Research Of Bulging Distortion On The Surface Of Unmelted Side In Thin Plates Partial Penetration Laser Welding

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 ?m to 23 ?m.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.