| Automotive light weighting is the inevitable trend with energy saving and emission reduction emphasized increasingly. In order to meet the requirements of lightweight, researchers and steel manufacturers have been developing higher and higher grade automotive steel. On the other hand, coating on the steel plate surface for anti-corrosion, such as zinc and aluminums, is generally processing. The continuous development of automotive steel also puts forward on the development of the joining process. Laser welding, as a kind of highly efficient welding method with high quality, can be adopted in fusion welding of a variety of materials with different surface states. After high power fiber laser come into the market, it has promoted the application of laser welding in automobile body manufacturing due to its stable beam quality, smaller volume of generator, lower maintenance costs and other advantages. In this paper, fiber laser is used to conduct tailored laser welding or lap laser welding of galvanized and Al-Si coating steel sheets and their welding performances is studied.Firstly, it is investigated about the fiber laser welding of galvanized steel sheet. Galvanized multi-phase steel with 1.5mm thickness and low alloyed steel with 1.0mm thickness are chosen to perform tailored laser welding. Thereafter, weld seam forming, microstructure, hardness, tensile strength, and formability are tested. Galvanized low alloyed steel with 1.35 mm thickness is chosen to perform laser lap welding. The influence of lap gap on weld seam forming and joint mechanical properties are investigated. The mechanism of presetting gap method, which can eliminate the influence of high pressure zinc vapor, is analyzed. Meanwhile the range of gap volume is discussed. The outcomes show that, tailored fiber laser welding of galvanized cold-rolled multi-phase steel and low alloyed steel can achieve sound weld seams, which are not influenced by zinc coating. Tempered softening zone exists in heat affected zone of multi-phase steel, whose width is mainly affected by welding velocity. The tailored blanks have good tensile strength and formability. The fracture location of tensile strength test is on the base metal of low alloyed steel, and that of LDH test is on the thinner sheet, parallel to the weld seam. For laser lap welding, zinc coating may cause severe destruction on weld seam. Gap exceeding 0.1mm can suppress the defects caused by zinc vapor effectively. Penetration reaches the maximum when gap is about0.15 mm. And greater laser power, which is 4k W, has better tolerance on gap from 0.05~0.2mm.Then, fiber laser welding of Al-Si coated steel sheet is investigated. Al-Si coated Usibor 1500 steel before hot stamping with 1.0mm thickness are chosen to perform tailored laser welding. Metallographic structure and mechanical property of the weld seam are analyzed, and the distribution of Al in the weld seams is investigated by EDS. Al-Si coated hot stamping steel with 1.35 mm thickness are chosen to perform laser lap welding. The influence of coating on the lap joint and the shear tensile strength of lap joint with different lengths and patterns are investigated. The results show that, Al enters into the weld seam and distributes in the whole area of weld seam, with local segregation. Al-Si coating which enters the weld seam decreases the mechanical properties of weld seam due the formation of brittle phase. After removing the top layer of the coating, the mechanical properties of the weld joint increase distinctly. For laser lap welding, the shear tensile test may cause two kinds of fracture location. The shear tensile strength of I-type weld seam can reach the shear tensile strength of the spot joint when its length is greater than 22.5mm. On the other hand, weld seams with patterns such as S-type, O-type or C-type have increased strength than I-type, among them which C-type pattern is the best. |
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