Welding (9/27/21)

Occasionally, an engineer needs something welded. One of the first questions a welder asks is the material, the gauge (thickness), weld type and application. Then they follow up with questions regarding heat treatment, time, and finally financials.

There’s 3 primary types of welding: TIG (Tungsten Inert Gas), MIG (Metal Inert Gas) which are both GMAW (Gas Metal Arc Welding) applications and there’s stick welding. Other more obscure types include spot welding, cold welding and friction welding with plenty of variations on each type depending on application. TIG is typically used often for aluminum and is the most difficult of the three. The first step to making a good weld is to nail the settings properly. When we weld at school we often look for a rainbow through the metal showing annealing and good penetration, meaning a strong bond was formed. The welder I spoke to last week mentioned that the strength of a good aluminum weld is about 22ksi (22,000 pounds per square inch) whereas heat treated welds can withstand around 35ksi. This was insightful for me because it showed me how much of a margin I have with my 700 psi requirements. I picked 700 because our maximum necessary pressure is 400psi tank pressure and a safety factor of 1.75. The reason this safety factor is so low is because a) the plumbing is configured to not allow pressure to reach above 400 at any time and b) we won’t be anywhere near the pressurized propellant tanks, nor will they be pressurized for more than a few minutes, assuming everything goes right...which it undoubtedly will not.

PWHT (Post Weld Heat Treatment) is the process of selectively heating a piece of welded material to near annealing temperatures for short but very specific periods of time to relieve the internal stresses as well which directly leads to better strength. This could be compared to pouring water onto some liquid nitrogen. The water would freeze nearly instantaneously, but it would freeze in a formation derived from the mechanics of the then falling water. PWHT would heat that water up to nearly melting point, allowing it to reform into a more compact and solid form. That is basically what happens when metals are heat treated.