Research On Resistance Spot Welding Process And Quality Control Model Of High Strength Steel B340LA

It has become the important target for vehicle body design to reduce vehicleweight and improve component strength. So the high strength low alloy steel turnsinto one of the lightweight materials which have most extensive application prospect.As the resistance spot welding with low-cost and high efficiency accounts for about90%of the body assembly work, controlling and improving the quality of resistancespot welding play a significant role for vehicle safety. This paper researches thenarrow resistance spot welding window and weld nugget fracture. At the same time, itis also important to choose the suitable quality control method, because there are morerandom disturbance factors in resistance spot welding process and it is more sensitivefor high strength steel when the welding quality fluctuates. An effective method ischosen to achieve real-time monitoring of spot welding for high strength steel throughcontrast test of various monitoring modes.1.2+1.2mm thickness high strength steels are used in this paper, and the simpletiming method, constant current method and IDRC method are taken in the contrasttests. Shunt is main disturbance factors in welding heat processes, sensitivity ofB340LA for random disturbance factor is investigated. Results of the tests indicatedthat it cannot get button-type fracture nugget by simple timing method and constantcurrent method when the shunt dot pitch is21.5mm. Only when the shunt dot pitch is65mm, the button-type fracture nugget is guaranteed by constant current method. Butthe superior quality button-type nugget can be achieved by IDRC method while theshunt dot pitch is just21.5mm. Shunt is more sensitive for this kind of steel, whileIDRC method has the higher compensation ability for shunt situation. A set ofcontrolling parameters have been chosen for IDRC method: electrode pressure P is3.6KN, the welding current I is8.6kA, current lag angle cumulative increase (i.e. agiven threshold) is260μs.The microstructures of spot welding joint are significant different from highstrength steel and low carbon steel with further research. The microstructures ofnugget without tempering are mainly coarse lath martensite. But the microstructures do not change obviously after single pulse tempering treatment. A part of columnarcrystals turn to equiaxed crystals, the finer temper martensite gets from lath martensiteand the microhardness decreases from HV360to HV270, after twice pulse temperingtreatment. The results of destructive testing of tempered nugget show that thesensitivity of shunt alleviates obviously because of the ductility increase of temperednuggets and the decrease of stress concentration. The superior quality button-typenugget is still achieved even the shunt dot pitch is only10mm. The twice pulsetempering spot welding is recommended for B340LA low alloy high strength steeland the IDRC method is chosen to ensure nugget quality in this paper.