Effect of welding variables and solidification substructure on weld metal porosity
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I.
INTRODUCTION
W E L D metal porosity is the most common defect for all welding processes. Therefore, control of porosity formation and the effect of porosity on weldment properties has been an area of great research interest. Porosity is defined as cavity-type discontinuities formed by gas entrapment during welding. In terms of size, shape, and location, porosity can be described as interdendritic porosity and bulk pores, tl,21 Interdendritic porosity occurs when gas bubbles are formed or entrapped between dendrite arms in the solidification substructure, whereas bulk pores are the spherical pores that result from supersaturation of gases in the weld pool. Therefore, the causes of porosity in weld metal are dependent upon both the J.E. RAMIREZ, Graduate Student, and S. LIU, Associate Professor, are with the Department of Metallurgical and Materials Engineering, Center for Welding and Joining Research, Colorado School of Mines, Golden, CO 80401. B. HAN, Principal Researcher, is with Samsung Heavy Industries, Daeduk Science Town, South Korea. Manuscript submitted November 15, 1993.
METALLURGICAL AND MATERIALS TRANSACTIONS A
amount of dissolved gases and the welding process variables. There are several methods that can prevent pore formation in welded joints. The most important methods, even though less investigated, are the metallurgical methods, which attempt to minimize porosity by means of controlling the solidification substructures through welding variables. Therefore, this research studies the effect of welding variables and solidification substructure on the size and distribution of porosity in weldments, as well as the ability of pores to detach from the solid/liquid interface and move to the weld pool surface.
II.
THEORETICAL BACKGROUND
The formation of gas pores in a weld metal is a nucleation and growth process. The nucleation of pores occur either homogeneously or heterogeneously in molten metal supersaturated with gas. The energy barrier for porosity formation is determined by the surface tensions of the solid/liquid, gas/solid, and gas/liquid interfaces and by
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the degree of gas saturation in the liquid. Equation [1] gives the ratio of heterogeneous to homogeneous energy barrier for nucleation and is graphically plotted in Figure 1. AEh~, (2 + 3 cos 0 - cos30) - AE.omo 4
[1]
In the case of wetting of liquid on the solid surface (0 < 1r/2), the energy required for heterogeneous nucleation of a gas pore is only slightly smaller than that of homogeneous nucleation. However, in the case of poor wetting (w/2 < 0 < 70, the ratio of energies decreases and heterogeneous nucleation becomes favored. At the solid/liquid interface, there is good wettability of the solidified weld metal by the molten weld metal. Additionally, because of solute atom partitioning and local supersaturation, the liquid adjacent to the advancing solid/ liquid interface is enriched of the gaseous element. These two factors increase the probability that the solid/liquid interface and the liquid i
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