Since many mechanics don’t have the TIG welding equipment, expertise, or comfort level to do an exhaust system repair, sending it out for repair is a common practice. There are repair stations like AWI that specialize in exhaust system repairs. As an alternative, some mechanics choose to take the exhaust part to their local welding shop to have it repaired. If you are having a local shop do the repair or if you are tackling the repair yourself, there are several things you need to know to help ensure you get a good repair.
Alignment. Proper alignment is important when repairing an exhaust system component. Most repairs need to be done in a jig in order for the component to fit properly during re-installation. Not using a jig can cause improper alignment, setting up stress after installation that can damage the part.
Lack of experience. Many welders don’t have experience working with aircraft exhaust systems. Exhaust systems are comprised mostly of 321 stainless or 601 or 625 inconel. There aren’t many other things that are made out of these alloys, and most general welders don’t have the experience of working with them. Even if the welder has welded stainless before, it is not the same as 321 stainless, which requires a specific rod and techniques. Using the wrong rods coupled with wrong procedures will result in a weak joint.
Proper cleaning. Thorough cleaning of the part is critical. The outside of an exhaust system component is typically dirty with oil and other deposits on it. But just cleaning the exterior of the part is inadequate. The inside of the part is full of carbon deposits left behind from burnt fuel and fuel additives. As soon as you start welding, the crack opens up from the heat and that contamination from inside the part is pulled right through into the weld puddle creating a weak weld. So the part needs to be thoroughly cleaned inside and out before welding.
Purging. A final tip for welding is to ensure the part is purged when welding. Purging is the process of providing a separate source of argon to the inside of the part. This pushes all of the air out of the part and creates a pure argon atmosphere inside the part. This pure argon atmosphere helps pull the weld puddle through the crack during the welding process, and the resulting weld is as clean on the inside as it is on the outside. Not purging will cause oxidation of the weld puddle on the inside, creating a rough, jagged bead. Not only is this a weaker weld joint, but the jagged edges will disrupt the gas flow creating hot spots that will set up spots for future failure.
Stainless vs. inconel
Another thing that mechanics need to be aware of is that exhaust system components can be manufactured of either stainless or inconel. These two materials are similar in appearance and can be difficult to differentiate without chemical tests or destructive (grinding) analysis. It is important to realize that these two materials have different characteristics that affect the visual indications of a pending failure.
Over time, stainless steel tends to deteriorate. The molecules of the metal start to break down and the metal starts to stretch, bulge, and deform. This is a good visual indication that the part is close to failure, and requires repair or replacement.
Because of this susceptibility of stainless to weakening and bulging over time, some engineers decided to go with a stronger metal that could withstand the heat and prevent these bulging failures. They chose inconel, a metal in the stainless-steel family that has more nickel and chrome in it, allowing it to withstand higher heat. Typically, inconel does not bulge and deform like stainless does. But what it does do is pit out from the inside. The metal properties of inconel aren’t very compatible with the mineral deposits that are left behind in AvGas. This causes severe pitting, almost like a cancer. Heid notes that 95 to 98 percent of the inconel parts that come through their door are severely pitted out.
The bottom line is that as a mechanic you may not know what material you are inspecting. Just because there are no cracks or bulges on the system, doesn’t mean that there aren’t any defects. You could be looking at an inconel part that is pitting from the inside.
Proper exhaust system inspection is critical to safe operation. By understanding inspection procedures and ensuring your parts are being properly repaired, you can do your part to ensure the aircraft you are maintaining doesn’t experience an exhaust system failure.
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