| Lightweighting has become an important strategic direction on the road of global green development.As the main material for heavy-duty vehicle manufacturing,high strength steel has better strength and toughness matching and higher safety factor compared with light materials such as aluminum and magnesium alloy.Enhancing strength has become the inevitable choice of high strength steel in car body lightweight application.However,as the strength grade reaches 1000 MPa,the performance of traditional welded joints of high strength steel deteriorates seriously,which hinders the popularization and application of1000 MPa high strength steel.Laser-MAG hybrid welding has the advantages of high welding efficiency and small heat input.Therefore,the welding joint performance and welding efficiency of HSS can be improved by welding wire matching combined with laser-MAG hybrid welding method,so as to promote the lightweight application of HSS in heavy-duty vehicles.In this paper,1000 MPa grade BS960 E high strength steel with 6.5mm thickness was taken as the research object.And the process optimization was carried out by laser-MAG hybrid welding method.Firstly,the weldability evaluation of BS960 E high strength steel was carried out by combining theoretical analysis with test.Then,according to the results of weldability,the equal strength matching and low strength matching welding wires were selected to study the adaptability of process optimization.And the microstructure and properties of BS960 E high strength steel welded joint are analyzed.The analysis of weldability shows that BS960 E high strength steel has a high tendency of quenching and cold crack sensitivity in theory,but the cold crack rate is much lower than20% with the change of line energy in laser-MAG hybrid welding.By comparing the sensitivity of two kinds of welding wire welding joint to cold crack and hot crack under three kinds of line energy conditions,it is shown that no hot crack appears and the cold crack rate meets the requirements of use.It shows that BS960 E high strength steel has good weldability under laser-MAG hybrid welding condition.The process research results show that under I and Y groove,the laser-MAG hybrid single-pass welding process optimization is achieved for the two kinds of welding wires,and the welding forming is good.The impact properties of three groups of optimized welding processes at-40°C under type I groove were compared.The results show that the impact properties of the two welding wires are better when the welding speed is 19mm/s,the laser power is 4600 W and the wire feeding speed is 11m/min.It shows that increasing the filling amount of welding wire and controlling the line energy reasonably can effectively improve the performance of welding joints.Through the analysis of the microstructure and properties of the joint,it is found that the two filling materials form a weld dominated by bainite,and the heat affected zone is influenced by the same line energy to form lath martensite.The hardness of 80 YM welding seam of low strength matched wire is lower than that of 90 G welding seam of equal strength matched wire,but it is equivalent to base metal,and the softening of heat-affected zone is effectively inhibited.Under the Y groove,80 YM and 90 G welding wires have better impact toughness at-40°C,and the tensile strength is equivalent to that of the base metal.The 80 YM welding joint has better elongation.Under the Y groove,the impact performance of the80 YM weld is better than that of the two welds.Compared with the impact properties of the two welds under type I groove,the impact properties of the weld under type Y groove are all higher.In the fatigue crack propagation analysis,it is found that the crack propagation resistance of the weld is the worst,and the heat affected zone is between the base metal and the weld.Compared with the two kinds of welds,the low strength matched wire has better ability to resist fatigue crack propagation. |
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