基于无量纲工艺图的激光熔化沉积制备Ti6Al4V工艺与性能研究

Process and Properties of Ti6Al4V Manufactured using Laser Melting Deposition with Dimensionless Processing Diagram

如何高效获取合适的工艺参数进行激光熔化沉积(LMD)制造高性能零件是一项艰巨的挑战。提出了一种有效进行参数选择的方法,建立了基于LMD工艺的无量纲参数组,利用文献中获取的LMD工艺数据构建无量纲工艺图,确定了本试验LMD制备Ti6Al4V的工艺范围。利用正交试验研究了不同激光功率q、扫描速率v和扫描间距h组合下的无量纲等效能量密度E;对LMD制备Ti6Al4V块状试样组织和性能的影响。结果表明,LMD制备的试样呈现出明显的柱状晶外延生长特点,柱状晶的宽度随E;增加而增大。在通过无量纲工艺图确定的最优参数E_(0)^(*)=3.74下LMD制备的Ti6Al4V试样无熔合缺陷,硬度为391.7 HV,抗拉强度为963 MPa,延伸率为13.4%。结果表明,利用构建的无量纲工艺图缩小工艺参数范围,可以获得综合力学性能优良的样件。

Objective Laser melting deposition(LMD) enables the generation of complex-shaped or customized parts that can be used in various engineering applications. The ultimate mechanical behavior of metallic parts is related to their thermal-history-dependent microstructure. To control their microstructure and resultant mechanical properties, understanding and predicting their thermal history during the layer-wise manufacturing is critical. Therefore, many research efforts have focused on determining the effects of process parameters on the microstructure and mechanical properties of LMD parts. However, obtaining optimal processing parameters for LMD to realize high-performance parts is challenging because the quality of these parts is affected by many parameters in the process. Few systematic attempts have been made to relate these parameters via characterizing underlying physical processes. In this paper, we propose a novel processing parameter screening strategy with a dimensionless processing diagram. Such a diagram defines a set of appropriate operating regions for LMD using identified dimensionless groups of processing parameters. The diagram provides a useful reference and methodology to aid the selection of appropriate processing parameters during the initial development stages.Methods To optimize processing parameters, we combined the approaches of dimensionless processing diagram and orthogonal design. Several key dimensionless processing parameters were derived considering the temperature field to describe LMD. Scattered processing parameters of different types of materials for LMD taken from the literature were normalized against material thermophysical properties and presented on the dimensionless processing diagram. The diagram presented isopleths of normalized equivalent energy density E_(0)^(*) and provided general process windows for a range of alloys. The orthogonal design was used to further narrow the processing window. Ti6 Al4 V samples were fabricated using LMD in a helium environment. The grain morphology was observed with a digital optical microscope(VHX-5000). Microhardness tests were conducted using an MH-6 microhardness tester with an applied load of 100 g and a dwell time of 15 s. Analysis of variance(ANOVA) was used to evaluate the effect of E_(0)^(*) in different combinations of laser power q, scanning velocity v, and hatching space h on microhardness. Uniaxial tensile tests were conducted in Instron 5967 machine at a strain rate of 5×10^(-4)/s. The tests were repeated six times for each type of sample, yielding standard error bars.Results and Discussions The microstructure of the as-built Ti6 Al4 V alloy is prior-β columnar grains grown epitaxially, and the higher magnified micrographs show a transition from a martensitic structure near the substrate to a basket-weave structure at the medium region of the build. The morphology of columnar grains is a function of the parameter-dependent solidification rate and thermal gradients during solidification, so the width of columnar grains tends to increase with an increase in E_(0)^(*). The ANOVA results show that the effect of q on the microhardness of deposits is more pronounced than that of v and h, and the maximum microhardness is 404.3 HV when E_(0)^(*)=2.34. The Ti6 Al4 V samples fabricated using LMD at the optimized parameters(E_(0)^(*)=3.74) determined using the dimensionless processing diagram are defect-free with ultimate tensile strength and elongation of 963 MPa and 13.4%, respectively. The mechanical properties of the as-built Ti6 Al4 V samples approach or surpass forgings and deposits after postprocessing in the literature. Undesirable microstructural aberrations are found in the components, including gas porosity and lack of fusion voids. Insufficient heat input at a lower E*0 introduces incomplete fusion between the continuous layers, leading to void formation. Excessive heat input at a higher E*0 makes the surface temperature exceed the vaporization temperature, forming gas pores.Conclusions Aiming at the problem that the optimized processing parameters of LMD are extremely complicated to obtain, an effective method is proposed by constructing a dimensionless processing diagram in this paper. A group of dimensionless processing parameters applicable to LMD has been defined, and a dimensionless processing diagram has been constructed on the basis of parameter data available in the literature. The practicability of the dimensionless processing diagram has been proved experimentally for the LMD of Ti6 Al4 V. The optical micrographs show that the prior-β columnar grain morphologies of the as-deposited samples are a function of E_(0)^(*). A high value of E_(0)^(*) leads to a relatively low cooling rate and coarse columnar grain. The cooling rate of the melt pool dictates the grain size formed in a deposited layer with a lower cooling rate, resulting in a coarse microstructure. By the experimental design of orthogonal array and ANOVA, the significance of processing parameters related to the microhardness of the LMD Ti6 Al4 V is q>v>h. The lack of fusion voids and gas pores is strongly affected by E_(0)^(*), and these defects, in turn, affect the mechanical property. The ductility of the as-built samples is compromised at a lower value of E_(0)^(*);however, owing to irregular lack of fusion voids, despite having comparable tensile strength with those samples at a higher value of E_(0)^(*), the yield strength, ultimate strength, and elongation reach a maximum value of 890 MPa, 963 MPa, and 13.4%, respectively, at E_(0)^(*)=3.74, which exceed the forgings standard and is close to those fabricated by additive manufacturing reported in the literature. High-performance parts are obtained because of the dimensionless processing diagram constructed in this paper, which effectively narrows the processing window.