Study On Effects Of Laser Welding Heat Input On Microstructures And Properties Of Stainless Steel Joints

With a variety of advantages, such as lightweight, good security, low cost,corrosion-resistance etc., stainless steel railway vehicle has become the first choice ofrailway vehicles in China at present. With the advance of contemporary science andtechnology, more strict requirements on the quality of railway vehicles are put forward toproducing high quality stainless steel railway vehicles, which can’t be satisfied byresistance spot welding technology. Laser welding technology is beneficial to improve thequality of railway vehicles obviously. Compared with traditional welding methods, laserwelding technology has irreplaceable advantages and will gradually replace the resistancespot welding technology applied in the stainless steel railway vehicle body.In this paper, the effects of laser welding heat input on weld depth, weld width,microstructures, hardness, tensile property and fatigue property of stainless steel lap jointswere studied. The research results show that both weld depth and width increase with theincrease of welding heat input. When the welding heat input is less than1.0000kJ/cm,weld surface performs a good forming and there are not obvious welding traces on theback side. As welding heat input increases, the directivity of cellular crystals and cellulardendrites in the weld zone enhances, the crystal zone width is bigger, the width of HAZincreases and the grain has coarsening tendency. The distribution of hardness is uneven inaustenitic stainless steel laser welding joint. The hardness of the weld and HAZ decreasewhile the tensile-shear strength of joint increases with the increase of welding heat input.Moreover, the fracture mode of laser welding joints has two kinds: HAZ fracture modeand interface weld fracture mode. Also, stainless steel lap laser welding process zone isbuilt in this paper, which is0.8636kJ/cm≤E≤1.0000kJ/cm.The welded joints with welding heat inputs of0.8636kJ/cm,0.9048kJ/cm,0.9091kJ/cm and0.9545kJ/cm are selected to test the fatigue property. Results indicate thatfatigue crack extends in weld seam and the lower plate under the condition of high load,and it fractures in weld seam. While crack extends and consequently fractures in the2mmlower plate with low and middle load. Furthermore, its propagation direction isapproximately parallel to that of the plate thickness. At low load, crack extends along thegrain boundary. Besides, the extension zone, in which the first stage time is long, is larger than the transient fault zone in area. Moreover, it is clear with the characteristics of awider fatigue stripe and deeper dimples. At high load, crack initiates in the persistent slipband (PSB). Additionally, with a long time of the second stage, the extension zone area issmaller than the transient fault zone area. And apparently, fatigue stripe width is smallerand dimples are more shallow compared with low load condition. At the early stage, crackexpands at a slower speed without fatigue striation on the fracture surface and the anglebetween crack propagation direction and stress axis is45°. At the following stage, crackexpands at a faster speed and there exists typical fatigue striation on the fracture surface.Besides, the propagation direction becomes perpendicular to the stress axis as well.