An investigation of heat flow, precipitate formation and grain growth in the HAZ of aluminium alloy weldments

Abstract

The modified Rosenthal's equation for heat flow during welding is shown to qualitatively predict the temperature distribution in the heat-affected zone during welding of aluminium alloy (AA) 2024. The effects of heat input and welding speed on the thermal cycles have been investigated. There is loss of hardness in the heataffected zone (HAZ) that is attributed to the effect of welding. Welding introduces micro-imperfections, which lead to reduced strength in the HAZ and fusion zone of the Weldment. These microstructural changes are of concern, because precipitation hardened aluminium alloys attain their strength through precipitate hardening-a delicate heat treatment process involving control of time and heating/cooling rates. The same degree of control cannot be achieved during,welding. Detailed information about the physical mechanisms involved is established by direct experimental observation. The experimental work involved in situ thermocouple measurements of the temperature-time pattern in single pass butt-welded panels of AA 2024 in T3 condition using gas tungsten arc welding GTAW). Vickers microhardness measurements, optical and transmission Electron Microscopy (TEM) examinations of characteristic HAZ microstructures were performed. The temperature in the HAZ is high enough to cause dissolution of the major metastable strengthening precipitate 8 which resulted in softening of this zone and hence a concomitant degradation in hardness. Upon welding the alloy shows 8 precipitate coarsening coupled with the formation of other plate or lath-like precipitates in the HAZ. Grain growth is pronounced in grains immediately adjacent to the fusion line and extends up to 10 mm from the fusion line. Welding processes having characteristically high values of qN (heat-input rate) generate coarse grains in the HAZ. In general, the hardness distribution in precipitation hardened aluminium alloys depends on the interplay between two competing processes (i. e. dissolution and re-precipitation). This study evaluates the effects of welding on microstructural evolution (i.e. changes in grain size/structure) in the HAZ of precipitation hardened AA 2024 in the T3 condition. Such effects will interest designers and manufacturers using or intending to use these alloys where the effects of welding are important design considerations. This is vital in identifying tolerant welding procedures designed to produce HAZ refmement and improve mechanical properties of welded joints. The ability to examine the effects of weld-parameter variations on the overall joint structure can be used as a design tool for weld-procedure optimization in a wide range of circumstances. The detailed examination of the welding phenqmena provides a basis for developing .- mathematical models and hence computer simulations that would be developed for advanced materials processing technologies.