| With the trend of energy conservation, environmental protection and safety, lightweight becomes a main measure of many carmakers to decrease the automobile pollution and energy depletion. Advanced ultrahigh-strength steels are widely applied in automotive industry to meet demands of lightweight due to the advantages of high strength, high energy absorption rate and hard distortion. Laser welding is a new-type technology which is high-energy-density, fast and precise in joining various materials with different thicknesses and types. In the present automotive industry, there is a growing interest in combining the laser welding and the ultrahigh-strength steels to meet the increasing demands of lightweight and safety.Lap and butt welding of ultrahigh-strength steel 22 Mn B5, as well as its tailor welding with DP980, DP590 and Q235 steels were all performed by a fiber laser. Microstructure, hardness, tensile and fatigue performance of these welded joints were studied in this paper.The study on the lap welded joints of 22 Mn B5 indicated that the width of the welded joints decreased with welding speed increasing. When the welding speed reached a certain value, the lap welded joints were no penetration. The weld zone(WZ) consisted of lathy martensite, with a higher hardness than that of base metal(BM). A soft zone appeared in the heat affected zone(HAZ) of 22 Mn B5. The Shear strength of welded joints decreased with the increase of welding speed. The fracture surface displayed typical dimple pattern, indicating the fracture mechanism to be micro-porous aggregation, but it displayed a very brittle fracture with a low elongation.The study on butt welded joints of 22 Mn B5 showed that the WZ still consisted of lathy martensite, with the similar microhardness distribution with lap welded joints. The HAZ can be divided into 3 parts: quenched zone, incomplete quenched zone and tempered zone. In the tempered zone, the martensite tempered into tempered martensite. With the welding speed increasing, the martensite in WZ became thinner and shorter and the width of tempered HAZ decreased. The tensile specimens of 22 Mn B5 welded joints were all broken in HAZ with a low elongation of about 2%. The fracture morphologies showed dimples and cleavage planes, as well as some tearing ridges, the fracture surface indicated quasi-cleavage fracture characteristics.Laser tailor welded blanks of 22 Mn B5, DP steels and Q235 studies indicated that good-quality joint can be obtained, and the defects such as gas hole and crack don’t appear in the WZ. The highest microhardness was still in WZ, but inside the WZ of dissimilar welded joints two hardness sub-regions were observed due to the difference in the alloying elements of these steels. Like the 22 Mn B5, DP steels in this study also owned the soft zones with dissimilar degree. The BM had a higher strength and the degree of its soft zone was deeper. However, Q235 BM was normalized, leading to no soft zones in its HAZ. Tensile specimens of 22 Mn B5-22 Mn B5 and 22 Mn B5-DP980 welded joints were all broken in HAZ, while the other dissimilar welded joints specimens were in the BM of the lower strength steels. At the same fatigue test parameter, the BM of 22 Mn B5 had the highest fatigue life and the similar 22 Mn B5 welded joints still failed in HAZ, while the the dissimilar 22 Mn B5-DP980 welded joints were all broken in BM of DP980. The typical fatigue fracture contained 3 parts: crack initiation region which occurred from the specimen surface, crack propagation region where the fatigue striations and secondary cracks can be seen and the final fast fracture region. |
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