Investigation of spot welding properties of pre-strained DP600 steel grades

Advanced high strength steels (AHSS) are sophisticated materials with carefully selected chemical compositions and multiphase microstructures. These materials have great strength combined with excellent ductility. One of AHSS is dual phase (DP) steel and usage of dual phase steel grades have been increased rapidly in the automotive industry due to high formability, good weldability and advantage of lightweight vehicles. Microstructure of DP steel consist of about 15-20% hard martensite particles dispersed in a soft ductile ferrite matrix. However, small amounts of other phases, such as bainite, pearlite, or retained austenite, may also be seen. Resistance spot welding (RSW) is one of the major joining methods of conventional and AHSS steels in the automotive industry. Resistance spot welding is a rapid and economic procedure to join steel sheets in automotive industry. It has wider application in body car assembly, in average a vehicle has 4000 to 7000 welded points. It is significant to characterize the spot welding behavior of DP steels to be able to use these steels properly. For body-in-white applications, dual phase steels are used with either galvanized (pure zinc) or galvannealed (zinc-iron alloy) coating for corrosion protection. Before welding process, body-in-white components of automobiles are deformed plastically. Due to the level of prestrain (plastic deformation), microstructure and mechanical characteristics vary locally and differ from the initial undeformed steel sheet. The material investigated in this study, is a cold rolled galvanized and uncoated sheets of DP600 steel grade with 1.2 mm thickness. In order to determine Imin and Imax values for different prestrain conditions, chisel test coupons with 45x45mm dimensions were used. What is more, for the mechanical strength tests, shear and cross tensile test coupons with 105x45mm and 150x50mm dimensions respectively were preferred. In order to obtain these test specimens with the required dimensions for distinct prestrain levels, a new tensile test specimen is designed based upon standard A80 tensile test specimen. New designed tensile test specimens were cold deformed at 7.5% and 15% prestrain levels by applying static tension on Zwick type static tensile test machine. GOM/Argus, optical 3D forming analysis system, was used to confirm homogenous plastic deformation occurred in the middle area of the new tensile test specimen. Spot welds were carried out by pedestal type AC spot welding machine with 250 kVA rated power at 50%. This machine is equipped with an air cylinder which has a maximum capacity of 6 bar. Chisel, shear and cross tensile test coupons are welded in accordance with SEP 1220-2 test standard procedure. As distinct from SEP 1220-2 standard, electrode caps were dressed according to the shape based on ISO 5821 standard, not to the dressing shape shown in Figure 1 of this standard. Electrode caps ISO 5821-F1-16 with 6mm face diameter made from Cu-Cr-Zr material were used during the tests. Tip dressing was performed with air operated manual tip dresser. After tip dressing, outer cross-section of the electrode caps switched to the shape of ISO 5821-BO-16 with 6mm face diameter. Before the spot welding tests, electrode force applied by air cylinder was measured and checked with a portable instrument designed for measuring the parameters of resistance welding. During the tests, force value, welding and holding times were kept constant in order to determine the minimum and maximum current values defined in SEP 1220-2 for each type of non-prestrained and prestrained steel sheets. Minimum nugget diameter 4√t formula, where t is the sheet thickness of steel, is used to determine Imin value. All the chisel test coupons were destroyed by manually. After that, I current value was increased in small steps until the expulsions were observed. Just below I current value that was seen expulsions was defined as Imax current value. Special molds were manufactured from kestamid material to prevent excess force on joint point and misalignment problems occurred before and after spot welding operation. Specially, in the not usage of any mold, alignment of chisel, shear and cross tensile test coupons are deformed during the welding operation. Imin and Imax current values were determined for non-prestrained and prestrained, 7.5% and 15%, conditions. For the determination of the current parameters SEP 1220-2 standard was taken as reference. It is seen that same Imax value is obtained for undeformed and deformed steel sheets. However, Imin value slightly changes depending on the prestrain levels. By the way, it is not observed any signification correlation between Imin and prestrain values. Imin value for 0% and 7.5% prestrained steel sheets may be accepted as the same. By the way, if the Imin value of undeformed sheet is compared with the Imin value of 15% prestrained sheet, 0.3kA difference can be described as a significant value. Thus, spot welds of shear and cross tensile test coupons of %15 prestrained sheets are also repeated for 7.5kA. Optical microscope and scanning electron microscope were applied to evaluate the microstructure. Etching was performed by using 2% Nital solution. Fusion zone (FZ), HAZ and base metal (BM) are observed in cross-section of microstructure. Fusion zone is melted and solidify again during the welding procedure. The fusion zone and a part of HAZ, which is near fusion zone have fully martensitic structure. The region near fusion zone divided into two side such as grain growth and recrystallized regions. Phase transformation can be occured in intercritical region based on temperature and cooling rate. The subcritical or tempered region, which is near the base metal, is so difficult to differ from the BM, does not demonstrate variance of microstructure. Fully martensitic microstructure was seen in fusion zone and a part of HAZ, which is near fusion zone. There is no phase transformation in the boundary of base metal and HAZ due to the lower temperature distribution. Tempering of martensite is occurred in subcritical zone. HAZ has great influence on the tensile properties of spot welded DP steel. Because rapture is occurred in the boundary of subcritical HAZ and the failure depends on HAZ softening significantly. The hardness value of base metal is increased by pre-strain. Although the region which is near base metal (subcritical zone) has lower hardness values, the region near fusion zone has higher hardness values due to the formation of martensite near fusion zone. Test procedures of ISO 14273 and ISO 14272 standards were followed for shear and cross tensile tests respectively. Tensile tests were carried out on electro-mechanical Zwick Z250 tensile testing machine at room temperature with the speed of 5 mm/min. The results showed that Fmax values obtained from shear testing is highly correlated with the prestrain values of sheet metal. With the increase in prestrain of sheet metal, strength of spot welds for both Imin and Imax current values increase significantly. These results showed that Fmax values obtained from non-prestrained (undeformed) sheet metal are not correct or adequate for the mechanical strength calculations of body-in-white of a car and also for the crashworthiness calculations. The hardness value of base metal is increased by prior deformation. Although the region which is near base metal (subcritical heat affected zone) has lower hardness values, the region near fusion zone has higher hardness values due to the formation of martensite near fusion zone.