Study On Microstructures And Mechanical Properties Of Resistance Spot Welded High Strength Steel B340/590DP

With the increasing awareness of environmental protection, vehicle weight reduction has received increasing attentions by vehicle manufacturers. Since advanced strength steels(HSS)have the advantages of high strength, good formability, high bake-hardening properties, high absorption rate, high fatigue strength, and good crashproof performance, the amount of its use keeps growing under the car lightweight construction. Resistance spot welding is a kind of connection method of materials with features of low power consumption, low cost, high efficiency, high quality, easy to get automated and to use, which is also a safe, economical, efficient, reliable, environment-friendly welding method. It has been mainly used to connect thin-walled parts. Because of its effective, reliable connection, resistance spot welding has received a wide range of popularity, especially in automotive industry. Generally a car approximately has 4,000 -6,000 solder joints, therefore, the reliability of resistance spot welding will affect the performance of cars directly. Thus, the study on the weldability of resistance spot welded high strength steel(HSS)is essential.This paper has systematically studied on the microstructures and mechanical properties of the welding joint of resistance spot welded high strength steel B340/590DP. Apart from that, this article has also discussed the effect of welding process parameters (welding current, welding time, electrode force) on the microstructures and mechanical properties of resistance spot welded joints of B340/590DP. Moreover, the tempering process parameters (tempering current, tempering time, cooling time) of resistance spot welded B340/590DP have been optimized by the method of orthogonal experiment to study the effect of the tempering process on the microstructures and mechanical properties of resistance spot welded joints of B340/590DP. Based on the experiment itself and the analysis of the experiment results, the main conclusions of this paper are summarized as follows:(1) The welding joint of resistance spot welded was mainly is composed of the nugget and heat affected zone (HAZ). The nugget formed from the border, having a characteristic of epitaxial growth, which belonged to a kind of non-homogeneous nucleation. The grains near heat affected zone which had experienced the welding process were equivalent to normalizing. Therefore, the grains in this area were refined, and the strength got increased too, which were elative to the base metal which was not melted.(2) B340/590DP nugget microstructures which were founded to be the columnar crystals, mainly contained a great deal of large lath martensite and a small amount of ferrite and pearlite.The phase structures of the nugget of resistance spot welded B340/590DP were mainly made up of Fe (δ-Mn) and a small amount of intermetallic compound Fe19Mn. The heat affected zone (HAZ) microstructures of resistance spot welded B340/590DP were mainly small lath martensite and a small amount of ferrite and pearlite. What's more, the distribution was uniform.(3) The influences of the welding current, welding time and electrode pressure on the microstructures and mechanical properties of resistance Spot Welded high strength steel (HSS) B340/590DP Joint were significant.When the welding current increased from 7.3kA to 9.5kA, weld nugget sizes and strength of welded joints increased with the increase of welding current. But when the welding current increased to a certain extent (greater than 9.5kA), the heat input was so much that the liquid metal in the nugget which had been melted got splashed ,as a result, the strength of spot welded joints got reduced. But when the welding current increased to a certain extent (greater than 9.5kA), the heat input was so much that the liquid metal in the nugget which had been melted got splashed ,as a result, the strength of spot welded joints got reduced.The spot welded nugget size increased with the increase of welding time (9 cycles to 17 cycles), so was the indentation depth of the welded joints. However, welded joint strength increased at first and then gradually stabilized. Under the experimental conditions, the welded joint strength began to stabilize until the welding time was more than 15 cycles. When the electrode pressure decreased from 6.0kN to 3.0kN, the nugget sizes increased, so were the strength of welded joints. But splash of the melting liquid metal in nugget area would emerge, when the electrode pressure reduced to a certain extent (less than 4.0kN), which made the spot welding joint strength decrease.By Previous studying the best parameters of resistance spot welding process for B340/590DP were determined as follows: the welding current was 9.5kA, the welding time was 13 or 15 cycles and the electrode pressure was 4.0kN. Under these conditions, the nugget diameter was 6.4mm and the spot welding joint tensile shear load was 15.991kN.(4) Under the tensile-shear conditions, resistance spot welded advanced high strength steel B340/590DP joints mainly had two kinds of fracture modes. They were respectively interfacial fracture which belonged to brittle fracture and button pullout fracture which belonged to ductile fracture. Button pullout fracture was the fracture mode requested in practice.(5) Through the experiment, we could see that several problems existed in the resistance spot welding process of high strength steel (HSS) B340/590DP. Furthermore, the common problems mainly included Shrinkage cavity, cracking and expulsion in the welding process. These issues above would affect the quality of spot welding joint significantly, therefore, corresponding measures should be taken in practice to prevent above problems from coming out and obtain more reliable welding joints.(6) Based on the existing experimental results and the original empirical formulas, the tempering process parameters of B340/590DP resistance spot welding process were determined by the method of orthogonal test. Through this test and analysis of the test results, the final choice of the tempered condition was a longer tempering time, moderate-intensity tempering current and short cooling time.Under those conditions,the best tempering process parameters were as follows: the electrode pressure was 4.0kN, the welding current was 9.5kA, the welding time was 15 cycles, the tempering current was 5.7kA, the tempering time was 45 cycles and cooling time was 20 cycles. In this case, spot welding joint tensile shear load was 19.139kN and nugget diameter was 6.8mm.And the nugget microstructures were mainly tempered martensite. Compared with the corresponding results of preceding experiment without adding the tempering pulse current to the spot welding process, the micro-hardness of spot welding joint zone (nugget and HAZ) decreased largely. At the same time, the tensile shear load increased. Thus, we could conclude that a appropriate tempering welding process could eliminate part of the internal stress, increase the toughness, meanwhile, maintain high hardness. In a word, the spot welding joint would have good mechanical properties under those conditions.