Sequential Design, Statistically Informed Multi-Objective Decision-Making, and Multi-Scale Quality Evaluation of Resistance Spot Welding Between Al-Si-Coated B1500HS and HC340/590DP Steels

Dissimilar resistance spot welding of Al-Si-coated B1500HS hot-stamped steel to HC340/590DP dual-phase steel suffers from a narrow process window and HAZ temper softening. A sequential orthogonal-central composite design strategy screened factors and constructed local second-order models for nugget diameter and tensile-shear force. Because the complete tensile-shear CCD dataset is unavailable for independent verification, the tensile-shear model is used strictly as an auxiliary local calibration and is not assigned the same validation level as the nugget-diameter model. Within-batch ANOVA showed that electrode force dominated diameter variation and first-pulse current dominated force variation. A model-assisted variance-aware compromise (7.8/8.5 kA, 2.9 kN, 13/17 cycles) was point-wise validated at 6.5065 ± 0.1366 mm and 15.053 ± 0.1899 kN (n = 20, CV 2.10%/1.26%). The measured performance-optimal orthogonal condition remained Run 11; thus, the compromise is interpreted as a stability-oriented choice rather than a global optimum. A joint-specific HAZ screening envelope (width < 0.7 mm; hardness loss < 50%) is proposed as a descriptive screening criterion only; because HAZ width and microhardness were not measured for the n = 20 validation condition, the envelope was not validated on that condition and remains conditional on the single-factor HAZ data. The framework integrates process optimization with transparent statistical qualification and reports its model calibration limits.