中文摘要：

利用6kw光纤激光器对1．5mm厚冷轧800MPa级双相钢进行激光拼焊试验，研究激光焊接接头的显微组织演变规律、显微组织对显微硬度及疲劳性能的影响规律。结果表明，焊接接头主要包括焊缝区（WZ）、粗晶区（CGHAz）、细晶区（FGHAz）、混晶区（MGHAz）和回火区（TZ），其中焊缝区和粗晶区显微组织均为马氏体，但焊缝区内的原始奥氏体晶界保留着柱状晶的生长形态，粗晶区内的原始奥氏体晶界呈多边形生长；细晶区和混晶区均为铁索体和马氏体，但细晶区的显微组织更为精细；回火区主要由铁素体和回火马氏体组成。混晶区和回火区显微硬度均低于母材，共同组成了焊接接头的软化区。由于软化区尺寸相对较窄（0．4mm）且硬度降低幅度低（～6．8％），拉伸断裂位置出现在母材。在应力比为O．1的拉一拉疲劳条件下，母材和焊接接头的疲劳极限分别为545MPa和475MPa，疲劳断裂未出现在软化区。母材中的疲劳裂纹在铁素体与马氏体两相界面萌生并扩展；而焊接接头中的疲劳裂纹则在焊缝中的奥氏体晶界上或马氏体板条内萌生，沿着焊缝中心处柱状原始奥氏体晶界的交汇处切断马氏体板条束扩展。

英文摘要：

Laser butter welding is implemented by 6 kW fiber laser on 800 MPa dual phase cold rolled steel sheet with 1.5 mm thickness. The microstrueture evolution of laser welding joint, its effect on microhardness and fatigue properties are studied. The results show that the weld joint consists of weld zone （WZ）, coarse-grain heat affected zone （CGHAZ）, fine grain HAZ （FGHAZ）, mixed-grain HAZ （MGHAZ） and tempering zone （TZ）. The microstructure of WZ and CGHAZ are martensite, and columnar crystals morphology is retained on original austenite grains in WZ, whereas polygonal original austenite grains are observed in CGHAZ. The microstructure of FGHAZ and MGHAZ are ferrite and martensite, and finer microstructure is obtained in FGHAZ. The microstructure of TZ consists of ferrite and tempering martensite. The microhardnesses of both MGHAZ and TZ are lower than that of base metal, and they form the softening zone of the welded joint together. Tensile fracture is in the base metal because of the narrow soften zone （0.4 mm） and low microhardness drop （-6.8%）. In tension-tension fatigue tests （stress ratio is 0.1）, the fatigue limit of base metal and weld joints are 545 MPa and 475 MPa respectively. Fatigue fractures are not in softening zone. The fatigue crack propagates along the phase interfacebetween ferrite and martensite in base metal. Whereas, the fatigue crack of welded joint occurs on the austenite grain boundaries or martrnsite lath in WZ, and propagates along the intersection of columnar original austenite grain boundaries in the center of WZ, which cuts off martensite lath beam to propagate.