Continuous resistivity fluctuation of rolled Al-Mg-Si-Cu alloy sheets

Abstract

To study the bake-hardening behavior of Cu added Al-1%Mg₂Si alloys by the tensile test after resistivity measurement, resistivity of many samples taken from a rolled sheet was measured in various stages of heat treatment. The resistivity ρ^STQ in the 848 K-3.6 ks solution treated and water quenched state (STQ) fluctuated within ranges of 0.9 to 1.7 nΩm. There were good linear relations between the ρ^STQ and resistivity after room temperature pre-aging for 1 day (ρ^RTP) for each of four alloys. The ρ^RTP values were maintained after short period final aging at 448 K. Possible causes of the fluctuation in ρ were checked. Effects of the STQ conditions on the ρ^STQ were negligible. Because the Matthiessen plot of the ρ^RTP gave good straight lines having acceptable gradient, the fluctuation is not due to errors in size factor and resistance. Change in room temperature showed no correlation with increment up to the ρ^RTP. Therefore, it is concluded that the ρ^STQ of rolled plates used in a series of study have a stripe shaped distribution of the ρ^STQ ranging ±0.25 to 0.5 nΩm in the transverse direction with width of 10 to 20 mm extending about 360 mm in the longitudinal direction. In a rough estimation, this fluctuation in ρ is corresponding to variation in Mg, Si or Cu contents of ±0.05 to 0.15 mass% from analysed compositions, though the species of alloying elements is not clear.