which indicates that the powder remained undissolved in the melt during laser irradiation. This is in conformity with established behavior of TiB2 during laser melting [16]. There is however isolated presence of Ti/Al intermetallics of various stoichiometry. The abundance of undissolved TiB2 particles in the clad suggests that the particle would be the driving phase influencing the surface property of the cladded titanium alloy.
771
Intensity (CPS)
M.O.H. Amudaet al./ Materials Today: Proceedings 4 (2017) 763–773
2θ Figure 8. X-ray diffraction spectra of Ti/TiB2 cladded Ti6Al4V alloy
3.4Analysis of Microhardness Profile Microhardness profile across the cladded alloy as a function of depth from the top of the cladding to the substrate alloy is shown in Figure 9. The figure shows that the microhardness value decreases with increasing distance from the surface to the substrate alloy. The microhardness hardness value is equally influenced by the energy density delivered to the samples. For the samples irradiated with a specific energy density of 50 J/mm2, the microhardness is in the range 1392 - 383 Hv0.5from the top surface to the substrate in the transverse direction. And for those treated with energy density of 75 J/mm2 and 150 J/mm2, the microhardness range 959 – 369 Hv0.5, and, 931 – 378 Hv0.5, respectively. The ranges of microhardness values obtained at the top surface is 3-4 times higher than that of the substrate. It is interesting to note that the lower end of the microhardness value is very close to the value for the uncoated substrate. This confirms that the laser cladding reaction was only restricted to the surface and near-surface regions of the substrate. These values are presumed to be influenced by the presence of aluminum-titanium intermetallics and the unmelted TiB2 particles in the cladded alloy. However, some few distance beneath the clad particularly within the LAZ and the HAZ, a drastic decrease in the microhardness value is observed. This further reinforced the postulation that the very high value obtained close to the surface is mainly influenced by the presence of the intermetallics and the TiB2 particles. This is because the LAZ and HAZ regions only experienced phase transformation and grain refinementsto a lesser extent compared to the resolidified zone, which may not be substantial enough to have greatly influenced the microhardness value. Furthermore, the differences in cooling rate as a result of the different specific energy densities delivered for the cladding process also influenced the variations in microhardness values. Substrate cladded at low specific energy density (50 J/mm2) experienced faster cooling rate which restricted the possibility of grain coarsening unlike that treated with energy density of 150 J/mm2. Samples with coarser grains typified by those treated at 150 J/mm2 provide less resistance to micro indentation, hence exhibit relatively lower microhardness values [17]. This apparently explain the difference in the microhardness values of the cladded alloy treated at different energy densities.
M.O.H. Amuda et al./ Materials Today: Proceedings 4 (2017) 763–773
772
Figure 9. Microhardness profile across the cross-section of the cladded Ti6Al4V alloy
4.0 Conclusion The geometrical and microstructural characteristics of Ti6Al4V alloy cladded with premix Ti/TiB2 powders have been elucidated. The aspect ratio and the degree of dilution range 12-18, and 0.28-0.34, respectively. For the aspect ratio, this range is outside the window recommended for the avoidance of porosity and as such isolated pores are present in the deposits. The degree of dilution is less than 0.5 and because of this, the interface region is very thin which assists in avoiding crack in the deposit. The microstructure in the alloy evolved from refined α/β pre-clad structure to one of dendritic and windmanstatten columnar structures in the resolidified zone and the LAZ, respectively. TiAl3, TiAl, Ti3Al, Ti and TiB2are identified as the phases present in the cladded alloy with TiB2 particles as the predominant phase. The microhardness of the cladded alloy is about 3-4 times higher than that of the substrate which is considered to be greatly influenced by the presence of TiB2and some aluminum-titanium intermetallics. Acknowledgement The laser materials processing centre, Council for Scientific and Industrial Research, Pretoria, South Africa is acknowledged for granting access to the laser facility for the cladding work. References [1]. [2]. [3]. [4].
Luo, J., Liu, X-B., Xiang, Z-F., Shi, S-H., Chen, Y., Shi, G-L., Wu, S-H. and Wu, Y-N. (2015). Synthesis of high-temperature self-lubricating wear resistant composite coating on Ti6Al4V by laser deposition. Journal of Materials Engineering and Performance, Vol. 24 (5), p. 1881-1889. Filip, R. (2006). Alloying of surface layer of the Ti6Al4V titanium alloy through the laser treatment. Journal of Achievements in Materials and Manufacturing Engineering, Vol. 15 (1-2), p. 174-180. Durdu, S., Deniz, O.F., Kutbay, I. and Usta, M. (2013). Characterization and formation of hydroxyapatite on Ti6Al4V coated by plasma electrolytic oxidation. Journal of Alloys and Compounds, Vol. 551, p. 422-429. Metikos-Hukovic, M., Kwokal, A. and Piljac, J. (2003). The influence of niobium and vanadium on passivity of titaniumbased implants in physiological solution. Biomaterials, Vol. 24(21), p. 3765-3775.
M.O.H. Amudaet al./ Materials Today: Proceedings 4 (2017) 763–773
[5]. [6]. [7]. [8]. [9]. [10]. [11]. [12]. [13]. [14]. [15]. [16]. [17].
773
Weng, F., Chen, C. and Yu, H. (2014). Research status of laser cladding on titanium and its alloys: A review. Materials and Design, Vol. 48, p. 412-425. Tian, Y.S., Chen, C.Z., Li, S.T. and Huo, Q.H. (2005). Research progress on laser surface modification of titanium alloys. Applied Surface Science, Vol. 242, p. 177-184. Azevedo, C.R.F. (2003). Failure analysis of a commercially pure titanium plate for osteosynthesis. Engineering Failure Analysis, Vol. 10, p. 153-164. Akgun, O.V. and Inal, O.T. (1994). Laser surface modification of Ti6Al4V alloy. Journal of Materials Science, Vol. 29, p.1159-1168. Jianing, L., Chuanzhong, C., Zhaoqing, L. and Squartini, T. (2011). Phase constituents and microstructure of laser cladding Al2O3/Ti3Al reinforced ceramic layer on titanium alloy. Journal of Alloys and Compounds, Vol. 509, p. 4882-4885. Cárcel, B., Serrano, A., Zambrano, J., Amigo, V. and Cárcel, A.C. (2014). Laser cladding of TiAl intermetallic alloy on Ti6Al4V: Process optimization and properties. Physics Proceedia, Vol. 56, p. 284-293. Pityana, S. L., Camagu, S.T. and Majumdar, J.D. (2015). Laser surface alloying of Al with Cu and Mo powders. The Journal of The Southern African Institute of Mining and Metallurgy, Vol. 115, p. 193-198. Mutiu, E.F., Akinlabi, E.T. and Pityana, S. (2014). Laser metal deposition of Ti6Al4V/Cu composite: a study of the effect of laser power on the evolving properties, in: proceedings of the world congress on Engineering, July 2-4, London, p. 1202-2017. Steen, W. M. (2003). Laser materials processing. 3rd edition, Springer-Verlag, Berlin, p. 320-329. Ravnikar, D., Mrvar, P., Medved, J. and Grum, J. (2013). Microstructural analysis of laser coated ceramic components TiB2 and TiC on aluminum alloy EN AW-6082-T651. Stronjniski Vetsnik-Journal of Mechanical Engineering, Vol. 59, p. 281-290. Chong, P.H., Man, H.C. and Yue, T.M. (2002). Laser fabrication of Mo-TiC MMC on AA 6061 aluminum alloy surface. Surface and Coatings Technology, Vol. 154 (2-3), 268-275. Anandkumar, R., Almeida, A. and Vilar. R. (2011). Wear behavior of Al-12Si/TiB2 coatings produced by laser cladding. Surface and Coatings Technology, Vol. 205 (13-14), p. 3824-3832. Amuda, M.O.H. and Mridha, S. (2012). Comparative evaluation of grain refinement in AISI 430 welds by elemental metal powder addition and cryogenic cooling. Materials and Design, Vol. 35, p. 609-618.
ScienceDirect Materials Today: Proceedings 4 (2017) 763–773
www.materialstoday.com/proceedings
5th International Conference of Materials Processing and Characterization (ICMPC 2016)
Microstructure evolution in Ti6Al4V alloy laser cladded with Premix Ti+TiB2 powders 1 2
M.O.H. Amuda1, 2*, E. T. Akinlabi1 and M. Moolla1
Modern and Advanced Manufacturing Systems Research Group, Department of Mechanical Engineering Science, University of Johannesburg, 2006, South Africa Materials Development and Processing Research Group, Department of Metallurgical and Materials Engineering, University of Lagos, Lagos, Nigeria 101017
Abstract:
Microstructure evolution in relation to the geometrical properties of Ti6Al4V treated with laser deposited premix Ti/TiB2 powders is presented. The microstructural characterisation is reinforced with X-ray diffractometry and scanning electron microscopy to identify the predominant phases responsible for the change inmicrohardness values of the cladded alloy. Geometrical analysis indicates that the aspect ratio is outside the range specified for the production of defect free claddings while the degree of dilution of XYZ ensured a thin interfacial layer which assists in avoiding crack in the cladded layer. A maximum microhardness value of WXY is achieved in the cladded layer which is about five folds greater than that of the substrate; and this is attributed to the presence of hard undissolved TiB2 particles in the cladded alloy. ©2017 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of Conference Committee Members of 5th International Conference of Materials Processing and Characterization (ICMPC 2016).
Keywords: Aspect ratio, bead geometry, dilution, microhardness, microstructure evolution, pre-mix powders, scan speed.
1.0 Introduction Titanium and its alloys are exotic materials that find application in major industrial fields such as aerospace, defense and armoury, marine and power, petrochemical, automobile and even in the medical field [1-3]. The wide acceptability of the material in the various fields is facilitated by a combination of attractive properties which are not readily obtainable in close competitor-material. These properties include lightweight, good corrosion and oxidation resistances in most common environments, good mechanical properties and excellent biocompatibility [4, 5]. In spite of these commendable properties, titanium alloys do perform poorly in service conditions requiring good tribological properties because of their low hardness, low galling resistance and high co-efficient of friction [6]. Additionally, unexpected early failures have been reported in medical implants produced from titanium materials essential due to corrosion-fatigue in the body. These E-mail address:[email protected] 2214-7853©2017 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of Conference Committee Members of 5th International Conference of Materials Processing and Characterization (ICMPC 2016).
M.O.H. Amuda et al./ Materials Today: Proceedings 4 (2017) 763–773
764
failures are further aggravated by the neurotoxicity and cytotoxity of aluminum and vanadium ions from the titanium alloys which harm the human body after long term immersion in body system [7]. These broaddeficiencies in titanium materials have sort of limited their application deficiencies in titanium materials have sort of limited their application to mild service conditions whereas they offer good potentials for extreme service conditions if these shortcomings could be addressed. Incidentally, these problems are surface phenomena based which do not affect the bulk properties of the material. Therefore, research efforts have focused on the means of manipulating the surface or near surface regions of titanium materials to improve their surface properties without altering the bulk features [2, 5, 6]. Conventional heat treatment methods cannot provide the necessary surface modification without affecting the bulk properties of the material. However, thermochemical treatments involving the introduction of gaseous alloying elements such as nitriding provided means for modifying both the composition and structure of the surface layer through the formation of hard TiN layer without altering the characteristics of the bulk material [8]. The depth of hardness achieved from the process is a function of temperature, time and nitrogen pressure; but the process requires high processing temperatures and long processing hours. This is the precursor to all known surface modification techniques in titanium and its alloys. Many other techniques have since then been applied to titanium materials for surface modification. These techniques include carburization, oxidation, physical vapour deposition, ion implantation, and laser surface modification (LSM). Among these techniques, LSM is widely applied for surface modification of titanium alloys because it is very flexible and offers minimum distortion of the substrate. Arising from these process attributes of LSM, many researchers have dedicated their investigations to the study of laser cladding of titanium materials to obtain defect-free coatings with improved surface properties particularly, excellent wear and oxidation resistances [5]. Progress is specifically being made in the cladding of titanium materials with ceramics to further improve the surface properties beyond what is obtainable with laser surface alloying through the formation of complex metal matrix ceramic composite coating [9]. In furtherance of the various works on laser cladding of titanium alloys with ceramics, the present work laser cladded aerospace grade titanium alloy with premix CpTi + TiB2 powders under different laser powers and characterized the resulting microstructure in relation to the microhardness of the substrate modified layer. The focus is to relate the degree of dilution and existence of defect to laser power in identifying powder range for preventing laser deposition defects. 2.0 Experimental Procedures Aerospace grade titanium alloy corresponding to Ti6Al4V commercial specification whose nominal composition is given in Table 1was used as the substrate material. The alloy was supplied by Titanium Metal Supply Inc. California in the annealed condition in 100 mm x 100 mm x 7.5 mm size dimensions. The working surface of the substrate was sand blasted with silica sand of grit size ~35 mesh size to brake-up oxide scales followed by vibratory rinsing in ethanol and acetone to dissolve-off descaled oxides and other dirt. The pre-deposition roughening was also used to improve absorptivity of the substrate. Commercially pure titanium (CpTi) and titanium diboride (TiB2) powders supplied by TLS Technik GmbH were premixed in the ratio 50/50 in a mechanical ball miller for 24 hours. The CpTi powder had a particle size ranging from 45 – 90 μm while that of the TiB2 powder was 40-120 μm. The premixed powder after mechanical ball milling is shown in Figure 1. Table 1: Compositional analysis of Ti6Al4V substrate material (%wt.) Elemental Composition
Substrate Ti6Al4V
Hardness(Hv)
C
H2
Fe
O
Al
V
Trace
Ti
0.030
0.006
0.120
0.170
5.98
3.70
0.4
Balance
380 ±2.75
M.O.H. Amudaet al./ Materials Today: Proceedings 4 (2017) 763–773
765
Laser surface cladding conducted using a continuous wave Rofin Sinar DY044, Nd: YAG laser with maximum power output of 4.0kW mounted on a KUKA articulated arm robot wasfacilitated by a 600 μm optical fibre which guided the raw laser beam to the cladding head. The powder nozzle and the laser beam were mounted 12 mm above the substrate and synchronized such that the powder stream coincided with the laser beam at the interaction zone. The pre-mixed powders were fed into the melt pool by means of an argon gas carrier which also acted as a shield against oxidation of the melt pool. A GTV powder feeder was used to feed the powder at a rate of 2.4 g/min while shielding was done at 10 l/min.The specimen clamped to the stage was traversed with respect to the laser beam in a single pass at a constant laser power but three different scan speeds using a XYZ translation stage. The energy density delivered to the cladding regions from the laser beam was estimated using Equation (1) [11];
Figure 1.Premixed Ti/TiB2 powders after mechanical ball milling with particle size averaging 60 μm.
P E = νD
(1)
where E is the energy density (J/mm2), P is the beam power (W), ν is the scan speed (m/min) and D is the beam diameter (mm). The schematic illustration of the laser cladding process is shown in Figure 2 while the process parameter details are presented in Table 2. After laser cladding, the samples were cross-sectioned in direction traverse to the laser track for microstructural examination. The samples hot mounted in resin were mechanically ground, polished and etched in Kroll’s reagent (100 ml H2O, 5 ml HNO3, and 2 ml HF) by swabbing for 15 seconds. The microstructures and geometric parameters were studied using Olympus Optical microscope (BX51M; Olympus) and TESCAN Scanning Electron Microscope (SEM) equipped with energy dispersive spectroscopy (EDS). The microhardness profiling of the clads was taken on the cross section of polished samples using MH-3 Vickers Microhardness indenter at a test load of 500 g for a dwell time of 15 seconds at indentation spacing of 100 μm. The phase identification in the laser cladded spots was conducted using CuK α radiation X-ray diffractometer model D8 Advance from Bruker Corporation supported with appropriate ICDD data file.
M.O.H. Amuda et al./ Materials Today: Proceedings 4 (2017) 763–773
766
Cladding Stage Figure 2. Schematic illustration of the laser cladding process (adapted from Cárcel et al. [10])
Table 2. Laser processing parameters for the cladding of premix CpTi/TiB2 powders on Ti6Al4V
Sample
Laser Power (kW)
Laser Spot Size (mm)
LC1 LC2 LC3
2.50
2
Scan Speed (m/min)
Energy Density (J/mm2)
0.50
150
1.00
75
1.50
50
Powder Flow Rate (g/min)
Gas Flow Rate (l/min)
2.40
1.50
3.0 Results and Discussion 3.1 Macroprofile and geometrical properties of clad The macrostructure of the cross section profile of a representative single track clad is shown in Figure 3 which depicts four distinct regions in the clad consisting of the resolidified zone as the fusion zone (FZ), the laser affected zone (LAZ), the heat affected zone (HAZ) and the unaffected substrate. The figure reveal good metallurgical bonding between the substrate and the deposited premixed powders but the presence of unmelted powder is also apparent in the figure but no cracks. The macroprofile describes the geometrical feature of the weld bead which provides a general reference for the cladding process in relation to the laser power and the scan speed. This profile is characterized in terms of the width and depth of the clad; and the area of the clad in relation to the area of the substrate melted. These characterizations are illustrated in Figure 4 where H (mm) is the clad height, W is the clad width, Ac is the area of
M.O.H. Amudaet al./ Materials Today: Proceedings 4 (2017) 763–773
767
5 μm Figure 3. Cross-section profile of a representative clad
Figure 4. Schematic illustration of main geometrical characteristics of substrate-clad profile [13]
the clad and Am is the area of the substrate melted. The areas of the clad and that of the melted substrate were estimated using the expression provided by Erinosho et al. [12]. The ratio of the clad width to clad depth is referred to as the aspect ratio (AR) as presented in Equation (2) and that of the area of clad region to the sum of area of the clad and area of the melted substrate as stated in Equation (3) is known as the degreeof dilution or dilution ratio (D). The AR represents a parameter that provides a range of quantitative value (3-5) for preventing porosity in laser cladding process particularly in overlapping tracks [13].The parameter D estimates the relative amount of the substrate that was molten during the laser irradiation and mixed with the clad material resulting in the formation of an interfacial bond which firmly bond the clad to the substrate.
W AR = H
(2)
Am D= Ac + Am
(3)
768
M.O.H. Amuda et al./ Materials Today: Proceedings 4 (2017) 763–773
The AR and parameter D of the three different clads are shown in Figure 5. The figure shows that the AR in the three clads range 12-18. This range is outside the window suggested for the avoidance of porosity stated earlier implying that under the process parameters considered for the laser cladding, the likelihood of incidence of porosity is very high and this is apparent in Figure 3. The figure equally indicates that the AR is a function of the energy density with the parameter increasing with increasing energy density. For a constant laser power, the energy density increases with decreasing scanning speed as shown in Equation (1). This is because a higher traverse speed reduced the beam interaction time with the melt ensuring that the powder fed was reduced and this translated to shorter deposit build. And the implication of this is that the cooling rate is equally high thus preventing the escape of entrapped shielding or in –process generated gas. This resulted in the presence of pores (Figure 3). In a single pass cladding, a high dilution ratio greater than 0.5 suggests that there was almost full depth melting of the substrate which mixed with the clad material producing a very wide interface region. This region has different thermochemical property relative to both the clad and the substrate; and thus represents a weak link for the initiation of cracks. In the present work, the interface region is quite thin and the dilution ratio range between 0.28 and 0.34 which is less than 0.5; hence the absence of cracks in the clad.
Figure 5. Aspect ratio and degree of dilution in cladded Ti6Al4V alloy under different energy densities
3.2 Microstructural Analysis
The microstructure of the uncoated substrate alloy and that cladded at specific energy density of 50 J/mm2 (P=2500W, ν=1.5m/min) is shown in Figure 6. The microstructure of the uncladded substrate (Figure 6a) consists of α and β composite phases with a refined grain structure elongated in the direction of rolling. This microstructure after laser cladding evolved to dendritic and columnar structure in the FZ and LAZ, respectively with equiaxed grains in the HAZ (Figure 6b). The columnar grains grown epitaxially from the interface which acted as the nucleation site towards the top surface. The figure revealed that there is a good adhesion of the coating to the substrate with no crack but isolated porosity. The growth direction of the grains suggests that solidification initiated from the substrate end towards the melt deposit. Fine interdendritic grain was observed within the columnar structure with isolated porosity. The region
M.O.H. Amudaet al./ Materials Today: Proceedings 4 (2017) 763–773
769
immediately outside the cladding zone is the LAZ which essentially consist of windmanstatten structure suggesting that melting and solidification also occurred in the zone except that it was not intense unlike the spot directly exposed to the laser beam. A magnified view of spot B in Figure 6b is shown in Figure 6c elucidating clearly the structural texture in the various zones of the cladded sample and the presence of isolated spores. The distribution of the dendritic structure is not-uniform indicating that the thermal flow across the cross-section of the samples is not uniform with different regions heated to different peak temperatures for different temporal times. Ravnikar et al. [14] reported similar structural condition in the LAZ of aluminum alloy laser coated with TiC/TiB2 powders attributing it to the existence of a temperature high enough to cause complete melting of the substrate under the coating. The dendritic arm spacings are narrow suggesting a very high cooling rate which is typical of laser melting process.
B
(b)
(a)
(c) Figure 6. Optical microstructure of: (a) uncladded Ti6Al4V substrate, (b) premix Ti/TiB2deposit on Ti6Al4V and (c) magnified view of circled spot B. spotat energy density of PQR (laser power=2500W, scan speed: 1.5m/min)
770
M.O.H. Amuda et al./ Materials Today: Proceedings 4 (2017) 763–773
The microstructural state observed in Figure 6 equally extends to samples laser treated with energy densities 50 J/mm2 except that the grain morphology in terms of the dendritic arm spacing differs owing to the difference in the cooling rate. Scanning electron microscopy image of the clad together with EDS established the presence of Ti, B, Al, and V (see Figure 7) which suggests the presence of Al-Ti intermetallics and undissolved TiB2. Ravnikar et al. [14] reported that no chemical reaction occurred between the molten substrate and the ceramic during laser remelting process. This implied that no dissolution of the ceramic component took place during the process. Chong et al. [15] and Anandkumar et al. [16] reported similar non-dissolution of TiC and TiB2 particles during the laser surface fabrication of metal matrix composite coating on the surface ofaluminum alloy, respectively. In the present case, it could be safely considered that there was no dissolution of the TiB2 powder during the laser cladding process. The TiB2 particles are however unevenly distributed in the resulting clad.
(a)
(b)
Figure 7. SEM micrograph and the corresponding elemental distribution: (a) image and (b)EDS spectrum
3.3 XRD Phase Identification The diffraction results were compared with the International Center Diffraction Data (ICDD) database and the measured values were close to the d values published in the database suggesting that the shift in the peak positions from the standard reference is not significant; and as such, the peaks can safely be considered as representing the identified phases. Therefore, the refined XRD spectra shown in Figure 8 indicate that the cladding surface consists of TiAl3, TiAl, Ti3Al, Ti and TiB2 phases. The intensity of the peaks show that the predominant phase in the cladding is TiB2 which indicates that the powder remained undissolved in the melt during laser irradiation. This is in conformity with established behavior of TiB2 during laser melting [16]. There is however isolated presence of Ti/Al intermetallics of various stoichiometry. The abundance of undissolved TiB2 particles in the clad suggests that the particle would be the driving phase influencing the surface property of the cladded titanium alloy.
771
Intensity (CPS)
M.O.H. Amudaet al./ Materials Today: Proceedings 4 (2017) 763–773
2θ Figure 8. X-ray diffraction spectra of Ti/TiB2 cladded Ti6Al4V alloy
3.4Analysis of Microhardness Profile Microhardness profile across the cladded alloy as a function of depth from the top of the cladding to the substrate alloy is shown in Figure 9. The figure shows that the microhardness value decreases with increasing distance from the surface to the substrate alloy. The microhardness hardness value is equally influenced by the energy density delivered to the samples. For the samples irradiated with a specific energy density of 50 J/mm2, the microhardness is in the range 1392 - 383 Hv0.5from the top surface to the substrate in the transverse direction. And for those treated with energy density of 75 J/mm2 and 150 J/mm2, the microhardness range 959 – 369 Hv0.5, and, 931 – 378 Hv0.5, respectively. The ranges of microhardness values obtained at the top surface is 3-4 times higher than that of the substrate. It is interesting to note that the lower end of the microhardness value is very close to the value for the uncoated substrate. This confirms that the laser cladding reaction was only restricted to the surface and near-surface regions of the substrate. These values are presumed to be influenced by the presence of aluminum-titanium intermetallics and the unmelted TiB2 particles in the cladded alloy. However, some few distance beneath the clad particularly within the LAZ and the HAZ, a drastic decrease in the microhardness value is observed. This further reinforced the postulation that the very high value obtained close to the surface is mainly influenced by the presence of the intermetallics and the TiB2 particles. This is because the LAZ and HAZ regions only experienced phase transformation and grain refinementsto a lesser extent compared to the resolidified zone, which may not be substantial enough to have greatly influenced the microhardness value. Furthermore, the differences in cooling rate as a result of the different specific energy densities delivered for the cladding process also influenced the variations in microhardness values. Substrate cladded at low specific energy density (50 J/mm2) experienced faster cooling rate which restricted the possibility of grain coarsening unlike that treated with energy density of 150 J/mm2. Samples with coarser grains typified by those treated at 150 J/mm2 provide less resistance to micro indentation, hence exhibit relatively lower microhardness values [17]. This apparently explain the difference in the microhardness values of the cladded alloy treated at different energy densities.
M.O.H. Amuda et al./ Materials Today: Proceedings 4 (2017) 763–773
772
Figure 9. Microhardness profile across the cross-section of the cladded Ti6Al4V alloy
4.0 Conclusion The geometrical and microstructural characteristics of Ti6Al4V alloy cladded with premix Ti/TiB2 powders have been elucidated. The aspect ratio and the degree of dilution range 12-18, and 0.28-0.34, respectively. For the aspect ratio, this range is outside the window recommended for the avoidance of porosity and as such isolated pores are present in the deposits. The degree of dilution is less than 0.5 and because of this, the interface region is very thin which assists in avoiding crack in the deposit. The microstructure in the alloy evolved from refined α/β pre-clad structure to one of dendritic and windmanstatten columnar structures in the resolidified zone and the LAZ, respectively. TiAl3, TiAl, Ti3Al, Ti and TiB2are identified as the phases present in the cladded alloy with TiB2 particles as the predominant phase. The microhardness of the cladded alloy is about 3-4 times higher than that of the substrate which is considered to be greatly influenced by the presence of TiB2and some aluminum-titanium intermetallics. Acknowledgement The laser materials processing centre, Council for Scientific and Industrial Research, Pretoria, South Africa is acknowledged for granting access to the laser facility for the cladding work. References [1]. [2]. [3]. [4].
Luo, J., Liu, X-B., Xiang, Z-F., Shi, S-H., Chen, Y., Shi, G-L., Wu, S-H. and Wu, Y-N. (2015). Synthesis of high-temperature self-lubricating wear resistant composite coating on Ti6Al4V by laser deposition. Journal of Materials Engineering and Performance, Vol. 24 (5), p. 1881-1889. Filip, R. (2006). Alloying of surface layer of the Ti6Al4V titanium alloy through the laser treatment. Journal of Achievements in Materials and Manufacturing Engineering, Vol. 15 (1-2), p. 174-180. Durdu, S., Deniz, O.F., Kutbay, I. and Usta, M. (2013). Characterization and formation of hydroxyapatite on Ti6Al4V coated by plasma electrolytic oxidation. Journal of Alloys and Compounds, Vol. 551, p. 422-429. Metikos-Hukovic, M., Kwokal, A. and Piljac, J. (2003). The influence of niobium and vanadium on passivity of titaniumbased implants in physiological solution. Biomaterials, Vol. 24(21), p. 3765-3775.
M.O.H. Amudaet al./ Materials Today: Proceedings 4 (2017) 763–773
[5]. [6]. [7]. [8]. [9]. [10]. [11]. [12]. [13]. [14]. [15]. [16]. [17].
773
Weng, F., Chen, C. and Yu, H. (2014). Research status of laser cladding on titanium and its alloys: A review. Materials and Design, Vol. 48, p. 412-425. Tian, Y.S., Chen, C.Z., Li, S.T. and Huo, Q.H. (2005). Research progress on laser surface modification of titanium alloys. Applied Surface Science, Vol. 242, p. 177-184. Azevedo, C.R.F. (2003). Failure analysis of a commercially pure titanium plate for osteosynthesis. Engineering Failure Analysis, Vol. 10, p. 153-164. Akgun, O.V. and Inal, O.T. (1994). Laser surface modification of Ti6Al4V alloy. Journal of Materials Science, Vol. 29, p.1159-1168. Jianing, L., Chuanzhong, C., Zhaoqing, L. and Squartini, T. (2011). Phase constituents and microstructure of laser cladding Al2O3/Ti3Al reinforced ceramic layer on titanium alloy. Journal of Alloys and Compounds, Vol. 509, p. 4882-4885. Cárcel, B., Serrano, A., Zambrano, J., Amigo, V. and Cárcel, A.C. (2014). Laser cladding of TiAl intermetallic alloy on Ti6Al4V: Process optimization and properties. Physics Proceedia, Vol. 56, p. 284-293. Pityana, S. L., Camagu, S.T. and Majumdar, J.D. (2015). Laser surface alloying of Al with Cu and Mo powders. The Journal of The Southern African Institute of Mining and Metallurgy, Vol. 115, p. 193-198. Mutiu, E.F., Akinlabi, E.T. and Pityana, S. (2014). Laser metal deposition of Ti6Al4V/Cu composite: a study of the effect of laser power on the evolving properties, in: proceedings of the world congress on Engineering, July 2-4, London, p. 1202-2017. Steen, W. M. (2003). Laser materials processing. 3rd edition, Springer-Verlag, Berlin, p. 320-329. Ravnikar, D., Mrvar, P., Medved, J. and Grum, J. (2013). Microstructural analysis of laser coated ceramic components TiB2 and TiC on aluminum alloy EN AW-6082-T651. Stronjniski Vetsnik-Journal of Mechanical Engineering, Vol. 59, p. 281-290. Chong, P.H., Man, H.C. and Yue, T.M. (2002). Laser fabrication of Mo-TiC MMC on AA 6061 aluminum alloy surface. Surface and Coatings Technology, Vol. 154 (2-3), 268-275. Anandkumar, R., Almeida, A. and Vilar. R. (2011). Wear behavior of Al-12Si/TiB2 coatings produced by laser cladding. Surface and Coatings Technology, Vol. 205 (13-14), p. 3824-3832. Amuda, M.O.H. and Mridha, S. (2012). Comparative evaluation of grain refinement in AISI 430 welds by elemental metal powder addition and cryogenic cooling. Materials and Design, Vol. 35, p. 609-618.