Novel modelling approach to optimisation of welding conditions and heat treatment schedules for age hardening Al alloys

Abstract

In the present paper the general purpose finite element code WELDSIM, equipped with a new natural aging model, is used to calculate the minimum heat affected zone (HAZ) strength level σ _min and the equivalent half width of the reduced strength zone Δy^eq _red of strength σ _min during single pass butt welding of Al–Mg–Si alloys. In particular, it is illustrated how the resulting strength loss depends on the interplay between the base metal chemistry and the initial temper condition on the one hand and the net arc power q ₀, the welding speed v, the plate thickness d and the effective heat transfer coefficient h between the Al plate and the steel backing on the other hand. Assuming one-dimensional heat flow and pseudosteady state, the former parameters can be combined in a single group variable q ₀/vd, which uniquely defines the HAZ thermal programme when welding is performed without the use of steel backing. Taking this as the main variable controlling the strength loss, the simulation results can be condensed into two-dimensional process diagrams, showing the variation in the design parameters σ _min and Δy^eq _red with q ₀/vd for different combinations of h and d. The output data from WELDSIM are, in turn, used as inputs to mechanical models to predict the resulting design stress under different loading conditions. It is concluded that significant weight reductions and cost savings can be achieved by minimising the strength loss after welding, provided that the design parameters σ _min and Δy^eq _red are calculated on the basis of the actual yield strength profile within the weld HAZ, as obtained from WELDSIM.