Ti6Al4V multi-material parts: Understanding thermal behavior evolution

Optics and Laser Technology 119 (2019) 105666

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Laser additive manufacturing of layered TiB2/Ti6Al4V multi-material parts: Understanding thermal behavior evolution

T

⁎

Caiyan Chen, Dongdong Gu , Donghua Dai, Lei Du, Rui Wang, Chenglong Ma, Mujian Xia College of Materials Science and Technology, Nanjing University of Aeronautics and Astronautics, Yudao Street 29, Nanjing 210016, Jiangsu Province, People’s Republic of China Jiangsu Provincial Engineering Laboratory for Laser Additive Manufacturing of High-Performance Metallic Components, Nanjing University of Aeronautics and Astronautics, Yudao Street 29, Nanjing 210016, Jiangsu Province, PR China

H I GH L IG H T S

finite element model was proposed to investigate the thermal behavior. • AThemultilayer thermal properties of diverse materials were considered. • The temperature-dependent bond forming mechanism at the interface was analyzed. • The metallurgical • layered TiB /Ti6Al4V multi-material parts were fabricated by SLM. 2

A R T I C LE I N FO

A B S T R A C T

Keywords: Additive manufacturing Finite element model TiB2/Ti6Al4V multi-material Thermal behavior

Selective laser melting (SLM) allows the fabrication of complex geometric shapes combined with improved functionalities and it has captured considerable attention for direct part production. While most studies conducted on the single materials, the multi-materials parts manufactured by this technology can offer more performance advantages of different materials to satisfy more demands in many engineering applications. Thus, TiB2/Ti6Al4V multi-material parts were fabricated to improve the properties of Ti6Al4V alloy components. In this study, a multilayer finite element model was proposed to investigate the complicated thermal behavior at the interface between the Ti6Al4V layer and TiB2 layer under varied process parameters. The temperature and temperature gradient distribution, remelting depth and liquid lifetime of the remolten pool at the as-fabricated Ti6Al4V layer during the SLM process of layered TiB2/Ti6Al4V multi-material parts were analyzed. The simulation results showed that the maximum temperature gradient was located at the interface. As the applied laser power increased from 300 W to 450 W, the maximum temperature gradient varied significantly, increasing from 24.920 °C/μm to 37.754 °C/μm while the maximum temperature gradient decreased slightly from 33.884 °C/μm to 31.478 °C/μm as the scan speed increased from 400 mm/s to 1000 mm/s. The interface temperature and liquid lifetime had an important influence on the wettability at the interface, which further impacted the metallurgical bonding at the interface. At the laser power of 400 W and the scan speed of 600 mm/s, a sound interfacial bonding could be achieved between the TiB2 layer and Ti6Al4V layer with a proper interface temperature of 2453 °C and liquid lifetime of 1.7 ms.

1. Introduction Ti6Al4V alloy has great potential applications in aerospace, chemical, marine and automotive industries owing to excellent properties including superior specific strength, high stiffness and exceptional corrosion resistance [1]. However, its application in severe environment is limited due to low hardness and poor wear resistance [2]. It

may cause inflammation and even implant failure because of wear resistance and the formation of oxide layer results in the severe adhesive wear in the surface layer owing to the high reactivity with oxygen [3,4]. Consequently, it is necessary to improve the properties of the Ti6Al4V alloy components to extend its possibility of application in severe abrasion environment. In which case, TiB2 can be introduced to join with Ti6Al4V in the form of TiB2/Ti6Al4V multi-material parts to

⁎ Corresponding author at: College of Materials Science and Technology, Nanjing University of Aeronautics and Astronautics, Yudao Street 29, Nanjing 210016, Jiangsu Province, People’s Republic of China. E-mail address: [email protected] (D. Gu).

https://doi.org/10.1016/j.optlastec.2019.105666 Received 17 April 2019; Received in revised form 9 June 2019; Accepted 24 June 2019 0030-3992/ © 2019 Elsevier Ltd. All rights reserved.

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models were conducted to investigate thermal behavior and further melting and solidification mechanisms of thermal behavior during selective laser melting. Roberts et al. [32] predicted three-dimensional temperature field for multiple layers of parts produced by additive layer manufacturing using an innovative simulation technique known as element birth and death. It suggested that the addition of more layers and subsequent laser scanning altered the temperature distribution in the preceding layers. Guo et al. [33] simulated the temperature gradient and thermal history in the multilayer by the buildup process. It was found that subsequent layers significantly affected the temperature gradient and the melting of previous layers which affected on the quality of SLMed parts. Meanwhile, Liu et al. [34] proposed a multilayer finite element model to study the thermal behavior as layered deposition during SLM process. They found that the neighboring underlying layer could be remelted thoroughly when laser irradiates a powder layer, thus forming an excellent bonding between neighboring layers. He et al. [35] established a multitrack and multilayer finite element model to simulate the temperature field and molten pool contours during SLM of 316L stainless steel powder. The results showed that the heat and stress were concentrated at the regions of bonding between adjacent layers and islands. All above studies were about the simulation of single material during multilayer manufacturing process of SLM. However, few existing reports were focused on the numerical simulation of the thermal behavior in the fabrication of multi-materials by SLM. In this study, a multilayer finite element model was proposed to predict the thermal behavior during the manufacturing of TiB2/Ti6Al4V multi-material parts with different process parameters. For the SLM fabrication of multi-materials, the mismatch of thermal-physical properties related with temperature results in the large temperature gradient and thermal stress at the interface and may cause pores and cracks in the final fabrication [36,37]. The process parameters have considerable effects on the transient thermal behavior and appropriate process parameters are crucial to alleviate the typical defects at the interface during SLM. Therefore, it is necessary to achieve a deep understanding of the relationship between process conditions and interfacial bonding of the multi-materials. The effects of laser power and laser scan speed on the thermal behavior at the interface about the temperature and temperature gradient distribution, liquid lifetime and remelting depth of the asprocessed Ti6Al4V layer were preliminary analyzed. Thus, the process parameters were optimized by the discussion of the formation of defects at the interface. In addition, the relevant experiments were conducted to investigate the interface bonding properties between different materials layers for validating the accuracy of the physical model.

improve the hardness and wear resistance characteristic of Ti6Al4V alloy components [5,6]. TiB2 has outstanding properties such as relatively high values of its melting point, hardness, elastic modulus, and wear resistance and especially almost equivalent thermal and physical properties (density and thermal expansion coefficient) with that of the Ti6Al4V alloy [7–9]. Various techniques have been employed to improve the properties of Ti6Al4V alloy parts by combining with the TiB2 such as pack boriding, tungsten inert gas cladding and magnetron sputtering etc. However, such approaches create challenges such as uncontrollable reactions, high porosity and easy oxidation [10–12]. Especially, it is inconvenient to prepare complex multi-material parts by conventional manufacturing method due to their restrictions in design of freedom [13]. In order to overcome these shortcomings, additive manufacturing (AM) which can produce free-form solid structures via layer by layer and under protection of argon atmosphere to avoid oxidation has been looked at for manufacturing TiB2/Ti6Al4V multimaterial parts as 3D printing continues to advance [14]. One of the most promising and popular systems that utilizes powder-bed fusion is selective laser melting (SLM) [15–17]. Selective laser melting allows the fabrication of complex geometric shapes combined with improved functionality that can be mass-customized without a need for part-specific tooling such as dies or mold [18–21]. Thus, SLM is expected to be a suitable process to solve the problem of multi-material connectivity. Al-Jamal et al. [22] adopted SLM to prepare Cu/H13 multi-material structure composite joints, which showed that multi-material structures have been obtained with a reliable interface and the resultant tensile strength of the formed part (288 MPa) was significantly higher than that of the tempered Cu alloy (235 MPa). In addition, Sing et al. [23,24] fabricated 316L/C18400 and AlSi10Mg/C18400 copper alloy multi-material samples by selective laser melting. It is demonstrated that good metallurgical bonding was obtained at the interface of multi-material laminates. Moreover, various excellent performance such as mechanical properties and corrosion resistance can be achieved by selective laser melting of multi-material [25,26]. Zhang et al. [27] have enhanced the corrosion resistance to meet the needs of implant applications by the SLM-produced Ti-5Cu alloy. Therefore, the SLM was adopted to accomplish the fabrication of multi-material laminated composite–Ti6Al4V and TiB2 in this study. During SLM, when the top surface of the powder bed is irradiated by incident laser beam, the interaction time between the laser and powder is so quite short that the fast phase transition involves complex nonequilibrium physical and chemical metallurgical process, exhibiting multiple modes of heat, mass and momentum transfer [28]. For multimaterial parts, the thermal behavior at the interface is particularly complex due to the differences in thermal-physical properties of different materials such as laser absorptivity, melting temperature, heat capacity, coefficient of linear thermal expansion, and thermal conductivity [29]. However, it is difficult to measure the thermal-physical statistics through experiment methods since the SLM process undergoes fast moving of laser beam, rapid melting and consolidation process and extremely limited liquid lifetime of molten pool. Accordingly, the numerical simulation approach can proffer an efficient and convenient way to investigate the thermal behavior of the SLM of multi-materials under different process parameters. In fact, some heat conduction models of SLM process have been established to study thermal behavior in the condition of the single layer. Hussein et al. [30] proposed a finite element model based on the powder bed to investigate the temperature fields in single layer built on the powder bed without support in SLM, and the results showed that the predicted length of the melt pool increased at higher scan speed while both width and depth of the melt pool decreased. Moreover, a threedimensional thermal model was developed to explore the heat transfer characteristics during the SLM considering the temperature-dependent thermal-physical properties by Yang et al. [31]. It was showed that the increased laser power was superior to the reduced scan speed in thermal performance. However, limited multilayer finite element

2. Model for SLM process and experiments 2.1. Physical description of the model The interaction between the incident laser beam and the dual materials is a complex process and the schematic of SLM process is presented in Fig. 1a. According to the defined scanning pattern, the heat flux from the laser beam is applied to the surface of the Ti6Al4V layer firstly and a great measure of laser energy is absorbed by powder particles while the remainder is scattered by radiation and convection [38]. A small sized molten pool is engendered after the rapid heating and consolidation, which is bonded with the substrate. When the TiB2 layer is irradiated by the laser beam, the TiB2 particles begin to melt as the temperature reaches the melting temperature of TiB2 (2980 °C). Meanwhile, the as-fabricated Ti6Al4V layer starts to remelt as the temperature on the Ti6Al4V layer raises to its melting temperature (1650 °C), giving rise to the interfacial bonding between different material layers. Thus, the thermal conduction, convection and radiation are the mainly heat transfer mechanisms during the SLM process, in which case, the thermal behavior at the interface of dual materials is quite complicated. 2

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Fig. 1. Schematic overview of the SLM physical model (a) the established finite element model (b) and laser scanning strategy (c) during SLM process (point 1 at the center of the Ti6Al4V layer; point 2 at the center of the TiB2 layer).

2.2. Basic setup of the finite element model

Table 1 Finite element simulation parameters.

A 3D finite element model was proposed for the numerical simulation of the thermal conditions of the multi-materials manufactured by SLM using ANSYS multi-physics finite element package, from which we can get a thorough understanding about the temperature evolution during SLM process and resultant interface bonding phenomenon of the multi-materials layers. The Fig. 1b depicts the 3D finite element model established during the SLM process with the dimensions of 1.4 mm × 0.7 mm × 0.4 mm including three layers. From the bottom to the top surface, which is followed by the titanium substrate, Ti6Al4V powder layer and TiB2 powder layer respectively. The height of each powder layer is 0.05 mm. Titanium alloy plate as the substrate for the powder bed is at the height of 0.3 mm. In order to improve the simulation efficiency and computational precision, the powder bed was meshed using the solid 70 hexahedron elements with the dimensions of 0.0175 mm × 0.0175 mm × 0.025 mm, while the substrate was coarsely meshed with a relatively coarsen tetrahedron elements. The whole 3D finite element model was meshed into 22,651 nodes and 54,819 elements totally. A reciprocating raster pattern was employed to complete the building of each layer in a track by track manner as shown in Fig. 1c. Point 2 situated at the center of TiB2 layer as the monitoring point while the point 1 situated at the center of the Ti6Al4V layer just below the point 2. The utilized process parameters are presented in the Table 1. In the model, the element birth and death method was applied to simulate the multilayers, which has been employed to model the threedimensional temperature field in multiple layers during SLM by Roberts et al [32]. The elements of TiB2 and Ti6Al4V layers were killed firstly and then the elements of the Ti6Al4V powder layer were activated. The Ti6Al4V layer was irradiated with high intensity heat flux until the selective scanning was finished. Meanwhile the TiB2 was visually present without addition of the overall stiffness to the Ti6Al4V layer. When

Parameter

Value

Absorptivity of Ti6Al4V, A Absorptivity of TiB2, A Powder layer thickness, d Laser spot size, D Hatch spacing, s Ambient temperature, T0 Process parameters used in the Ti6Al4V layer, P and v Laser power used in the TiB2 layer, P Scan speed used in the TiB2 layer, v

0.77 [39] 0.8 [39] 50 μm 70 μm 50 μm 20 °C 250 W, 600 mm/s 300, 350, 400, 450 W 400, 600, 800, 1000 mm/s

the calculation of the Ti6Al4V layer was completed, there was 4 s paused to activate the elements of the next TiB2 layer to mimic the spreading the next powder layer. 2.3. Governing equations During SLM process, the temperature of the whole powder system changes drastically as a result of the rapid movement of the laser beam. The spatial and temporal distribution of the temperature field analysis is a typical nonlinear transient heat conduction problem, which satisfies the following heat condition equation:

ρCP

∂T ∂ ∂T ∂ ∂T ∂ ∂T = (k ) + (k ) + (k ) + Q ∂x ∂x ∂y ∂y ∂z ∂z ∂t

(1)

where ρ is the material density, Cp is the specific heat capacity, T is the temperature of the powder system, t is the interaction time between the laser beam and the powder bed, k is the thermal conductivity, and Q is the heat generation per unit volume. The initial temperature distribution in the powder bed at time t = 0 3

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can be expressed as:

T (x , y, z , t )|t = 0 = T0 (x , y, z ) ∈ D

Table 2 Thermal physical parameters of Ti6Al4V [33,41,42].

(2)

where the ambient temperature (T0) is considered as 20 °C. In SLM process, the thermal boundary conditions can be given by [40]:

k

∂T − q + qc + qr = 0(x , y, z ) ∈ S ∂n

(3)

where S is the surfaces contacted with imposed heat fluxes, radiation and convection, n is the normal vector of surface S, the input heat flux q is showed in the following by Eq. (6). The heat loss qc caused by convection between the top surface of the powder bed and the ambient atmosphere can be expressed by:

qc = h (T − T0) b2 − 4ac

qr =

−

T04 )

(4)

(5)

The heat flux put on the powder bed is high intensity laser energy nearly distributed as Gaussian relationship, which can be mathematically described as [44]:

600

800

1000

1200

ks of powder Ti6Al4V (W/ m·°C) ks of solid Ti6Al4V (W/ m·°C) c of Ti6Al4V (J/kg·°C) ρ (kg/m3) Tm1 (°C)

0.145

0.104

0.083

0.167

0.279

0.813

1.09

7.07

9.44

11.8

14.5

17.4

19.79

25

580 4428 1650

610

650

720

765

936

1016

T (°C)

20

200

500

1000

2000

3000

ks (W/m·°C) c (J/kg·°C) ρ (kg/m3) Tm2 (°C)

96 617 4520 2980

90 800

81 1073

78.1 1160

77.3 1296

77.1 1400

keff kf

(6)

= (1 −

where A is the laser energy absorptivity of the powder as presented in the Table 1, P is the laser powder utilized, R is the effective laser radius, which represents the distance from the center of the laser spot to the position where the laser irradiance reduces to 1/e2 of its core and r is the radial distance of the point away from the center of the laser beam. The absorptivity of ceramics is distinct from this of the metallic materials, so there is an apparent difference in thermal history between Ti6Al4V layer and TiB2 layer, which can also affect or complicate the molten pool behavior of the interface between Ti6Al4V layer and TiB2 layer. The laser utilized in this study is a continuous Nd: YAG laser with wavelength of 1060 nm. Due to the limitation of experimental data, the absorptivity for Ti6Al4V powder was assumed to be the same as pure titanium powder equaling to 0.77, while the absorptivity for TiB2 powder was assumed to be the same as non-oxide ceramics with a relatively high absorptivity reaching 0.80 [39]. During SLM process, the powder particles start to melt when they are irradiated by the laser beam and the subsequent solidification will take place with rapid cooling. The latent heat occurred in the phase change, such as melting and solidification phenomenon in laser processing. At the melting point, there is a rapid change in the enthalpy due to the released latent heat during solidification [45]. Therefore, the latent heat in the phase phenomena should be taken into consideration for precise simulation. The enthalpy for the latent heat can be described as the function of the temperature:

∫ ρcdT

400

In order to ensure the efficiency of the simulation results, the utilized effective thermal conductivity of powder bed is very important. The thermal conductivity (ks) and specific heat capacity (c) of materials depending on temperature is shown in Tables 2 and 3. The influence factors for the thermal conductivity of powder bed are mainly including laser wavelength, powder particle size and morphology, as well as the packing structure and density as reported by Rombouts et al [46]. The effective thermal conductivity of the loose powder bed can be estimated using [47]:

2.4. Design of Gaussian heat source model

H=

200

2.5. Thermal properties used in the model

where σ is the Stefan-Boltzmann constant and ε is the emissivity of the powder particles.

2AP 2r 2 exp (− 2 ) q= 2 πR R

20

Table 3 Thermal physical parameters of TiB2 [43].

where h is the thermal convection coefficient, T is the temperature of the surface S. The heat loss qr owing to the laser radiation to the powder bed can be descried as:

σε (T 4

T (°C)

1 − ϕ )(1 +

⎧

2 ⎨1 − ⎩

kf ks

ϕkr )+ kf

1−ϕ

k f ⎞ ⎛ ks ⎞ kr ⎫ ⎤ ⎡⎛ ⎢ 1 − k ln ⎜ k ⎟ − 1⎥ + k ⎬ s⎠ ⎝ f ⎠ f ⎦ ⎣⎝ ⎭ ⎜

⎟

(8)

where φ is the porosity of the powder material, ks is the thermal conductivity of the bulk materials, kf is the thermal conductivity of the ambient atmosphere surrounding the powder and substrate and kr is the thermal conductivity part of the powder bed owing to radiation, which is donated by:

kr = 4FσTp3 Dp

(9)

where F is a view factor, which is approximately about 1/3, σ is the Stefan-Boltzmann constant, which is equal to 5.67 × 10−8 W m−2 K−4, TP is mean absolute temperature of the powder bed, Dp is the mean diameter of powder particles. In this model, the employed TiB2 ceramic powder and Ti6Al4V metallic powder had dramatic difference in thermo-physical properties. The Ti6Al4V powder bed was used as the first layer above the substrate, which was melted by the laser radiation firstly. The TiB2 powder bed would be scanned then after the previous Ti6Al4V layer was calculated completely. It was worth noting that the Ti6Al4V layer has been solid layer when the laser irradiated the TiB2 layer, in which case, the thermal physical properties would be changed because of phase transition. Other physical properties of powder materials used in this model are listed in the Tables 2 and 3.

(7)

where ρ is the density of the powder material, c is the specific heat capacity and T is the temperature of the melting occurred for the powder.

2.6. Experiment The multi-materials powder system included spherical Ti6Al4V 4

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Fig. 2. The 3D profile of transient temperature distribution (a) and temperature contour plots of the center of the TiB2 layer (point 2) (b); the 3D profile of transient temperature distribution (c) and temperature contour plots of the center of the Ti6Al4V layer (point 1) (d) during the SLM process when laser irradiated the center of the second TiB2 powder layer (point 2) (P = 400 W, ν = 600 mm/s).

95 μm in width respectively, which was much larger than that of the TiB2 layer and the depth of the TiB2 layer with the value of 11.9 µm was also much smaller than that of the Ti6Al4V layer with the dimension of 16.3 µm. Furthermore, the isotherm curves at the Ti6Al4V layer were more intensive than those at the TiB2 layer in the depth direction and the depth to width ratio of the molten pool at the TiB2 layer was much larger than that at the Ti6Al4V layer. The simulation results show that the isotherm curves were more intensive at the fore part of the ellipses than that at the tailed region of the ellipses. This is mainly due to the change of thermal conductivity caused by the transition from powder to solid, which in turn contributes to heat transmission in the powder layer. Moreover, the combination of the heat accumulation effect and the change of thermal conductivity as a result of powder to solid transition will cause that the laser spot center is ahead of the temperature maxima [48]. When the laser beam reached the point 2, the TiB2 layer was affected by the combination of the heat accumulation and the increased laser energy density. However, the molten pool size of the TiB2 layer was much smaller than that of the Ti6Al4V layer. It can be explained by that the melting temperature of TiB2 is far greater than that of Ti6Al4V. On the other hand, though a great deal energy is absorbed by the TiB2 powder located at the supper layer, the thermal conductivity of the TiB2 is much larger than that of the Ti6Al4V, which causes the heat accumulation at the interface and dissipates slowly. During SLM process, as the thermal conducted along the depth direction, the isotherm curves at the Ti6Al4V layer became more intensive than those at the TiB2 layer because the thermal conductivity of as-fabricated Ti6Al4V solid is much lower than that of TiB2 power. The optical properties of TiB2 are distinct from those of metallic materials with respect to the laser beam. While the interaction between laser beam and metallic powder is restricted to the vicinity of powder bed surface, the laser radiation can penetrate much deeper inside TiB2 powder media due to the high optical transparency and reflectivity of TiB2 powder [49]. Thus, the depth to width ratio of the molten pool at point 2 was much larger than that at the point 1. Therefore, different properties for multi-materials, such as the melting temperature, thermal conductivity and absorption have great impact on the distinct thermal behaviors at different material layers.

powder with a mean particle size of 25 μm and irregular TiB2 powder with the particle size distribution of 1–3 μm. The SLM equipment consisted of YLR-500-SM ytterbium fiber laser with a power of ∼500 W and a spot size of 70 μm, an automatic system for powder delivery and the argon as the protective atmosphere. The applied process parameters were same as those used in the simulations (Table 1). The fabricated samples for metallurgical examinations were cut, grounded and polished according to standard procedures and then etched with a solution composing of distilled water, HNO3, and HF with a volume ratio of 50:3:1 for 30 s. The characteristic cross-section morphologies study of the SLM-processed such as the metallurgical bonding and defects were conducted by a field emission scanning electron microscopy (FESEM). 3. Results and discussion 3.1. Temperature distribution at different material layers Fig. 2 depicts the transient temperature distribution of TiB2 layer and Ti6Al4V layer when the laser beam reached point 2 (Fig. 2a) with the P of 400 W and ν of 600 mm/s. It can be seen in Fig. 2a that an elongated horizontal contour emerged when laser beam moved along the X-axis. The maximum temperature was higher than the melting temperature of the TiB2 (2980 °C), which slightly fell behind the center of the laser spot. As shown in Fig. 2c, the isotherm curves on the surface of TiB2 layer were a series of ellipses and the fore part of which was more intensive compared with those at the tailed region. The dashed line circle presented in the temperature contour plot was the isotherm of the melting point of the TiB2 (2980 °C), which led to a small molten pool. The predicted temperature deceased from 3169 °C in the center of the molten pool to 2980 °C at the edge of the molten pool. Owing to the heat conduction from the TiB2 layer to the as-fabricated Ti6Al4V layer, the Ti6Al4V layer would remelt as the temperature exceeded the melting point of the Ti6Al4V (1650 °C). The remelting phenomenon at the as-fabricated Ti6Al4V layer resulted in a remolten pool as shown in Fig. 2d. The operative SLM temperature decreased from 2559 °C in the center of the remolten pool to the 1650 °C at the edge of the remolten pool and the maximum temperature was lower than the evaporation point (2976 °C). It can be found that the size of the molten pool on the surface of the as-fabricated Ti6Al4V layer reached 125 μm in length and 5

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Fig. 3. Temperature distribution along the Z axis from the center of the second TiB2 layer to the bottom of the Ti6Al4V layer (a) for different laser powers (ν = 600 mm/s) and (b) for different scan speeds (P = 400 W) when the laser irradiated the center of the second layer (point 2).

Fig. 4. Temperature gradient distribution along the Z axis from the center of the surface of the TiB2 layer to the bottom of the Ti6Al4V layer (a) using different laser powers (ν = 600 mm/s) and (b) using different scan speeds (P = 400 W) when the laser irradiated the center of the TiB2 layer (point 2).

TiB2 layer and Ti6Al4V layer were not melted completely under the circumstances of the interface temperature below the high melting temperature of the TiB2 (2980 °C). Furthermore, the temperature at the interface exceeded the melting temperature of the Ti6Al4V (1650 °C) while the maximum temperature did not exceed the evaporation temperature of the Ti6Al4V (2976 °C). The temperature distribution of the powder bed is affected by the laser power directly while the scan speed indirectly influences the temperature distribution by changing the interaction time between the powder particles and laser beam. Therefore, the laser power has a more pronounced influence on the temperature distribution than the scan speed. Furthermore, it can be found that a very steep temperature gradient is generated due to the rapid heating of the upper surface by laser and slow heat conduction to the underlying layers. As the heat conducted to as-fabricated Ti6Al4V layer, the temperature at the interface was continuously rising when the point 2 was irradiated. The asfabricated Ti6Al4V began to remelt when the temperature at the interface exceeded the melting temperature of the Ti6Al4V. Furthermore, the formation of TiB by the in-situ reaction of Ti6Al4V with TiB2, which leads to the formation of good interfacial bonding between the Ti6Al4V and TiB2, thus forming metallurgical bonding between neighboring layers [50,51]. Fig. 4 shows the temperature gradient along depth direction using varied P and ν during SLM of TiB2/Ti6Al4V multi-materials. The

3.2. Thermal behavior during SLM Fig. 3 shows effects of laser parameters on the temperature distribution during SLM of Ti6Al4V/TiB2 biomaterial along Z direction from the center of the surface of the TiB2 layer (point 2, Fig. 1c) to the bottom of as-processed Ti6Al4V layer (point 1, Fig. 1c) when the laser spot scanned at the point 2. The Tm1 represents the melting temperature of Ti6Al4V (1650 °C) while the Tm2 represents the melting temperature of TiB2 (2980 °C). The Te1 is the evaporation temperature of the Ti6Al4V (2976 °C). As the increase of the distance from the point 2 along the depth direction, the temperature monotonously decreased rapidly. The slope of the curves presented the temperature gradient of the powder bed and a steep slope meant a relatively high temperature gradient, which would be clarified in detail below. As shown in Fig. 3a, the temperature at any given location increased with the increase of laser power. In particular, the maximum temperature at the surface of the TiB2 layer increased from 2353 °C to 3515 °C as the laser power increased from 300 W to 450 W. On the other hand, it can be concluded from the Fig. 3b that the temperature at any given location decreased with the increase of scan speed. Meanwhile, the maximum temperature decreased from 3335 °C to 2858.5 °C with the increase of the scanning speed from 400 mm/s to 1000 mm/s. The maximum temperature was lower than the melting temperature of the TiB2 when ν reached 800 mm/s. In all cases, the TiB2 particles at the interface between the 6

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the applied P. As the laser power increased from 300 W to 450 W, the maximum interface temperature elevated from 1851 °C to 2765 °C because of a higher laser energy density input. Meanwhile, the liquid lifetime lengthened from 0.6 ms to 2.3 ms (Fig. 5a). On the other hand, enhancing the applied ν from 400 mm/s to 1000 mm/s resulted in the slight decrement of the maximum temperature from 2812 °C to 2190 °C. Consequently, the liquid lifetime also shortened from 2.79 ms to 0.95 ms. Moreover, it should be noted that the decreasing tendency of maximum temperature and liquid lifetime were alleviated when the scan speed was increased above 600 mm/s (Fig. 5b). In addition, comparing the data from Fig. 5, it revealed that the laser power had a more pronounced effect on the maximum temperature compared with the scan speed, and the liquid lifetime of the molten pool during SLM was more sensitive to the scan speed than the laser powder. Due to the extremely short interaction time between the laser beam and the powder particles, the SLM process involves highly non-equilibrium physical and chemical metallurgical phenomena, including the rapid melting and solidification process and liquid-solid phase transformation [54,55]. The dramatically different thermal-physical properties between the TiB2 and Ti6Al4V contributed to different melting station and complex phenomenon at the interface. In this condition, the as-fabricated Ti6Al4V started to melt as input laser energy conducted to the Ti6Al4V layer is sufficient to remelt Ti6Al4V while the TiB2 particles have not been melted completely during the SLM process. Therefore, a sound wettability is beneficial to lower the melt viscosity and further favors the spreading of liquid material around the surrounding ceramic particles to enhance the interfacial bonding ability [56,57]. As the applied P increased or ν decreased, the liquid lifetime of molten pool increased because of the enhancement of the amount of energy absorbed by the powers. When an overwhelmingly high scan speed (1000 mm/s) or an exceedingly low laser power (300 W) was utilized, an extremely short liquid lifetime (0.60 ms) and low temperature (1524 °C) occurred, which were unfavorable to produce sufficient liquid to moisten the unmelted ceramic particles and caused the aggregation of the unmelted particles. As the applied laser power increased or the scan speed decreased, the interface temperature increased and liquid lifetime prolonged, giving rise to the increase of the amount of particle rearrangement. It may cause the appearance of spherical pores and attendant high porosity at the interface. However, as the TiB2 layer was irradiated with a low scan speed (400 mm/s) or high laser power (450 W), it yielded an extremely high interface temperature (2812 °C) and long liquid lifetime (2.79 ms), which led to a large amount of liquid. It is difficult to control because of the high capillary instability of the melt and may generate the microcracks at the interface owing to heat accumulation. In conclusion, scan speed (600 mm/s) combined with a suitable laser power (400 W) played a paramount role in the

vertical dash line represented the interface between the TiB2 layer and Ti6Al4V layer. The temperature gradient increased from the surface of the TiB2 (Z = 0 μm) to the interface between the TiB2 layer and Ti6Al4V layer (Z = 50 μm) while that decreased from the interface to the bottom of Ti6Al4V layer (Z = 100 μm). When the applied P increased from 300 W to 450 W, the maximum temperature gradient varied significantly, increasing from 24.920 °C/μm to 37.754 °C/μm (Fig. 4a), which changed near linearly with the enhancement of the applied P. However, as the ν increased from 400 mm/s to 1000 mm/s, the maximum temperature gradient decreased slightly from 33.884 °C/ μm to 31.478 °C/μm (Fig. 4b). The simulation results illustrated that there was a slight decrease in the temperature gradient with the increase in ν during SLM of TiB2/Ti6Al4V multi-materials. For the data compared in Fig. 4, it can be concluded that laser power possesses a greater effect on the variation of the temperature gradient along the Z-direction than scan speed, mainly because the powder bed is controlled by the laser power directly. Additionally, as the distance from the point 2 along the depth direction increased, the temperature gradient gradually increased until it reached the maximum value and the temperature gradient started to decrease at the Ti6Al4V layer. Therefore, the maximum temperature gradient was at the interface between the TiB2 layer and Ti6Al4V layer. It can be explained by that the thermal conductivity of the TiB2 powder decreases with the raise of the temperature while the thermal conductivity of the solid Ti6Al4V layer increases in such conditions [33]. Because the heat is mainly dissipated through the previously fabricated layer, the heat flow direction during the SLM process is approximately perpendicular to the surface of as-fabricated layers and the temperature gradient along the depth direction is higher than that along the laser scanning direction. As the laser power increases, more thermal energy accumulation will occur at the interface due to low thermal conductivity of Ti6Al4V. In this situation, the temperature gradient at the interface will increases. [52] The high temperature gradient induced at the interface between the TiB2 layer and Ti6Al4V layer may lead to the formation of large thermal stress, which would further cause the appearance of the thermal cracks and/or deformation at the interface as a result of the release of the stresses [53]. As a relatively high power (450 W) was utilized, it might suffer from residual stress due to the high temperature gradient (37.754 °C/μm) at the interface between the TiB2 layer and Ti6Al4V layer, which may cause the crack and even delamination at the interface. The variation in the maximum temperature and the liquid lifetime of the molten pool at the as-processed Ti6Al4V layer with different parameters is illustrated in Fig. 5 when the laser irradiated the point 2 (Fig. 1c). It was obvious that the maximum temperature and the liquid lifetime of the molten pool at the Ti6Al4V layer were generally linear to

Fig. 5. The maximum temperature and the liquid lifetime of the as-processed Ti6Al4V layer during SLM using (a) different laser powers (ν = 600 mm/s) and (b) different laser scan speeds (P = 400 W). 7

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Fig. 6. Temperature distributions on the cross-section of the molten pool when the laser irradiated the center of the TiB2 layer (point 2) under different laser powers (ν = 600 mm/s): (a) 300 W, (b) 350 W, (c) 400 W, (d) 450 W. Variation of the remelting depth of the as-processed Ti6Al4V layer to the depth of the TiB2 layer ratio (Rd) under (e) different laser powers (ν = 600 mm/s).

the value of Rd as ν changed with a fixed P of 350 W. It can be seen that the shapes of the liquid pools on the cross-section were similar in various processing conditions though the size of the molten pool decreased with the increase of the applied ν. At a relatively lower ν of 400 mm/s, the molten pool with dimensions of 127.45 μm in width and 23 μm in depth was generated and corresponding Rd was calculated to be 0.46 (Fig. 7e). As the applied ν increased to 600 mm/s, the obtained molten pool size decreased to a smaller width value of 88.72 μm and depth of 16 μm, accordingly yielding a lower Rd of 0.32 (Fig. 7e). Furthermore, as the scan speed reached 600 mm/s, the decrement of the Rd became moderate as shown in Fig. 7e. On increasing the applied ν from 600 mm/s to 800 mm/s, the obtained molten pool size shortened by 19.55 μm and 3.4 μm whereas the Rd was slightly decreased from 0.32 to 0.252 (Fig. 7e). Owing to the too short remaining time of the laser beam, a considerably small molten pool was obtained with the dimensions of 63 μm in width and 10.7 μm in depth as the scan speed increased to 1000 mm/s (Fig. 7d). Furthermore, a relatively small value of Rd of 0.214 was generated as shown in Fig. 7e. As the applied P enhanced or ν decreased, the size of the crosssection molten pool at the Ti6Al4V layer increased gradually due to the resultant higher laser energy absorption. When the used P and ν were 350 W and 1000 mm/s, a narrow molten pool was obtained, in which case, the width of 34.5 μm was so small that the unmelted ceramic particles cannot be wetting by the molten metal liquid totally. It resulted in the poor interfacial bonding between TiB2 layer and Ti6Al4V layer. As the laser power increased to 400 W, the width size was larger

interfacial bonding between the TiB2 layer and Ti6Al4V layer with appropriate temperature of 2453 °C and a proper liquid lifetime of 1.7 ms. 3.3. Molten pool configuration of multi-material layers In order to observe the interface remelting degree, the as-fabricated Ti6Al4V layer to the depth of the TiB2 layer ratio was donated as Rd. Fig. 6 shows the change of cross-sectional temperature contours and Rd with variation of P at a fixed ν of 600 mm/s. The zone encompassed by the black dashed line (1650 °C) depicted the molten pool at the asfabricated Ti6Al4V layer, while the width and depth of which were specially noted in the temperature contours. At a lower P of the 300 W, a small molten pool was produced with a width value of 34.5 μm and depth of 5.5 μm. In this situation, the Rd was calculated to be 0.11 (Fig. 6e). For the case with a higher P of 350 W, the width and depth of the molten pool respectively increased by 29.81 μm and 6 μm respectively (Fig. 6b), obtaining the value of 0.23 in Rd (Fig. 6e). Moreover, the distribution of the isotherm curves was found to be similar to that of 350 W. On the increasing the applied P to 400 W, the molten pool with width of 88.72 μm and depth of 16.3 μm was formed. The resultant Rd reached a higher value of 0.326 (Fig. 6e). As applied P increased to 450 W, the intensified laser energy conducted from the TiB2 layer yielded a larger molten pool with length of 107.55 μm and depth of 20 μm, resulting in a significantly elevated Rd of 0.4 (Fig. 6e). Fig. 7 shows the change of cross-sectional temperature contours and 8

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Fig. 7. Temperature distributions on the cross-section of the molten pool when the laser irradiated the center of the TiB2 layer (point 2) under different scan speeds (P = 400 W): (a) 400 mm/s, (b) 600 mm/s, (c) 800 mm/s, (d) 1000 mm/s. Variation of the remelting depth of the as-processed Ti6Al4V layer to the depth of the TiB2 layer ratio (Rd) under (e) different scan speeds (P = 400 W).

than the hatch space (70 μm) while the laser penetration depth was found to be larger than the power layer thickness. Therefore, as the applied P was lower than 400 W or the ν was higher than the 600 mm/s, the neighboring layers cannot be bonded admirably since the required overlap rate between the TiB2 layer and Ti6Al4V layer was not achieved. A superior SLM-fabricated component rests with the sound mechanical bonding between adjacent tracks and the neighboring layers, especially for the combination of TiB2/Ti6Al4V multi-materials. To form a perfect bonding between neighboring layers, it is important to remelt the neighboring underlying layer thoroughly. In order to illustrate the remelting degree of the as-fabricated Ti6Al4V layer during the SLM of multi-materials, the remelting depth of the as-fabricated Ti6Al4V layer to the depth of the TiB2 layer ratio (Rd) is introduced. The simulation results indicate that Rd increased evidently with the enhancement of P, while varied slightly with increment of ν. In other words, the morphology of molten pool cross-section was more sensitive to the applied P compared with the utilized ν. At a relatively high P (P ≥ 400 W) or lower ν (ν ≤ 600 mm/s) utilized, the Rd had been enough to achieve a sound interfacial bonding between the TiB2 layer and Ti6Al4V layer.

Ti6Al4V layer. At the laser power of 300 W, insufficient laser energy input caused the presence of unmelted particles at the TiB2 layer, in which case, the remelting depth of the Ti6Al4V layer was 6.4 μm and the value of Rd was 0.128 (Fig. 8a). As the laser power further increased to 350 W, large unmelted TiB2 particles disappeared and few small unmelted TiB2 particles existed. Moreover, the deeper penetration was obtained with the Rd of 0.246 (Fig. 8b), indicating an improved interfacial bonding between neighboring layers and adjacent tracks. When the P reached 400 W, the remelting depth of the as-fabricated Ti6Al4V layer increased to 17 μm and the bonding of the adjacent layers was appropriate with relatively high Rd of 0.34. Moreover, the values obtained in the experiment are basically consistent with that in the simulation above (Fig. 6). During the SLM process, the laser energy input has a pronounced influence in the thermal behavior at the interface of the TiB2/Ti6Al4V multi-materials. As the laser power enhances, the higher temperature causes the more liquid lifetime with low viscosity, which in turn increases the dimensions of the remolten pool at the Ti6Al4V layer and promotes the liquid spreading around the ceramic particles. Combining the experiment analyses and the simulation results, it can be found that the metallurgical bonding at the interface between the TiB2 layer and Ti6Al4V layer is considerably affected by the laser power. At a lower laser power of 300 W, a considerably small amount of liquid phase of the as-fabricated Ti6Al4V layer with higher viscosity was formed due to lower temperature of 1851 °C at the interface, resulting in a remelting depth of 5.5 μm (Figs. 6b and 7a). It caused the appearance of lagersized unmelted particles among layers and resultant poor metallurgical

3.4. Experimental validation The cross-sectional metallographic microstructures of shaped samples with varying laser powers are shown in Fig. 8, in order to investigate the interfacial bonding properties between the TiB2 layer and 9

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Fig. 8. SEM images showing microstructures on the etched cross-section view of samples at different laser powers (ν = 600 mm/s): (a) P = 300 W; (b) P = 350 W; (c) P = 400 W.

creating sound interfacial bonding [60]. At a comparatively higher scan speed of 800 mm/s, a small amount of liquid phase of molten pool at the Ti6Al4V layer with a relatively high viscosity was yielded, resulting from the lower operating temperature of 2285 °C and extremely short liquid lifetime of 1.2 ms (Fig. 5b). It is disadvantageous that the formed liquid diffuses smoothly to fill the voids between the particles, which in turn emerges large-sized pores. Moreover, the existence of the unmelted TiB2 particles near the interface also results from the lower working temperature.

bonding behavior between adjacent layers (Fig. 8a). When the laser power enhanced to 350 W, a high interface temperature (2152 °C) and more liquid lifetime (1.3 ms) were generated, however, it was not capable to prevent the existence of the unmelted particles (Fig. 8b) to achieve a sound mechanical bonding between the TiB2 layer and Ti6Al4V layer. On increasing power to 400 W, the volume of liquid phase and remelting depth increased, resulting from the enhancement of molten pool temperature of 2453 °C. Appropriate dimensions with the width of 88.72 μm and the depth of 18 μm (Fig. 8c) were achieved, showing a superior interfacial bonding between the TiB2 layer and Ti6Al4V layer. Moreover, the values obtained in the experiment are basically consistent with that in the simulation above (Fig. 6). Fig. 9 shows the SEM images showing low-magnification microstructures on the etched cross-sections of samples and photographs of the spark during the SLM process at different scan speeds. At a relatively low scan speed of 400 mm/s, a considerably large spark was observed in the laser irradiating region, resulting in a severe splash of the heated powder. In this situation, the microcracks were presented at the interface, which hindered interfacial bonding to a large extent (Fig. 9a). As the applied ν increased to 600 mm/s, spark was smaller without any apparent splash. The near-full dense bonding was obtained at the interface between the TiB2 layer and Ti6Al4V layer, where there were no obvious pores and unmelted particles. When the ν reached 800 mm/s, a large spark was generated with a significant splash of molten materials. Therefore, an inferior interface quality characterized by lager gaps and pores formed along the interface. Moreover, unmelted TiB2 particles also appeared near the interface, which severely impeded densification response to a great degree (Fig. 9c). During the SLM process, a lower scan speed of 400 mm/s led to a longer abidance time of the laser beam on the surface of the powder layer. The interface temperature reached 2812 °C as a result of attendant higher laser energy input, which caused a high degree of overheating of liquid. Furthermore, the huge temperature gradient (33.884 °C/μm) was generated at the interface, which brought about the formation of the microcracks [58,59]. As the applied scan speed increased to 600 mm/s, the increasing interface temperature (2453 °C) made Ti6Al4V liquid with lower viscosity sufficient to spread around the unmelted ceramic particles and fill the pores of the solid phase,

4. Conclusions A multilayer finite element model for multi-materials was proposed to study the effects of laser power and scan speed on thermal behavior at the interface between Ti6Al4V layer and TiB2 layer during SLM process, including the temperature and temperature gradient distribution, remelting depth and liquid lifetime of the remolten pool at the asfabricated Ti6Al4V layer. Furthermore, the corresponding experiment was conducted in this study, and the following conclusions were drawn. (1) As the laser beam irradiated at the point 2 (Fig. 1c), the length and width of the molten pool on the surface of the as-fabricated Ti6Al4V layer reached 125 μm and 95 μm respectively, which much larger than that of the TiB2 layer and the isotherm curves at the Ti6Al4V layer were more intensive than those at the TiB2 layer in the depth direction. This is mainly due to the different thermal-physical properties between the TiB2 and Ti6Al4V materials. (2) The maximum temperature gradient was located at the interface between the TiB2 layer and Ti6Al4V layer. As the applied P increased from 300 W to 450 W, the maximum temperature gradient varied significantly, increasing from 24.920 °C/μm to 37.754 °C/μm while the maximum temperature gradient decreased slightly from 33.884 °C/μm to 31.478 °C/μm with the ν increased from 400 mm/s to 1000 mm/s. (3) The interface temperature and the liquid lifetime are important for the wettability at the interface. As the laser power increased from the 300 W to 450 W, the maximum temperature elevated from 1851 °C to 2765 °C and liquid lifetime lengthened from 0.6 ms to 10

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Fig. 9. SEM images showing low-magnification microstructures on the cross-sections of samples and photographs of the spark during the SLM processing at different scan speeds (P = 400 W): (a) ν = 400 mm/s; (b) ν = 600 mm/s; (c) ν = 800 mm/s.

2.3 ms (Fig. 5a). On the other hand, enhancing the applied ν from 400 mm/s to 1000 mm/s resulted in the slight decrement of the maximum temperature from 2812 °C to 2190 °C and the liquid lifetime also shortened from 2.79 ms to 0.95 ms. (4) The dimensions of the remolten pool at the as-fabricated Ti6Al4V layer increased as the P increased or the ν decreased. At the laser power of 400 W and the scan speed of 600 mm/s, a sound interfacial bonding can be achieved between the TiB2 layer and Ti6Al4V layer with the Rd of 0.326. (5) The experiment was conducted on the SLM of the TiB2/Ti6Al4V multi-material parts to validate the simulation results. A sound interfacial bonding was achieved at the combination of P = 400 W and ν = 600 mm/s, due to appropriate remolten pool size with 88.72 μm in width and 17 μm in remelting depth, which proved the validity of the simulation results.

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Acknowledgments

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The authors gratefully acknowledge the financial support from the National Natural Science Foundation of China (No. 51735005), the National Key Research and Development Program “Additive Manufacturing and Laser Manufacturing” (No. 2016YFB1100101), Funding of Jiangsu Innovation Program for Graduate Education (KYLX16_0344) , the Graduate Innovation Base (Laboratory) Open Fund of Nanjing University of Aeronautics and Astronautics (No. kfjj20180620), and the Priority Academic Program Development of Jiangsu Higher Education Institutions.

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Appendix A. Supplementary material

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Supplementary data to this article can be found online at https:// doi.org/10.1016/j.optlastec.2019.105666.

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