BUTT-WELDED ALUMINIUM JOINTS: A NUMERICAL STUDY OF THE HAZ EFFECT ON THE ULTIMATE TENSION STRENGTH

摘要

The use of aluminium alloys structural members represents nowdays a modern trend in structural applications exhibiting a lot of advantages. The corrosion resistance and the high strength to weight ratio are certain characteristics of such an alloy which are of great significance in the design of lightweight and transportable bridges, constructions in a marine environment, curtain-wall systems, etc. Although the strength of pure aluminium is very low for structural applications, the strength and other characteristics of aluminium are improved when it is alloyed with other elements, and its mechanical properties can be further improved as soon as heat treatment is applied. As a matter of fact the heat input in welds removes some of the beneficial effects due to the above mentioned treatments and leads to a decrease in the elastic limit f_(0.2) near the welding, whereas the ultimate limit strength f_u is not so hardly affected. The limited knowledge about the structural response of aluminium structural members exhibiting a nonlinear character and the weakening of the metal around welds, known as HAZ (Heat-Affected Zone) softening has induced hesitation about the use of aluminium alloys as a structural material. This effect is taken into account through the definition of a reduced-strength zone, which extends immediately around the weld, beyond which the strength properties rapidly recover to their fully unwelded values. The severity of this type of softening and the extent of this zone has to be in any case considered in the calculation and design of welded connections. The severity is largely a function of the parent material used, while the extent is affected by the control of temperature during fabrication; so it depends mostly upon the welding procedure (velocity, voltage, number of passes, thickness of the joint). In general, the extent of the HAZ is greater for TIG than for MIG welding procedure, both being the most commonly used welding techniques in aluminium connections. In addition, the TIG process is more dependent on the operator's technique, as compared to the semiautomatic MIG process. The most common method for the estimation of the extent of the HAZ is the 'one-inch rule' that assumes that the HAZ extends within the parent metal 25mm in all the directions around the weld and has been widely used since the decade of '60s. The EC9 relates the extent of the HAZ with the thickness of the connection members. In both methods, the subject of the extent of the HAZ is not based on exact scientific results. In many cases it has been found that the effect of the HAZ on the resistance is small enough, making the 'one-inch rule' quite acceptable. However, in other cases experimental tests have shown that the HAZ softening significantly reduces the resistance and therefore, economy can be made by the use of more refined methods. Such a method that has been proposed in order to estimate the extent of the HAZ in a more precise way is the RD Method, based on the classic heat flow equations of Rosenthal. In the present paper, a butt weld connection is numerically studied by means of the Finite Element Method (FEM). The HAZ is limited to a region in the vicinity of the weld where reduced mechanical properties are considered. The extent of the HAZ is a parameter that is determined using the 'one-inch rule', the EC9 and the RD Method. The obtained results are compared and discussed, showing that the extent of the HAZ does have a significant influence on the ultimate yield tension strength that the weld connection can sustain.