MECHANICAL CHARACTERISATION OF 6082 Al WELDS BY FIMEC TEST

摘要

The mechanical strength of Al-Mg-Si alloys, which are widely employed in welded joints, are enhanced by ageing after solution heat treatment. The sequence of precipitation is well known (1-3). After welding softening occurs in the welded zone (WZ) and in the heat affected zone (HAZ) of joints (4-6). Overageing in the HAZ induces modifications of precipitate morphology (7) and liquation cracking has been also observed for particular compositions of the filler metal (8-9). When it is possible to perform a post-welding treatment, mechanical properties of joints may be improved by ageing (10).In previous works (11-12) the structural evolution of 6082 Al joints obtained by gas metal arc welding (GMAW) has been investigated by SEM observations and by Vickers microhardness, tensile, Charpy and fatigue tests. The present paper describes results of FIMEC (Flat Top Cylinder Indenter for Mechanical Characterisation) test, which permitted to achieve local mechanical properties in WZ, HAZ and base metal on as-welded and post-welding T6 treated samples.FIMEC is a non destructive penetration test employing a cylindrical WC punch with flat head (radius r = 0.5 mm, height = 1.5 mm). Applied force F and penetration depth #delta# are measured step by step during the test. The advancement speed of the punch is maintained strictly constant by a feed-back system. Under the assumption that the force F is uniformly distributed on the sample-punch contact area, the curve applied load ( q = F/ (#pi#r~2)) vs. #delta# is obtained. The trend of the q-#delta# curve has been extensively described and discussed in refs. (13-15): an initial linear stage is followed by a work-hardening stage with loads tending to a saturation value q_s. The load q_y is the limit between first and second stage. In certain standard conditions (penetration rate in FIMEC of 0.1 mm/min and deformation rate in tensile tests of 10~(-3) s~(-1)) #sigma#_y approx = q_y /3 and #sigma#_R approx = q_S/3 (being #sigma#_y and #sigma#_R yield stress and ultimate tensile strength respectively). The given relationships have been assessed for a large number of materials (15). The investigated material is the T6-6082 Al alloy. As shown in fig. 1, round and elongated particles, identified by microanalysis EDS as (Fe,Mn)_3SiAl_(12), are present in the Al matrix. The size of (Fe,Mn)_3SiAl_(12) particles is in the range 1-10 #mu#m. Finer particles (0.1-0.5 #mu#m) of Mg_2Si not completely dissolved during the solution treatment, and micropores produced.