| In this study,the resistance spot welding test of 1.2 mm thick third generation advanced high strength steel Q&P980 steel was carried out.The effects of welding time,welding pressure and welding current on the macroscopic structure,microstructure,hardness,shear strength,peel strength and energy absorption properties of the spot welded joints were investigated.Based on the optimum primary spot welding parameters,the influence of secondary welding current on the microstructure and properties of spot welded Q&P980 steel joints is studied.It was found out that:Welding current,welding pressure and welding time had a significant influence on the weld size and microstructure of spot welded Q&P 980 steel joints.The joint microstructure can be divided into three zones: base zone,thermal affect zone and welding zone according to the microstructure characteristics.The martensite structure in the welding zone is gradually coarsened with prolonging welding time,increasing welding current and raising welding pressure.Too small welding pressure,too large welding current will produce material expulsion,welding zone cracking and shrinkage defects.The welding process window with qualified nugget size and avoiding material expulsion is 6-8 k A welding current and 4-8T welding time.The joints obtained in the welding process window had fine and uniform martensite structure and excellent mechanical properties.Welding parameters had an influence on the hardness and strength of spot welded Q&P980 steel joints.The hardness of welding zone increased first and then decreased with the increase of welding time,decreased continuously with the increase of welding pressure,and changed little with the increase of welding current.The joint strength and energy absorption properties increased first and then decreased with the increase of welding time,welding pressure and welding current.To obtain high strength spot welding joint,it is necessary to increase the size of nugget,refine the martensite structure in welding zone,increase the uniformity of martensite structure in welding zon,and avoid the deffects of gas hole,shrinkage and crack in welding zone.The joints produced with the welding parameters combination of 8T welding time,1754 N welding pressure and 8k A welding current had the largest nugget size,the finest and most uniform martensite structure and the least welding defects in welding zone.Hence,the spot welded Q&P980 steel joints produced with the parameter combination not only had the highest shear strength and peeling strength,but also had the best ductility and energy absorption properties.Therefore,it had the best mechanical property.Secondary welding remelted the primary welding structure of spot welded Q&P980 steel joints and formed a secondary welding zone.It refined the martensite structure of the welding zone and suppressd or reduced the shrinkage defects in the welding zone.Therefore,the hardness,shear strength,peel strength and energy absorption properties of the joint were improved after secondary welding except for the decrease of hardness uniformity.An improvement in the mechanical properties of spot welded Q&P980 steel joints caused by secondary welding was determined by the increase in the size of the secondary nugget and the improvement of the martensite structure in welding zone.The larger secondary welding nugget,fine martensite structure of the welding zone and the reduction of defects such as shrinkage,crack in welding zone increased the bearing capacity of welding nugget,enhanced the martensite refinement strengthening effect and decreased the stress concentration caused with shrinkage,crack respectively.All them were beneficial to the improvement of the shear strength,peel strength and energy absorption properties of the secondary welded Q&P980 steel joints.The spot welding joints produced with 6 k A secondary welding current had the largest secondary nugget,the finest martensite structure and the least shrinkage defect.Therefore,the secondary spot welded Q&P980 steel joints had the highest hardness,joint strength and energy absorption properties. |
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