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Engineering
Materials and Chemical Engineering
Why weld joints are stronger than the base material itself?
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[QUOTE="Baluncore, post: 6200130, member: 447632"] Energy propagating as noise waves along the tubes is reflected from the lug or weld discontinuities. That increases the local stress at a stepped junction as multiple waves are present in the tube. That is why gentle tapers are important in the welds. Notice the shape of the old lug joints that were silver soldered to the tubes, there were external tangs that coupled energy into the tube over some distance. That prevented step reflections and concentrations of forces. Surrounding every weld in the base metal is a “Heat Affected Zone”. Cracks may form in that zone which weaken the parent material. To prevent that cracking, the joint should first be preheated, then after the weld is placed, allowed to cool slowly in the oven, or under a blanket. Some removal of material from the tube surface may take place adjacent to a weld. Unfortunately, that reduces tube thickness at the point where it needs to be thickened. The same thinning of the tube can occur if the area of the weld is mechanically ground to taper the profile. Select a welding method that does not reduce tube thickness, and one that does not require mechanical finishing that may damage the tube surface. There was a time when joining tubes with lugs and silver solder achieved the best results. Working in a dark environment made it possible to monitor the solder penetration into the joints between the lug and tube. The style of the lug and the meniscus of the solder formed the taper. Some aluminium alloys will work harden. Be sure to select an aircraft grade tube that is specified for TIG welding, if that is the procedure you will use. The choice of filler rod is important. You might cut your own rods from the same alloy. Practice your welding on cheap tube. Then destructively test the weldment to identify weaknesses. Repeat the trying, testing, learning process cycle until you can get consistently good results. Only then should you weld your frame. [/QUOTE]
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Why weld joints are stronger than the base material itself?
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