Comparative Study On Mechanical Properties Of Resistance Spot Welded Joints Of Q235B/301L And 09CuPCrNi/301L Dissimilar Steels

301L series austenitic stainless steels is widely used in lightweight stainless steel passenger car bodies,the car body adopts plate-beam combination integrated load-bearing spot welding structure,there have dissimilar steel connection of low carbon steel-stainless steel and weathering resistant steel-stainless steel locally.This paper used Q235 B low carbon steel,301L-DLT stainless steel and 09 Cu PCr Ni weathering resistant steel as the welding base materials,designed lap shear resistance spot welding specimens with two thickness combinations of 2.0+2.0 mm and 4.0+1.5mm.The static mechanical properties,fatigue resistance under high and low fatigue loads and failure modes were compared and analyzed,the effect of stress and strain distribution of spot welded specimens on the fracture mode was analyzed.The nugget of dissimilar steel spot welded specimens presented an asymmetrical structure.Among the specimens with equal thickness plates,the microhardness value in301 L stainless steel nugget was increased from the nugget center to the edge of the nugget which had the highest hardness value,the Q235B/301 L specimens showed better static mechanical properties,the fracture load and fracture absorption energy were 169%and 139% of that of 09 Cu PCr Ni/301 L specimen respectively,the maximum plastic strain value was less than 09 Cu PCr Ni/301 L specimen;Among the specimens with unequal thickness plates combination,the microhardness distribution in 301 L stainless steel nugget was gentle and there was no increasing trend,the 09 Cu PCr Ni/301 L specimens showed excellent static mechanical properties,the fracture displacement was four times more than that of Q235B/301 L specimens,and the fracture absorption energy is much higher than that of Q235B/301 L specimens.The median fatigue limit of spot welding specimens with the same plate thickness combination was similar,the fatigue curves in the high and low fatigue loads areas were approximately parallel and the fatigue resistance was similar.Among the specimens with unequal thickness plates combination,the slope of the fatigue curves of specimens was quite different in the high and low fatigue loads areas.in the low-cycle fatigue life zone,the fatigue performance of Q235B/301 L specimens was better,the fatigue performance of 09 Cu PCr Ni/301 L specimen in the high cycle fatigue life area was better than Q235B/301 L specimen.Through comparative analysis of the static and fatigue properties of different specimens,it was determined that the Q235B/301 L dissimilar steel spot welded specimen was more suitable for the 2.0+2.0 mm combination,and the09 Cu PCr Ni/301 L dissimilar steel spot welded specimen was suitable for the 4.0+1.5mm combination.when the equal thickness specimens were under high and low fatigue loads the stress concentration occured at the edge of the nugget and the base metal near the edge of the nugget had concentrated plastic strain,the position of stress concentration was consistent with the crack initiation position of the specimens.The 09 Cu PCr Ni/301 L specimen and Q235B/301 L specimen had double-plate fatigue fracture under high fatigue loads,the bearing capacity of two plates were equivalent.Under low fatigue loads,Q235B/301 L specimen had double-plate fatigue failure and 09 Cu PCr Ni/301 L specimen had 301 L stainless steel plate fracture;The thickness of the base materials of unequal thickness plates combination specimens were quite different,the stress concentration mainly occured on the 301 L stainless steel side and concentrated plastic strain occurred in the base metal which near the side of 301 L stainless steel nugget.The failure modes of specimens was caused by fatigue fracture of 301 L stainless steel plates,the initial fracture position was consistent with the position of maximum stress and strain.