焊前预处理对钛合金厚板焊接残余应力的影响

Effect of Prewelding Pretreatment on Welding Residual Stress of Titanium Alloy Thick Plate

焊接残余应力是影响焊接成型及焊接部件服役的重要因素。为了优化焊接工艺和焊接接头微观结构,有效地减小焊接残余应力,本工作研究了焊前预处理工艺对电子束焊接钛合金厚板焊接接头组织、性能和残余应力的影响机制。对不同预处理工艺制备的钛合金焊接接头进行组织形貌表征,发现焊前预热显著增宽了焊接接头的熔合区和热影响区,粗化了两区域内的α片层,提高了焊接接头的强韧性匹配。采用中子衍射法、深孔法和轮廓法检测钛合金厚板电子束焊接头残余应力,对3种方法的检测结果进行对比分析,获得了焊接接头不同区域沿不同方向的残余应力分布。另外,对比了2种预热工艺焊接接头采用深孔法获得的残余应力结果,发现焊前预热可以显著降低焊接接头的残余应力。采用数值模拟计算了不同预热温度下温度场和应力场的变化情况,获得了不同预热温度条件下热应力的动态变化规律,揭示了预热降低残余应力的内在机理。对比2种预处理工艺焊接接头熔合区和热影响区的显微组织、元素分布和晶粒取向,进一步阐明了预热可以通过促进元素扩散和诱发变体选择降低β→α相变产生的应力集中。

Welding is an essential means of joining structural components to form a new structure.Welding residual stress mainly results from materials expanding or contracting due to temperature variations,which can reduce the life of titanium alloys.Therefore,to reduce undesired residual stress,the welding process and microstructure of the materials involved should be optimized.Titanium alloys play a crucial role in marine and aviation fields due to their excellent corrosion resistance and high specific strength.This work investigates the influence mechanism of the prewelding pretreatment process on the structure,mechanical properties,and residual stress of the electron beam welding joint of a titanium alloy thick plate.The macrostructure and microstructure of titanium alloy welding joints prepared using different pretreatment processes are characterized.Results showed that preheating before welding substantially widens the fusion zone(FZ)and heat-affected zone(HAZ)of the welding joint,coarseningαlamellae in both zones.Thus,the hardness of the FZ and HAZ of the preheated welding joint is reduced to close to that of the base metal.Simultaneously,the strength and toughness of the welding joint is considerably improved such that it is similar to the base metal.The neutron diffraction,deep-hole drilling,and Rostenthal-Norton contour methods are used to measure the residual stress of the electron beam welding joint.The neutron diffraction method exhibits high detection accuracy and can achieve stress monitoring in different zones of the weld seam.Deep-hole drilling is a mechanical strain relief technique for measuring transverse and longitudinal residual stress through component thickness.The Rostenthal-Norton contour method can obtain a three-dimensional stress on the welding joint.A combination of these three measurement techniques can complement and be used to verify each other,providing reasonable data for the residual stress evaluation.The detection results of unpreheated welding joints are compared and analyzed,and the residual stress distribution in the FZ and HAZ zones along different directions is obtained.The FZ is subjected to tensile residual stress along all three directions.Alternatively,the HAZ is subjected to compressive stress along the transverse and longitudinal directions and tensile stress along the normal direction.The residual stress at base metal is small.Additionally,the residual stress results obtained by the deep-hole drilling method for the welding joints using two preheating processes are compared.The results showed that preheating before welding can considerably reduce residual stress at the weld.The reason is discussed in depth.Numerical simulation is used to calculate the changes in the temperature and stress fields under different preheating temperatures.The dynamic change rules of thermal stress under different preheating temperatures are obtained.Results showed that increasing the preheating temperature reduces thermal stress and the thermal expansion mismatch in different areas of the welded joint.Moreover,the microstructure,element distribution,and grain orientation of the FZ and HAZ of joints welded using two pretreatment processes are analyzed.Preheating coarsens theαlamellae and promotes the redistribution of alloy elements,thereby reducing the stress concentration betweenαandβphases.Besides,variant selection of the HAZ is induced by the preheating process.The number and differences in the orientation ofαvariants are decreased,thereby reducing the stress concentration between variants.