Dilution in single pass arc welds
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I.
INTRODUCTION
THE weld deposit which develops during fusion welding of two dissimilar alloys will attain a chemical composition intermediate to the two alloys. The final deposit composition will depend on the individual compositions of the materials and the degree of mixing between the alloys. Figure 1 shows this schematically in the simplest form, where a single pass of a filler alloy is deposited onto a substrate of a different composition. The degree of mixing for the simple case illustrated is defined by the percentage dilution, D: As • 100 As+As,,
Pct D = ~
[1]
where A~ is the melted cross-sectional area of the substrate and Asm is the cross-sectional area of the deposited filler metal. The weld deposit composition can be determined if the dilution is known along with the filler metal and substrate compositions. This approach assumes sufficient mixing occurs in the liquid state so that composition gradients are not formed in the weld deposit. Previous worklt.21has shown that concentration gradients in dissimilar metal welds typically exist only near the fusion line over distances on the order of a few hundred microns. (This discussion does not deal with the relatively small scale concentration gradients in the weld metal which result from solidification segregation.) We provide some additional evidence which supports this claim in a later section. In cases where concentration gradients are insignificant, Eq. [I] can be used to determine the weld deposit composition accurately. Even when composition gradients exist over the entire deposit, knowledge of the dilution permits an estimation of the weld metal composition. Last, it should be noted that Eq. [ 1] assumes that losses by evaporation are negligible, which may not always be the case for alloying elements which have a high vapor pressure (e.g., Mg in A1 alloyst31). Control of dilution can be important in dissimilar metal
J.N. DuPONT, Associate Research Scientist, and A.R. MARDER, Professor, are with the Department of Materials Science and Engineering, Lehigh University, Bethlehem, PA 18015. Manuscript submitted October 10, 1995. METALLURGICAL AND MATERIALS TRANSACTIONS B
welding applications. In cladding, where a corrosion resistant filler metal is deposited on a less corrosion resistant material, a low dilution is typically desirable. With a low dilution value, the final deposit composition is close to that of the filler metal and the corrosion resistance of the cladding is maintained. Dilution is also important to avoid undesirable phase formation, most notably in joining of ferritic steels to austenitic stainless steels.C41 Despite the importance of dilution, relatively little work has been published which defines, in a quantitative manner, the role of material properties and processing parameters on dilution. ChandeltS] derived empirical equations for A~ and Aj,, as a function of the welding parameters in order to predict dilution of bead-on-plate welds on carbon steel deposited by the submerged arc welding (SAW) process. Dilution values from 2
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