Research On Laser Welding Procedure Of High Strength Steels

Automotive lightweighting is one of the most important methods to meet therequirements of energy conservation and emission reduction. And it can be achieved byusing laser welded high strength steels. A fiber laser was used to weld dual-phase steelDP600and interstitial free (IF) steel. The weld geometry, microstructure, microhardnessand uniaxial tensile properties of the welded joints were studied and the process windowswere optimized. Also, the effect of welding parameters on heat affected zone (HAZ)softening behavior of laser welded DP780and DP980were investigated. The mainconclusions are as follows.The full penetration welds of DP600/DP600(0.7mm/0.7mm) similar or DP600/IF(0.7mm/1.2mm) dissimilar joints include the keyhole-type mode and the melt-typemode. As decreasing laser power and increasing welding speed the melt-type modeoccurs and it can achieve smaller heat input and higher energy efficiency. Under a highlaser power and welding speed combination, the keyhole is elongated and leads to theunusual weld geometry in similar weld of joint, but it can be destroyed in the dissimilarweld of joint.DP600/DP600(0.7mm/0.7mm) similar joints have microstructure consisted offerrite and martensite. Gradually, the size and fraction volume of martensite decreasefrom the fusion zone (FZ) to the base metal (BM). The average microhardness of the FZvaries from365HV to390HV, and increases with the decreasing heat input. The averagemicrohardness of the BM is about200HV. There is no soft area in HAZ and thetransverse joints all fracture at the BM, showing ultimate tensile strength (UTS) equal tothat of BM. The elongation (El) of the joint welded at1.5kW,4.0m/min is the largestand reach95.6%of the BM.DP600/IF (0.7mm/1.2mm) dissimilar joints have mixed microstructure consistingof bainite and ferrite in the FZ. The FZ average microhardness varies from230HV to280HV, and increases with a decreasing heat input, but decreases obviously when it changed into a melt mode even with less heat input. The joints all fracture at the IF steelBM with average hardness of90HV. The tensile properties change little with differentparameters, The UTS of the joint welded at2.5kW,3.0m/min achieve85.06%of IFsteel while the El is about96.63%of DP600steel.DP780/DP780and DP980/DP980(1.4mm/1.4mm) similar joints have the HAZsoft areas in all joints and the latter combination has a more serious soft degree. Thelarger heat input parameters also lead to a more severe soft zone. The joints all fracture atthe HAZ soft area, and their positions become farther away from the weld center linewhen the heat input increases. When the laser power is3.4kW and the welding speed is5.0m/min, both the DP780/DP780and DP980/DP980combinations obtain the besttensile properties, and the UTS reaches98.6%and99.5%, the El attains72.2%and66.2%, respectively.