TiNi形状记忆合金激光焊接焊缝金属相变温度的控制

PHASE TRANSFORMATION TEMPERATURE CONTROL OF WELD METAL OF LASER WELDED TiNi SHAPE MEMORY ALLOY JOINT

采用镀Ni后焊接以改变焊缝金属的Ni含量,再经时效处理的"复合法"工艺对焊缝金属的相变温度进行控制.研究了镀层厚度和时效时间对焊缝金属相变温度的影响规律,利用DSC,EDS,SEM,OM和XRD分析了焊缝金属的相变温度、化学成分和组织形貌,并从析出相、晶粒择优取向及晶格畸变对马氏体弹性能和相变阻力的影响几方面分析了"复合法"控制相变温度的机理.结果表明,增加焊缝金属Ni含量仅能增加马氏体的弹性能,导致相变温度降低;而时效处理在增加相变阻力的同时也降低了马氏体的弹性能,导致相变温度提高;控制"复合法"工艺参数则可以将焊缝金属中马氏体的弹性能和相变阻力控制在合理的范围内,使焊缝金属的相变温度A_s和A_f与母材相当.

The phase transformation temperature （PTT） of laser welded TiNi alloy joint is mainly determined by the PTT of its weld metal. A two-step approach is developed to control the PTT of weld metal, including Ni-plating and aging treatment. The Ni-plating increases the Ni content of weld metal, and the aging treatment improves the formation of precipitation phase. The effect of plating thickness and aging time on the PTT of weld metal is studied. The chemical composition, microstructure, crystal structure and PTT are analyzed by DSC, EDS, OM, SEM and XRD, respectively. The PTT controlling mechanism of the developed two-step approach is understood by investigating the effect of precipitation phase, grain preferred orientation, and lattice distortion on the elastic energy resilience of martensite and the resistance of phase transition. The increasing of Ni content in weld metal by Ni-plating can just rise up the elastic energy resilience of martensite, which causes the drop of PTT. The aging treatment not only increases the resistance of phase transition, but also reduces the elastic energy resilience of martensite. The compound effect increases the PTT of weld metal. By adjusting the plating thickness and the aging time of the two-step approach, both the elastic energy resilience of martensite and the resistance of phase transition can be controlled within a reasonable range. These measures keep the austenite start temperature （As） and finish temperature （Af） of weld metal as the similar as that of base metal, thus the PTT of TiNi alloy joint is well controlled.