动车组车轴增材再制造材料选择和性能评价

目的 选择最佳材料用于动车组车轴的再制造，以符合车轴的力学性能及轮对压装技术要求。方法 以激光熔覆技术作为增材再制造技术工艺方法，选择不同化学成分的合金材料，通过熔覆金属的力学性能、线膨胀系数、过渡熔合区成分、稀释区组织以及硬度突变情况对比分析，确定最佳车轴再制造材料。对所选材料激光熔覆试件的宏观组织、微观组织、化学成分、硬度、力学性能进行检测，并开展轮对压装试验，通过光学显微镜、扫描电镜、纳米压痕法进行分析。结果 Schaeffler组分图预测Fe310、Fe314的激光熔覆金属熔合过渡区组织为奥氏体A+铁素体F组织，但是实际过渡区的硬度值高于600HV，说明有硬质马氏体相析出，而Fe310和NiCrMo合金的熔合区硬度值未发生突变，Fe310的力学性能略低于EA4T钢，且线膨胀系数与基体差距较大，因此不适用于车轴的再制造。选择NiCrMo合金作为车轴再制造增材材料，其熔覆金属的抗拉强度为790 MPa，屈服强度为542 MPa，冲击韧性为68 J/KU5，且具有相近的线膨胀系数。另外，NiCrMo合金纳米压痕的压缩弹性模量Er为180~185 GPa，与基体EA4T钢(185~190 GPa)相近，最终经再制造车轴的轮轴压装试验，其压装曲线的最大压装力在680~1160 kN范围内，曲线也符合标准要求。结论 选择NiCrMo合金作为动车组车轴再制造激光熔覆材料，其热膨胀系数、力学性能以及压缩弹性模量与基体EA4T钢相近，且激光熔覆金属过渡区域无脆硬的马氏体组织产生，并通过了轮对的压装试验，满足动车组压装曲线要求。

Material Selection and Mechanical Property Evaluation for Additively Remanufactured EMU Axles

The work aims to select the optimum material for remanufacturing of EMU axles and meet the mechanical property requirements of the axle and the conditions of the wheelset press-fitting. The appropriate material for the remanufacturing of the EMU axles was determined through selecting alloy materials of different chemical composition as well as comparing and analyzing mechanical properties, coefficient of linear expansion, composition in the dilution zone of fusion area, and hardness variation of the deposited metal by laser cladding technology. The cladded specimens were analyzed in terms of macroscopic structure, microstructure, chemical composition, hardness, mechanical properties and wheelset press test by optical microscopy, scanning electron microscopy and nano indentation. The microstructure of the laser cladding fusion area for Fe314 and Fe316 should consist of A+F as predicted by Schaeffler composition diagram, but the hardness value of the actual fusion area was higher than 600HV, which implied the existence of martensite formation. On the other hand, the hardness of the fusion zone was consistent with the deposited Fe310 and NiCrMo alloy. The mechanical properties of Fe310 were lower than those of EA4T steel, and the coefficient of linear expansion was much different from that of the EA4T steel. As such, Fe310, Fe314, and Fe316 were regarded not suitable for remanufacturing of EMU axles. NiCrMo was selected instead because the tensile strength was 790 MPa, the yield strength was 542 MPa, the impact toughness was 68 J/KU5 for the deposited NiCrMo alloy, and it had a similar coefficient of linear expansion compared with EA4T steel. In addition, the compressive elastic modulus Er in nano indentation was 180~185 GPa for the NiCrMo alloy, which was close to the compressive elastic modulus Er of 185~190 Gpa for the EA4T steel. The press-fit curve of remanufactured axles showed the maximum pressing force in the range of 680~1160 kN, which was deemed acceptable. NiCrMo alloy is selected as the laser cladding material for remanufacturing of EMU axle because its thermal expansion coefficient, mechanical properties and compressive elastic modulus of cladding metal and substrate EA4T steel are similar, and there is no brittle martensite structure in the transition zone of laser cladding metal, it has passed the press-fitting test of wheelset, and it meets the requirements of press-fitting curve of EMU.