materials cannot. These aluminum/ magnesium/silicon alloys are highly crack sensitive because they contain approximately 1 to 2% magnesium silicide (Mg2Si), which falls close to the peak of the solidification crack sensitivity curve — Fig. 2. This is why there are no 6xxx filler alloys. Fortunately, the arc welding process and addition of filler alloys dilutes the weld pool with enough magnesium (5xxx series Al-Mg filler alloys) or silicon (4xxx series Al-Si filler alloys), which reduces the cracking tendency to acceptable levels. A related problem occurs when welding 6xxx materials in a squarebutt configuration. For example, one fabricator working with 3⁄8-in.-thick 6061-T6 base material and 5356 filler material noticed that his GTA welds developed a centerline crack immediately after welding. Figure 3 illustrates that a square edge preparation does not allow sufficient amounts of the filler alloy to mix with the base material. As shown, a V-groove preparation introduces more filler alloy into the weld metal mixture, which increases the Mg content of the weld so that it is well beyond peak crack sensitivity. The Not-So-Cheap Boat Casting Repair Every boating enthusiast is aware that the little cast metal fin on the bottom of the outboardmotor’s lower unit (called a skeg) breaks frequently. Since a new lower unit can cost thousands of dollars, boat owners usually opt to replace the skeg. A quickie do-ityourself approach involves brazing on a new skeg using a “3-in-1” brazing rod and an oxyacetylene torch with a welding attachment. The flaw in this repair method is the composition of the brazing rod: 93% zinc, 4% aluminum, 3% copper, and 0.050% magnesium. Outboard motors incorporate a small zinc anode button that acts as a sacrificial corrosion point for the motor. When a zinc-based filler metal is used to repair the skeg, the welded joint also becomes a sacrificial anode that will quickly corrode. True, these welded joints are painted, but the paint lasts only as far as the nearest sandbar. To correctly repair a skeg, first identify the alloy, which is typically a 3xx series material. For example, some Mercury® engines use a 356 casting. In 44 WELDING JOURNAL / APRIL 2015 Fig. 2 — The 6xxx series of aluminum/magnesium/silicon alloys are highly crack sensitive because they contain about 1 to 2% magnesium silicide (Mg2Si). this case, use a 356 filler material, or alternatively a 4008 casting rod, and repair the skeg using a GTAW power source with an AC output. ‘Nonweldable’ Alloys Sometimes, there is no “correct” filler material. Such a situation arose when a fabricator asked us what welding wire he should use for welding a box made from 7075 that he planned to pressurize. We immediately advised him to scrap the project, as the problem with welding 7075 is stress corrosion cracking. It is a matter of when the welds will break, not if they will break. Stress corrosion cracking is particularly dangerous because it is often not immediately detectable right after welding. The cracking develops at a later date, often when the component is in service. What happens is that changes occur within the base material adjacent to the weld during welding. These changes produce a metallurgical Fig. 3 — Use a V-groove preparation on thick sections of 6xxx series to increase the Mg content of the weld so that it is well beyond peak crack sensitivity.
Welding Journal | April 2015
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