Influence of Microstructure Adjacent of Grain Boundary on Intergranular Cracking in Al-Mg-Si Alloys

The Al-Mg-Si alloys with excess Si without copper addition has recently been found to cause intergranular cracking. In the present study, we have investigated the effects of microstructure adjacent to grain boundaries on the intergranular. Table 1 shows the composition of the specimens used in this study. It was confirmed that the alloy with excess Si without Cu addition (No. 7) had lower elongation to failure caused by higher fraction of intergranular fracture surface (Fig. 1) consisting of many dimples than balanced alloy (No. 4) and alloy with excess Si and Cu (No. 9). Fractographic observations on the pair of fractured test pieces revealed that a grain boundary precipitate was present at the bottom of a dimple in either piece. In the alloy with excess Si without Cu, difference in the number of grain boundary precipitates was found within the pair of the pieces (an example of this is indicated by a pair of circles in the figure), while equal number of precipitates were found in the pair in the other two alloys. Both the size of the precipitates and the amount of precipitation were larger in the alloy with excess Si without Cu than the other two alloys (Fig. 2). The larger amount of precipitation meant the larger width of PFZ in the alloy. Thus, the larger fraction of intergranular fracture was closely correlated with the larger size of grain boundary precipitates and wider PFZ (total volume of grain boundary precipitates per unit area), as shown in Fig. 3. Monolithic Si phase precipitate was not observed on the grain boundaries of the two alloys with excess Si.