| According to a survey by German Ministry of Environment, simply by adding 212 euros to per vehicle (about 280 U.S. dollars) for the cost to purchase high-strength steel or the new lightweight materials, they can save more than 5% fuel. Compared with any other kinds of installation of hybrid power systems, the unit cost is lower. Except for hybrid technology, The fuel-efficient technology also include variable valve and intake manifold, Turbo charger, individual cylinder flameout etc., but no one can achieve a 5% fuel saving effect. Vehicle equipment mass is positive proportional to the fuel consumption. According to relevant institutions'statistics, each vehicle equipment mass to reduce 40kg, fuel consumption for per 100 km can be reduced by 1%. Therefore, in the case of fixing other variables, the less of body weight, the more fuel is saved. But the car design emphasizes on the balance, the vehicle need good stability and security. Using of high strength steel, which could lighten vehicle equipment mass and enhance the vehicle safety in the same time. With the continuous high requirements on anti-collision and security of car, ultra high strength steel (UHSS), which be of high strength and yield strength (1200MPa), high anti-collision and energy absorption properties., UHSS can be used to manufacture anti-collision components of car, such as door crashworthy beam, pillar etc.. As one of most promising lightweight structural materials, UHSS has a broad application for vehicle manufacturers.Due to the special physical and chemical properties of ultra-high strength steel, its welding process is very difficult to control, and the traditional spot welding will cause serious spatter problem. The strength of UHSS welding joints cannot meet actual production. Improve the spot welding process usually by the following two ways:firstly, increasing the electrode pressure. Solution is to increase the electrodes diameter and electrodes pressure. However, these improvements will lead to greatly increase the weight of spot welding device. It is difficult for the workers' actual operation. Secondly, increasing the electrode current. Solution is to increase the electrodes diameter and increasing current (up to 12～14KA). However, these improvements will lead to big weight of welding machine and a very serious splash, deep indentation. The joints' quality cannot meet actual production needs. Material history shows that welding performance determine whether a new material could be extensive used or not. The welding process must be adjusted to get the welded joints with good quality. Currently, it is believed that IF spot welding technology is a feasible solution in the actual production. The IF spot welding technology could meet the new welding demands from ultra-high strength, that is, the welding electrical source must be of higher energy and precise parameter adjustment, dynamic adjustment and reduce the heat and mechanical pressure electrode. In addition, laser welding technology also meet the new welding demand of UHSS. Because the laser welding technology be of many unique merits, such as low distortion, good accessibility, flexibility and high efficiency. And Laser welding technology has broad application prospects in the near future. Therefore, it is of great practical significance for the investigation for the welding performance of UHSS.In this paper, The IF steel welding performance of ultra-high strength steel is detailed researched. High-quality joints with good mechanical properties are obtained by optimizing the welding process parameters. Optical microscope test, hardness test, fracture analysis is underwent to analyse the welded joints and we get the following conclusions:1) Two ultra-high strength steel plates are welded by using the optimized spot welding parameters (welding pressure 2.0KN, pre-press time:600ms, welding current: 8.0KA, welding time:350ms, hold time:200 ms, tempering current:5.4 KA, welding pulse number:2). The welding joint meet the actual production's requirement. No holes and shrinkage defects were found in the joints nugget. The microstructure of base material is small dense martensite. Optical microscope shows lath martensite consist of the microstructure in the nugget. The microstucture in heat affected zone (HAZ) is granular martensite; Hardness test showed that the hardness distribution in the nugget is uneven. The nugget center is of the highest hardness, because the microstructure in nugget center is thick lath martensite. The hardness value near the fusion line is lowest because the rapid organization transition due to the less dense microstucture in this zone; Surface hardness test of weld joints show that the hardness in plastic ring region decline evidently, Which is caused by the ferrite generated in plastic ring region; IF spot welding joints with high shear strength 29.7KN. Shear strength close to the base metal tensile strength, but toughness decreased significantly (down about 50%); IF the shear strength increased with IF nugget diameter; Fracture morphology of utral-high strength steel base metal is equiaxed dimples; Fracture mode of IF spot welding joints constituted by the small cleavage plane, vein patterns, and secondary crack.2) The welded joints between UHSS and Micro-alloy steel have the same welding parameters to the welded joints between two ultra-high strengh steel(welding pressure: 2.0KN, pre-press time:600ms, welding current:8.OKA, welding time:350ms, hold time: 200ms, tempering current: 5.4, welding pulse number:2). The welded joints with good quality. No shrinkage, cracks, welding defects are found in the joints. Hardness tests show that the hardness distribution in the nugget is uneven, which mainly caused the martensitic and ferritic mixed microstructure in the nugget. The hardness decline sharply in the fusion line zone is mainly caused by the rapid transition microstructure and formed a less dense microstructure. Fracture mode is a mixed mechanism. The joints in micro-alloy steel side belongs to dimple fracture, and in the ultra-high strength steel side belongs to intergranular brittle fracture.3) Laser welding technology is adopted for two ultra-high strength steel plates lap welding. Using appropriate process parameters (welding rate:4000W; spot diameter:1mm; welding speed:20～35mm/s) access to quality laser welded joint, the joint strength to meet the FAW-Wolkswagen factory standard about ultra-high strength steel laser welding. The joints with good internal quality. No cracks, shrinkage and other defects are found in the nugget; The microstucture of the joints is still martensite, but grains are more coarse than the base metal. Under the same welding power, the growth speed of grain in nugget will accelerate if the welding speed increases. Hardness test show that hardness distribution in nugget of laser welded joints are uneven. Laser welding speed has an significantly effect on the nugget hardness. The hardness decreased in the fusion line zone caused by the decarburization phenomena in the welding process and a large number of ferrite in this region; Fracture mode of the joints in nugget region belongs to equiaxed dimple fracture, and in the fusion line zone is a hybrid mechanism.
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