Response-Specific Effects of Strontium and Copper–Tin Modification on Cutting Force and Surface Roughness in the Dry Turning of an Al-12Si Casting Alloy

Abstract Purpose Modification of Al-12Si casting alloys is usually treated as a single intervention, and cutting force and surface roughness are usually assumed to move together. This study asks whether the two additions used in modification act on the same machinability criteria, and whether the two criteria share a mechanism. Methods A 36-run full-factorial dataset from the dry turning of Etial 140 (Al-12Si) modified with Al10Sr, CuSn5 and Al10Sr+CuSn5 master alloys was analysed. The four groups form an embedded 2 × 2 design with respect to strontium (0.155 wt%) and copper–tin (1.05 wt% Cu), allowing the modification effect to be decomposed into orthogonal components. All inference is conducted at the experimental-run level. Bootstrap confidence intervals were used to bound null effects, and the contribution of built-up edge was assessed by conditional-association analysis using ordinal scoring of insert SEM images. Results The effects are response-specific. Strontium raises the cutting force by 44% (F(1,16) = 26.38; p < 0.001) with no detectable change in surface roughness (p = 0.65); the CuSn5 addition degrades surface roughness by 71% (F(1,16) = 182.52; p < 0.001) with no detectable change in cutting force (p = 0.94). Both null effects are bounded below half the magnitude of the opposing effect. Including built-up edge attenuates the CuSn5 effect on roughness by only 11%. Microscopy shows discrete angular second-phase particles in the CuSn5-bearing groups alone; their identity was not established. Conclusion Machinability cannot be summarised by a single criterion: cutting force and surface roughness respond to different additions, so alloy selection must follow from which criterion takes priority.