【作者】 崔金鹏；

【导师】 何国；

【作者基本信息】 上海交通大学 ， 材料学， 2009， 硕士

【摘要】 随着汽车工业的蓬勃发展,以及汽车轻量化的大趋势,优质高强度钢的使用成为了潮流。激光焊接是汽车工业中主要的连接方式之一,解决高强度钢激光焊接的工艺问题具有重要的意义。本文针对CP和DP系列汽车用高强度钢,研究了激光焊接工艺对焊缝成形性,接头组织和力学性能的影响。选取1.8mm厚DP600钢板和2.8mm厚CP800钢板,分别采用四种不同的激光焊接工艺进行焊接试验,结果表明DP600钢激光焊接线能量增加3.5%,焊缝的上宽度从2.02mm增加到2.48mm,而下宽度从1.13mm减小到1.06mm,而形状因子σ减少了21.8%,焊缝倒梯形上宽下窄的趋势更明显。CP800钢激光焊接线能量增加了7.3%,焊缝的上下宽度都在增加,而形状因子σ增加了16.3%。从焊缝形状上来看,随着线能量的增大,焊缝的倒梯形有向矩形转变的趋势。激光焊接后的样品进行组织观察,结果表明DP600钢中焊缝中间主要是板条状马氏体组织,在热影响区有马氏体,铁素体和残余奥氏体生成。在CP800钢的焊缝中主要形成板条状马氏体以及板条之间的残余奥氏体组织,其中马氏体晶粒尺寸约为20-30μm,同时在焊缝中发现了孪晶马氏体。在热影响区中主要有铁素体,马氏体,贝氏体和残余奥氏体组织生成,热影响区细晶区的晶粒尺寸约为3-5μm,粗晶区晶粒尺寸约为约为15-23μm。测试焊接后的样品的显微硬度,从母材到热影响区再到焊缝区,DP和CP钢激光焊接后样品显微硬度逐渐提高。DP钢激光焊接后焊缝区的显微硬度大约为母材硬度的1.5-1.9倍。CP 800钢激光焊接后焊缝区硬度大约为母材硬度的1.4-1.5倍。DP600和CP800钢采用四种不同的激光焊接工艺,结果表明材料表面成形质量很好,熔合的很整齐,焊缝比较美观,而且没有发现气孔,裂纹等焊接缺陷。更多还原

【Abstract】 High strength steel (HSS) becomes more and more popular in the automobile production, because the lightweight is always important for the energy consumption and the circumstance protection. Its weld process is required to be more accurate and higher efficient so as to meet the demand of the volume-produce in automobile industry. Laser welding, being an effective method to join the HSS sheets, is very suitable for such application. In this dissertation, two grades of the high strength steel, DP600 and CP800, are selected for the laser welding. The weldments have been investigated in terms of geometry, defects, and the microstructure in the weld and the thermal effect zone. Some relationships have been established between the laser welding parameters and the weldments.Based on these experiments, the up-width of the weld in DP600 steel increases from 2.02mm to 2.48mm and the down-width of the weld decreases from 1.13mm to 1.06mm when the heat input of the laser-welding improves 3.5%. However, the shape factorσdecreases 21.8%. The up-width and the down-width of the weld in CP800 steel increase and the shape factorσalso increases 16.3% when the heat input of the laser-welding improves 7.3%. The shape of the weld has the tendency to change from inverted trapezoid to rectangle with the increasing of the linear energy in laser welding. The microstructures of the weldments were observed after welding, and the results show that the structures of the weld in the DP600 steel samples are mainly composed of Martensite. And Ferrite, Austenite and Martensite are also observed in the heat affected zone (HAZ). Martensite and retained Austenite which is formed between lath Martensite are observed in the weld of the CP800 steel sheets, and the grain size of the weld is in the range of 20 and 30μm. Twin Martensite is also found in the weld of the samples. The structures of the HAZ consist of Ferrite, Bainite, Martensite and retained Austenite. The grain size of the FG HAZ is in the range of 3 and 5μm and the size of CG HAZ is in the range of 15 and 23μm.The results of the hardness experiments show that the microhardness of the samples increases gradually from the parent metal to the weld in both DP600 and CP800 steel. And the hardness of the weld is half time higher than that of the base metal. We also found that the qualities of the welding joints are excellent. The weldments are beautiful and the defects (pore or crack) do not exist in the joints.更多还原