激光选区熔化制备CuSn10/AlSi10Mg功能梯度材料组织演变及裂纹产生机理

Microstructure Evolution and Crack Formation Mechanism of CuSn10/AlSi10Mg Functional Gradient Materials Prepared by Selective Laser Melting

激光选区熔化(SLM)制备功能梯度材料(FGMs)可以对性能进行局部定制。通过SLM制备CuSn10/AlSi10Mg功能梯度材料,研究材料成分比对CuSn10/AlSi10Mg过渡层显微组织的影响。通过相图计算(CALPHAD)过渡层的物相及其数量,并结合背散射电子衍射仪(EBSD)结果讨论梯度材料界面区域组织演化规律,揭示界面区域裂纹的形成机制。结果表明,CuSn10/AlSi10Mg过渡层显微组织由基体、Al_(4)Cu_(9)相和Al_(2)Cu相组成,形貌呈柱状晶粒和细小等轴晶粒。在过渡层区(从铜合金侧到铝合金侧),随着AlSi10Mg含量的增加,基体含量变化不大,Al/Cu金属间化合物含量变化剧烈,Al_(4)Cu_(9)相先析出且含量逐渐减少,Al_(2)Cu相后析出且含量逐渐增加,裂纹周围有大量Al/Cu金属间化合物生成。过渡区域内产生宏观开裂的主要原因是直接生成的Al_(4)Cu_(9)相引起高体积变化(体积变化率为4.4%)容易首先在质量分数为0~20%的AlSi10Mg区域内形成应力集中,产生微裂纹;Al_(2)Cu相与基体中多余的铜转变为Al_(4)Cu_(9)相(间接生成),其引起的高体积变化(体积变化率为4.3%)在质量分数为80%的AlSi10Mg区域内进一步加重应力集中,从而造成宏观开裂。避免Al_(4)Cu_(9)的生成(直接和间接生成)是解决裂纹的主要途径。过渡层区域的显微硬度高于两侧基体,最高硬度在裂纹处(804 HV),与Al_(4)Cu_(9)的硬度相近。

Objective Selective laser melting(SLM)can be used to prepare functionally gradient materials(FGMs)for local customization of performance.In this study,CuSn10/AlSi10Mg functional gradient materials were prepared by SLM,and the effect of the material composition ratio on the microstructure of the CuSn10/AlSi10Mg transition layer was investigated.The phase and quantity of the transition layer were calculated using CALPHAD,the microstructural evolution of the interface region of the gradient materials was discussed based on electron backscattering diffraction(EBSD)results,and the formation mechanism of cracks in the interface region was revealed.The results show that the microstructure of the CuSn10/AlSi10Mg transition layer consists of a matrix of Al_(4)Cu_(9) and Al_(2)Cu with columnar and fine equiaxed grains.In the transition layer zone(from the copper alloy side to the aluminum alloy side),with an increase in the AlSi10Mg content,the matrix content does not change significantly,whereas the content of Al/Cu intermetallic compounds changes sharply.The Al_(4)Cu_(9) phase first precipitates and its content gradually decreases,whereas the Al_(2)Cu phase precipitates later and its content gradually increases,and a large amount of Al/Cu intermetallic compounds are generated around the cracks.The main reason for the formation of severe cracks in the transition zone is that the directly generated Al_(4)Cu_(9) phase is prone to large volume changes(4.4%),leading to stress concentration and initial microcracks.The large volume change(4.3%)caused by the transformation of the Al_(2)Cu phase and Cu enriched in the matrix into the Al_(4)Cu_(9) phase(indirectly generated)further exacerbates the stress concentration and ultimately leads to macrocracking.Avoiding the direct and indirect generation of Al_(4)Cu_(9) is the primary means of solving the problem of cracking.The microhardness of the transition layer is higher than that of the matrix on both sides.The highest hardness is observed at the crack(804 HV),similar to that of Al_(4)Cu_(9).Methods In this study,CuSn10/AlSi10Mg gradient functional materials are prepared by SLM through two gradient paths(19 and 16 layers of different compositional gradients are designed for samples 1 and 2,respectively).The microstructures of the different transition regions are observed by optical microscopy(OM)and scanning electron microscopy(SEM)equipped with energy dispersive spectroscopy(EDS).To reveal the microstructural evolution,the phase compositions of the transition regions are measured using Xray diffraction(XRD)and EBSD.Finally,the microhardness is measured using a microhardness tester to understand the changes in mechanical properties.Results and Discussions Sample 1(19-layer transition composition)prepared using SLM forms more cracks,generates transverse cracks,and causes macroscopic cracking throughout the sample.Sample 2(16-layer transition composition)forms slight cracks,and the transverse cracks disappears.Although both transition compositions have cracks,the 19-layer transition is significantly more severe than the 16-layer transition.More importantly,nearly all the cracks are generated in the Al-rich transition region(Fig.2).During the printing process,the distribution of Al along the deposition direction gradually increases from zero at the beginning to a uniform distribution at the end,which is consistent with the spot scanning results.However,both Cu and Sn are uniformly distributed throughout the transition region,which further confirms that the CuSn10 alloy is continuously remelted and is then diffused to the upper layer during the printing process,resulting in the enrichment of Cu in the region of the Al alloy(Fig.4).In the transition region,the phases mainly consist of the matrix phaseα-Cu/α-Al and Al/Cu intermetallic compounds,and the intermetallic compounds are mainly Al_(4)Cu_(9) and Al_(2)Cu.From the Cu alloy side to the Al alloy side,the content of the matrix does not change significantly with the addition of the Al alloy.In addition,Al_(4)Cu_(9) first precipitates and then gradually decreases,and it is dominant at 40%AlSi10Mg.With a continuous increase in the Al alloy content,the Al_(2)Cu phase precipitates later and gradually increases,exceeding the Al_(4)Cu_(9) phase at 50%AlSi10Mg content(Figs.5‒7).Conclusions The microstructure of the SLMed CuSn10/AlSi10Mg gradient material is composed of columnar and fine equiaxed grains that grow in the direction of the center of the molten pool,and the equiaxed grains close to the boundary of the molten pool have a random grain orientation.In the transition region,the phases mainly consist ofα‑Cu/α‑Al matrix and Al/Cu intermetallic compounds,and the intermetallic compounds are mainly Al_(4)Cu_(9) and Al_(2)Cu.From the Cu10Sn side to the AlSi10Mg side,with the addition of the Al alloy,the content of the matrix does not change significantly,but Al_(4)Cu_(9) first precipitates and gradually decreases,and it dominates at 40%AlSi10Mg.With a continuous increase in the aluminum alloy,the Al_(2)Cu phase precipitates later and gradually increases,exceeding the content of the Al_(4)Cu_(9) phase at 50%AlSi10Mg.A large amount of the Al_(4)Cu_(9) phase is generated around the microcracks in the transition region.However,a large amount of the Al_(2)Cu phase is generated around the macrocracks,and nearly all cracks mainly occur in the Al-rich transition region.The volume change of the generated Al_(4)Cu_(9) is the highest(4.4%),and the reaction between Al_(2)Cu and the Cu matrix forming the Al_(4)Cu_(9) phase exhibits the second-highest volume change(4.3%),whereas the volume change forming the Al_(2)Cu phase is only 0.3%.The Al_(4)Cu_(9) phase nucleates in both Al-and Cu-rich solid solutions,whereas the Al_(2)Cu phase can only nucleate in the Al-rich region.Therefore,the reason for crack formation is that the direct generation of the Al_(4)Cu_(9) phase in the transition region is prone to forming a stress concentration that generates the initial microcracking.The indirectly formed Al_(4)Cu_(9)(the reaction between Al_(2)Cu and excess Cu in the matrix)causes a large volume change and further aggravates the stress concentration,resulting in severe macrocracks.Avoiding the generation of the Al_(4)Cu_(9) phase(including direct and indirect formations)is the primary means of solving the cracking problems.The microhardness of the transition layer region is affected by the intermetallic compound content.From the Cu alloy side to the Al alloy side,the microhardness first increases and then decreases,and it is higher in the transition region than that of the substrate.This trend is consistent with the number of intermetallic compounds.The highest microhardness(804 HV)is observed at the cracks,which is very close to that of Al_(4)Cu_(9).This further verifies that enriched intermetallic compounds are the main reason for crack formation.