||With the introduction of advanced high strength steels (AHSS), welding has become more difficult to model and execute, as the material data is more complex than the more common materials in use today. Especially car manufactures are interested, as they would utilise the materials in the supports that make the chassis, which is welded together by resistance welding.
Three sheet spot welding of advanced high strength steel studies the option of welding a thin sheet of mild steel with two thicker sheets of AHSS. Industry wise the thin mild steel could be the sheets covering the car body.
Initially a simulation study was conducted using SORPAS so establish welding parameters for experimental welding of seven different material combinations. The three first were a combination of DC06(0.6mm), Docol350yp(0.8mm), DP600(1.5mm) and TRIP700(1.2mm): DC06-Docol350yp-DP600, DC06-Docol350yp-TRIP700 and a comparison DC06-Docol350yp-DP600/TRIP700. The last four were a combination of DC06, DP600 and TRIP700: DC06-DP600-TRIP700, DC06-TRIP700-DP600, DC06-TRIP700-TRIP700 and DC06-DP600-DP600.
Based on the simulation results and with the use of 2k Factorial Design, the weld experiments were planned and executed. The test welds were then shear tested to find the weld strength for later comparison and analysis
To analyse the experimental results a variance analysis was used to identify key parameters influence on the weld strength. A comparison of the measured IRMS value and weld strength together with splash was also conducted.
It was found that changing the lower electrode size from a smaller type to a larger had positive effect on the weld strength for DC06-Docol350yp-DP600. Alternately it had no effect with the DC06-Docol350yp-TRIP700 combination. For the DC06-Docol350yp-DP600/TRIP700 comparison test, splash occurred with TRIP but not with DP. This shows that the TRIP weldability range is narrow compared to the DP. Changing the material from TRIP to DP showed a clear connection between weld time and sheet thickness. In all of the tests the DC06 was successfully welded to the two sheets of AHSS.
For the last four material combinations, TRIP showed to have a detrimental effect on the welding when in contact with the DC06. The combinations: DC06-TRIP700-DP600 and DC06-TRIP700-TRIP700 were highly unstable, with no welding of the DC06 occurring at certain points. DC06-DP600-TRIP700 was more stable and generating the highest average weld strength of all the tests. The combination DC06-DP600-DP600 was close to completely stable in stark contrast to the tests involving TRIP. The problems with the TRIP are two fold: One is the fact that TRIP is coated; this slows down the nugget growth and adds some instability to the experiment. The second problem is TRIP it self, as TRIP has a very narrow window of weldability and does not take much to either hit splash or not get any weld.