PorosityPorosity results when bubbles of gas are nucleated in the weld pool and trapped during solidification. The source of the gas may be a chemical reaction, such as the formation of carbon monoxide from carbon and oxygen, or the expulsion of gases which have been dissolved by the liquid metal but are relatively insoluble in the solid phase such as nitrogen in steel. In general, spherical cavities are formed, the stress intensification factor of individual pores is low and a large amount of porosity is required to cause a significant loss of joint cross section. In spite of these factors the acceptable level of porosity is often restricted in design codes and must be controlled. The problem is best controlled by eliminating the source of the pore generating gas. This may be achieved by effective shielding - the choice of an appropriate shielding gas, the use of the correct gas shroud, the prevention of leaks or ingress of air or moisture to gas lines, adequate pre-and post-surge provisions and removal of contaminates such as oil, grease or moisture.
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