I have a magnesium part that I want to reduce the residual stress after welding without heat treating. I want to see what are the ways to do this. A friend suggested using liquid nitrogen during tig welding. Is it possible to reduce the residual stress in this way and what are the ways in general? To what extent is the selection of the welding process effective in reducing this residual stress? I would be grateful if you could explain.
The greater the temperature gradient in the component during welding, the greater the residual stress. This means that welded components must be as hot as possible before welding. It would be optimal to weld on a component that is so hot that it is about to melt. In this case the residual stress would be zero. Of course, in practice the preheating of welded parts cannot be carried out to such a high level, but in any case it must be as high as possible. In this sense, welding with extreme cooling is pure nonsense. If preheating is not possible, the residual stress can be reduced by subsequent annealing.
Good day, Mehdi Moslemi
Minimizing residual stresses coused by TIG welding without the application of pre- or post-weld heat treatment is a challenge, and depends on the specific characteristics of the structure being welded. First of all, it is imperative to reduce clamping forces and permit unrestricted deformation of the structure during welding. For thin-walled structures, mechanical treatment such as shot peening can be used. Alternatively, employing a high-density current through the structure may serve to diminish the value of residual stresses.
In the case of smaller structures, application of low welding rate can be advantageous. It facilitates the gradual accumulation of heat within the structure, effectively simulating a preheating effect.
The suggestion of utilizing liquid nitrogen is rather unusual.
I doubt that you can weld the magnesium part while in liquid nitrogen due to the massive heat loss, but assuming you can, then you will end up with increased residual stresses due to the increased thermal gradient across the weld joint, as explained by Vadim Verlotski
Preheating is generally used to reduce cooling rate so as to prevent formation of undesirable micro-structure in the heat affected zone in steels. Since you are interested in residual stresses, one should first understand the nature of residual stresses. These are tensile stresses (up to the yield point) in weld zone and region near to it. In rest of the object these are compressive in nature but smaller in magnitude. Irrespective of the cooling rate this nature does not change and residual stresses in weld zone are tensile up to the yield point. What changes with the change in cooling rate is the spread of the region of tensile residual stresses, which is wider with low cooling rate and narrower with rapid cooling rate. What is generally implied by higher or lower residual stresses is the spread of this zone of tensile residual stresses. In this sense, rapid cooling leads to smaller residual stresses (i.e. less width of zone of tensile residual stresses) and because of this welding processes which lead to narrow arc and rapid cooling, such as Laser and Electron-beam welding, result in lower residual stresses (i.e. narrower zone of tensile residual stresses). One can see other welding processes and use of preheat etc. in the light of this. Besides, materials like Magnesium have other welding problems which have to be tackled and only feasible welding processes and welding parameters should be assessed with the above factors taken in to consideration for minimizing residual stresses ( i.e. zone of tensile residual stresses). Constraints (such as clamping or the restriction provided to weld contraction by the other parts of the structure) also affect a type of residual stresses, which are called reaction stresses.