Why good parts go bad
A quality engineer recently asked us why parts that have been welded successfully for several production lots sometimes suddenly develop weld quality problems. The types of problems he encountered included defects such as inconsistent weld line (shown above), cracking, spitting, pin holes, and failed X-ray or leak check tests. The engineer was looking for a quick solution as his company struggled to decrease rework and scrap costs. If the welding equipment is operating correctly, then there are three likely culprits creating the majority of problems: Contamination, Fit, and Material.
Keep it Clean
Contamination involves any foreign material that is in the area of the weld joint. When the weld pool touches this material it vaporizes instantly, sometimes explosively causing metal to erupt out of the joint. The contaminate can also melt into the weld pool, thereby, changing the alloy of the metal at the area of the joint. Contamination comes from many different sources including cutting fluid, metal chips, oil, saw dust, rubber O-rings, sealant, or even sealing gel from leak checking. Oxidation and rust can also contaminate the weld pool. Proper cleaning methods and procedures are the best way to eliminate problems from contamination.
Precision Fit = Precision Weld
Energy beam welding generally works best when the parts have a tight fit. Fit is important because the energy beam is very small and concentrated and does not rely on a rod or filler material to transfer heat to the joint such as the case with TIG or MIG welding. If the beam misses the part slightly, the energy applied to the weld pool will change. Air gaps behind the joint, such as from a chamfer, can also cause problems as hot gases try to escape the joint through the weld pool. Fit problems generally occur due to variations in machining processes from one lot of material to the next. Designing and manufacturing parts with proper geometric tolerances should help minimize the chances of fit problems.
Not All Material Lots Are Created Equal
Welding characteristics can differ significantly from one alloy of a metal to another. Welding problems can arise because the metal alloy in one lot differs from another. This can happen for a number or reasons including errors in material sourcing, changes in prior processes such as heat treatment, or simple human error. Welding filler material is one area in particular to watch. This material is often provided in wire form and is easily mixed up with other similar looking wires. It is very easy for a welding operator to mistakenly grab the wrong piece of material. For this reason, many companies, especially in the aerospace and defense industries, adopt exceptionally tight controls for material lot tracking, and particularly for welding filler material.