Abstract
A quantitative study has been made of the effects of welding and material variables on porosity formation in the tungsten inert gas arc welding of copper. It has been confirmed that porosity can result from the occurrence of the steam reaction in the weldpool. The extent of this porosity is increased by increasing water vapour content of the arc atmosphere, increasing oxygen content of the base material, and decreasing welding speed. All three factors interact so that the threshold porosity levelfor eachfactor is strongly influenced by the value of the other two factors. The steam reaction has been investigated during arc melting by determining the oxygen and hydrogen contents of copper buttons which have reacted with argon–water vapour atmospheres. It is shown that hydrogen ispreferentially absorbed from the arc following dissociation of the water vapour. The final levels of the oxygen and hydrogen in the button correspond to the equilibrium for the steam reaction in a high-temperature zone under the arc. Preferential absorption of hydrogen leads to very low levels of oxygen in the button and results in deoxidation of high-oxygen buttons such as electrolytic tough-pitch copper. A model is proposed for the steam reaction in the arc melting of copper in argon–water vapour atmospheres. This is used to account for steam porosity in arc welding and to explain the effects of the major factors. The implications of the investigationfor weldingpractice are considered and some recommendations made.
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