【作者】 刘恩泽；

【导师】 李铸国； 姚成武；

【作者基本信息】 上海交通大学 ， 材料科学工程， 2020， 硕士

【摘要】 奥氏体镜面不锈钢因耐腐蚀和美观等优点常用于各类薄板非熔透搭接焊装饰性结构中。这种装饰性结构强度要求较低,但对非熔透侧表面质量要求较高。目前在薄板激光搭接非熔透焊接件中,非熔透侧的奥氏体不锈钢镜面会出现一微凸起痕迹,这极大限制其应用。为研究这一凸起痕迹的变形机制,本文开展DC01镀锌钢-SUS304奥氏体镜面不锈钢非熔透激光搭接焊接试验,并使用非接触式激光表面轮廓仪表征非熔透侧不锈钢镜面微凸起痕迹的变形量和挠度。采用光学显微镜、扫描电子显微镜和EBSD技术表征微凸起变形区域的微观组织形貌,并以DC01镀锌钢-Q235碳钢非熔透激光搭接焊做对比试验。同时,针对冷轧SUS304不锈钢进行不同温度下保温、折弯后水淬处理,表征折弯处内侧、外侧的显微组织状态,研究拉应力和压应力对SUS304不锈钢显微组织形态的影响。根据上述研究结果,分析非熔透侧表面凸起痕迹的变形机制。最后分别开展强制冷却、反变形和预应力条件下的激光搭接焊接试验,探索SUS304不锈钢非熔透侧表面凸起变形的控制方法。结果表明,DC01镀锌钢-SUS304不锈钢激光搭接焊非熔透侧奥氏体不锈钢镜面表面的微凸起变形轮廓特征为:1)在垂直于焊缝方向,自不锈钢表面向上凸起宽度约8 mm的拱形轮廓,该拱形轮廓中心处约2 mm范围宽的区域变形程度陡升,由于变形高度的剧烈变化使光线发生明显偏折而产生视觉上的凸起痕迹。在中心2 mm范围外,拱形轮廓较为平缓;2)在沿焊缝方向,也出现自不锈钢表面向上凸起的拱形轮廓,但变形平缓,未出现拱形轮廓中心的变形陡升的微凸起;3)随着激光功率增加,微凸起峰值高度增加,1.9 k W~2.6 k W的激光功率范围内,峰值高度由约3.5μm增加至23μm;4)拱形轮廓中心的微凸起区出现数量较多、密度集中的微细孪晶簇群,这些孪晶呈透镜状,其长轴垂直于冷轧轧制方向,为发源于晶界、终止于晶内的变形孪晶。DC01镀锌钢-Q235碳钢激光搭接焊非熔透侧Q235碳钢表面微凸起变形轮廓特征为:在垂直于焊缝方向,Q235碳钢表面也出现向上凸起的拱形轮廓,但该轮廓整体变化较为平缓,并未出现变形集中于中心区域的凸起陡升特征,且相同功率下Q235碳钢表面凸起变形的峰值高度更小。与SUS304不锈钢镜面相比,Q235碳钢表面未出现视觉上的凸起痕迹,表明材料的物理性质是影响微凸起变形轮廓的重要因素。对冷轧SUS304不锈钢薄板,经不同温度保温、水淬处理后,通过显微组织对比分析,薄板激光搭接焊非熔透侧SUS304不锈钢背面微凸起处焊接热循环瞬时温度应在900℃以上。在经1000℃、5 min保温、折弯和水淬处理后,在折弯外侧拉应力作用区域,SUS304不锈钢再结晶的晶粒沿拉伸应力方向拉长,且晶内退火孪晶也发生变形而平行于拉应力方向。在折弯内侧压应力作用区域,SUS304不锈钢再结晶的晶粒垂直于压应力方向,且在该折弯内侧也产生了微凸起区。该微凸起变形区生成了大量微细孪晶簇群,为压应力作用下的择优变形孪晶,其形态类似于不锈钢表面微凸起变形区生成的孪晶簇群。由此推断非熔透侧SUS304不锈钢表面微凸起处应经受与折弯内侧相类似的压应力作用,而这种压应力是激光焊接过程中产生的热应力造成的。由此得出结论,薄板激光搭接焊非熔透侧奥氏体不锈钢表面视觉上的微凸起痕迹是由焊接过程中压应力引起的,其微凸起变形机制可描述为:在激光焊接过程中,SUS304不锈钢热影响区金属热膨胀受到相邻低温区的约束而受到压应力,金属可能被向上表面或向下表面挤压。由于高温下SUS304不锈钢屈服强度低,故向下“挤压”应力使SUS304不锈钢发生塑性变形而被“挤出”表面形成凸起。当冷却至室温时,此微凸起变形便保留下来。在强制冷却条件下,不锈钢表面微凸起变形的峰值高度有所降低,但无法消除微凸起变形,即使减小激光功率至两板未焊合时,不锈钢表面仍出现微凸起痕迹。在激光搭接焊接前,对SUS304不锈钢待焊位置的背面分别施加0~4 mm的反变形,相比较强制冷却法,非熔透侧SUS304不锈钢表面微凸起变形峰值高度减小更为明显。随着反变形量的增加,微凸起峰值逐渐减小,但形成反变形夹具凹坑深度也逐渐增加,损伤了非熔透侧SUS304不锈钢镜面。在激光搭接焊接前,针对薄板DC01镀锌钢-SUS304不锈钢搭接试样施加预拉伸应力,结果表明随着预应力的增大,微凸起变形的峰值高度逐渐降低,视觉上的非熔透侧SUS304不锈钢表面微凸起变形痕迹显著减弱。更多还原

【Abstract】 Austenitic mirror stainless steel thin plates are often used in various decorative structures which are composed of partial penetration laser lap welding joints due to its corrosion resistance and beautiful appearance.Under these condition,the required structural strength is relatively low,but the surface quality of the unmelted side is higher.At present,as for a thin plates laser lap partial penetration weldment,a micro bulging mark appears on the austenitic stainless steel mirror surface on the unmelted side,which greatly limits its application.In order to study the deformation mechanism of this bulging mark,a partial penetration laser lap welding test of DC01 galvanized steel-SUS304 austenitic mirror stainless steel joints was carried out in this paper.And a non-contact laser surface profiler was used to characterize the micro bulging distortion of the stainless steel deformation height and deflection of the marks.Optical microscopy,scanning electron microscopy,and EBSD techniques were used to characterize the microstructural morphology of the micro bulging distortion area.DC01 galvanized steel-Q235 carbon steel partial penetration laser lap welding was used for comparison test.At the same time,cold-rolled SUS304 stainless steel was heated and preserved at different temperatures,then water quenched and bent to characterize the microstructure state at the inside and outside of the bend to study the effect of tensile and compressive stress on the microstructure of SUS304 stainless steel was studied.Based on the results of the above experiments,the deformation mechanism of the marks on the unmelted side surface was analyzed.After that,laser lap welding tests under cooling,pre-strain and pre-stress conditions were carried out to explore the control method of bulging distortion on the surface of unmelted side SUS304 stainless steel.The results show that the bulging distortion profile characteristics of the laser lap welding of the thin plate DC01 galvanized steel-SUS304 stainless steel on the partial penetration side austenitic stainless steel are: 1)The profile of bulging distortion perpendicular to the weld direction are An arched profile with a width of about 8 mm.The area with a wide of about 2mm at the center of the arched shape has a sharp rise.The dramatic change in the height of the micro bulging distortion significantly deflects the light and produces visually marks.Outside the center 2 mm,the arched profile is gentler.2)In the direction of the weld,the peak height of the micro bulging distortion in the middle section of the weld is higher than the start and end sections.3)As the laser power increases,the peak height of the micro bulging distortion increases.In the laser power range,1.9 k W~2.6 k W,the peak height increased from 3.5 μm to 23 μm.4)Through microstructure analysis,it is found that there are a large number of dense clusters of micro bulging in the center of the arched contour.These twins are lenticular and their long axis is perpendicular to the cold rolling direction.Deformation twins originating from grain boundaries and terminating within the grain.Micro bulging distortion profile characteristics on the surface of DC01 galvanized steel-Q235 carbon steel laser lap joints are as follows: the surface of Q235 carbon steel also appears upward convex arch profile perpendicular to the weld seam.But the overall height change of the profile is relatively gentle.And there is not steep feature concentrated in the central area.And at the same laser power,the peak height of micro bulging distortion on the surface of Q235 carbon steel is smaller.Compared with SUS304 stainless steel mirror surface,there are no obvious visual bumps on the surface of Q235 carbon steel,which indicates that the physical properties of the material are an important factor affecting the deformation profile of micro bulging distortion.As for cold-rolled SUS304 stainless steel sheet,after heat preservation and water quenching at different temperatures,microstructure comparison analysis shows that the laser lap welding of the unmelted side of SUS304 stainless steel,the micro bulging distortion on the mirror surface of the SUS304 stainless steel,the instantaneous thermal cycling temperature should be above 900℃.After 1000℃,5 min heat preservation,bending,and water quenching,in the tensile stress area,the recrystallized grains of SUS304 stainless steel are elongated in the direction of tensile stress,and the transgranular annealing twins which are parallel to the direction of tensile stress are deformed.In the compressive stress area,the recrystallized grains of SUS304 stainless steel are perpendicular to the direction of the compressive stress,and micro bulging distortion are also generated on the inner side of the bend.A large number of fine twin crystal clusters are generated in the micro bulging distortion zone,which are preferentially deformed twins under the action of compressive stress.The morphology is similar to the twin clusters generated in the micro bulging distortion zone on the stainless steel surface.It can be inferred that the bulging distortion on the surface of unmelted side of 304 stainless steel should be subjected to a similar compressive stress effect as the inner side of the bend,and this compressive stress is caused by the thermal stress generated during the laser welding process.It is concluded that the visual micro protrusions on the surface of the unmelted side of austenitic stainless steel were caused by compressive stress during welding.Therefore,the micro bulging distortion mechanism can be described as: the thermal expansion during the welding is constrained by the adjacent low temperature zone,then the area is subject to compressive stress.So the metal may be squeezed up or down.The stainless steel has low yield strength at a high temperature,and the unmelted side is less restrained.Therefore,the downward squeezing stress causes the SUS304 austenitic stainless steel to plastically deform and be extruded to form protrusions on the surface.When the temperature is cooled to room temperature,the microbump deformation is retained.Under the condition of forced cooling,the peak height of the bulging distortion of the stainless steel surface is reduced,but the micro bulging distortion cannot be eliminated.Even when the laser power is reduced where two plates are not welded,the micro bulging marks still appear on the stainless steel surface.Before laser lap welding,pre-strain value of 0 to 4mm were applied to the back of the SUS304 stainless steel to be welded.Compared with the forced cooling method,the peak height of the micro bulging distortion of the surface of the SUS304 stainless steel on the partial penetration side was reduced more significantly.Before laser lap welding,pre-stress was applied to the DC01-SUS304 lap joints.The results show that as the pre-stress increases,the peak height of the micro bulging distortion gradually decreases.Visually,the surface of the SUS304 stainless steel on the non-permeability side has slightly reduced deformation marks.更多还原