热塑挤压改善镁合金熔焊接头疲劳性能及其微观机制

Improving fatigue properties with thermo plastic extrusion and micro-mechanism for fusion welded joint of magnesium alloy

以AZ31B镁合金为试验材料,用同质成分丝材作填充材料进行TIG对接焊.将获得的熔焊接头在真空热压炉中实施400℃恒温热塑挤压变形,然后再进行室温疲劳性能试验.对试验试样借助于扫描电镜、微观金相及能谱分析等手段探讨热塑挤压改善镁合金熔焊接头疲劳性能方法及其微观机制.结果表明,热塑挤压能够有效地提高镁合金熔焊接头疲劳寿命,其改善机制主要基于400℃恒温挤压变形有助于熔合区氧化物夹杂的破碎及微裂纹的密实、愈合,并促使熔合区粗大α晶粒形成亚晶界以细化组织;同时热塑挤压变形又使焊缝组织通过动态再结晶实现组织重构,并改变β相沿晶界网状分布为重溶后在α相晶内呈弥散分布.

As an example of experimental material,AZ31B magnesium alloy was butt welded with filling wire of similar composition by TIG welding method.The fusion welded joints attained were put in a vacuum furnace to be thermo plastically extruded at constant temperature of 400 ℃ and then the fatigue properties of joints were tested at room temperature.With the help of scanning electronic microscopy,optical metallographic microscope and energy spectrum analyzer etc.,the experimental specimen were analyzed to explore the way and micro-mechanism of improving fatigue properties with thermo plastic extrusion for fusion welded joint of magnesium alloy.The results show that with thermo plastic extrusion,the fusion welded joint fatigue life of magnesium alloy can be improved effectively,this mechanism is chiefly due to beneficial effecting for oxide to be broken and micro-crack to be closed or healed with constant temperature extrusion at 400 ℃,also the coarse α grain in fusion zone may be promoted to transform into sub grain boundary,therefore a refining microstructure can be attained.Meanwhile it causes weld microstructure to reconstruct by way of dynamic re-crystallization and changing network-like distribution of β-Mg17Al12 precipitate on α-Mg grain boundary of weld microstructure and making it solid solve to re-precipitate in way of dispersed distribution inside α grain.