2000 Volume 18 Issue 4 Pages 519-526
JIS A6061P T651 (Al-Mg-Si alloy) was electron-beam-welded without filler metal and with Al-Si filler metal. Composition of crack, segregation and eutectic melting in welds was examined to clear mechanism of weld crack formation. Si content and its distribution in weld metal were investigated to find out effective Si content for prevention of weld crack. Obtained conclusion are summarized as follows :
(1) Al, Si and Mg are recognized in the boundary of dendrite of weld metal and in eutectic melting of HAZ from result of qualitative analysis of EPMA. It is presumed that these elements form Al-Mg2Si eutectic alloy when filler metal is not used. On the other hand, it is also presumed that these ones form Al-Mg2Si-Si eutectic alloy in boundary of dendrite of weld metal and Al-Mg2Si eutectic alloy in eutectic melting when filler metal is used.
(2) It is furthermore presumed that solidification crack of weld metal occurrs due to formation of Al-Mg-Si eutectic alloy and that liquation crack in HAZ occurrs due to formation of eutectic alloy of Al and Mg2Si in base metal. Crack is hard to occur because Al-Mg-Si eutectic alloy in weld metal fully fills in crack when this eutectic alloy of lower melting point exists in larger amount.
(3) When depth of penetration exceeds height of filler metal, Si content of weld metal decreases suddenly in root area. And its standard deviation increases with its meanvalue increase. However, standard deviation decreases as the ratio of penetration depth to filler metal height approaches one.
(4) Crack dose not occur when Si content of surface weld layer exceeds 1.6% and content of root weld layer exceeds 2.7%. As far as liquation crack in HAZ is concerned, it decreases with increase of Si content of weld metal. It also disappears when Si content of weld metal exceeds 4.8%.