We are expieriencing what appears to be liquation cracking in the HAZ of the base metal. A 0.110" wall tubular extrusion is being joined to a 0.030" wall drawn tube as a 'T' joint. The drawn tube forms the vertical and the extrusion forms the horizontal member of the 'T'. The drawn tube is coped 'fish mouth' style to fit the extrusion. The weld is a fillet type. The cracking is occurring in the 0.110" extrusion on the opposite side from the weld. Does grain size in the extrusion play an important role? If so, what would be an acceptable range for grain size. Is there any doccumentation regarding grain size and crack sensitivity in 6061 aluminum?

Thanks,
Perry