Welding of dissimilar metals

Welds of dissimilar metals result in (1) Warping, buckling, and/or excessive residual stresses caused by different thermal expansion coefQcients. (2) Hard spots in heat-affected zones and fusion zones are caused by formation of hard intennetallic compounds such as carbides. The heating/cooling cycle can harden HAZs and can cause a sensitized HAZ in nonstabilized alloys. 

The interleaving of the two flow paths proposed by Nunes is illustrated in the plan view showninFig. 3.12. Similar macrostructures have been observed in welds of dissimilar metals (Ref 23). The occurrence of stick-slide modes due to the interface between the weld tool and the work-piece would explain the origin of interleaving. The residue of the straight-through current metal flow predominates on the RS and the upper... 

E.E. Murr, Y. Li, R.D. Rores, E.A. Trillo, and J.C. McClure, Intercalation Vortices and Related Microstructural Features in the Friction-Stir Welding of Dissimilar Metals, Mater. Res. Innovat., Vol 2,1998, p150-163... 

BE-7301 Life optimization of dissimilar metal welds for high temperature components Mr. V. Bicego CISE SpA... 

In buried pipeline installations, avoid contact of piping with structures of dissimilar metals. Also, where possible, specify uniform quality, grade and surface conditions. Various quality sections should not be welded together in buried installations. 

The electrochemical examination of fusion joints between nine pairs of dissimilar metal couples in seawater showed that in most cases the HAZ was anodic to the weld metals" . Prasad Rao and Prasanna Kumarundertook electrochemical studies of austenitic stainless steel claddings to find that heat input and 5Fe content significantly affected the anodic polarisation behaviour under active corrosion conditions whilst Herbsleb and Stoffelo found that two-phased weld claddings of the 24Cr-13Ni type were susceptible to inter-granular attack (IGA) as a result of sensitisation after heat treatment at 600°C /pa was unaffected by heat input. 

Rowe, M.D., Nelson, T.W., and Lippold, J.C., Hydrogen-induced cracking along the fusion boundary of dissimilar metal welds, Welding Res., February, 1999, p. 31. 

A thermocouple consists of two wires of dissimilar metals or alloys insulated from each other by placing them in a two-channel porcelain tube. The wires are welded together at one end (the hot or measuring junction of the thermocouple)... 

M. Fukumoto, T. Yasui, Y. Shimoda, M. Tsubaki, and T. Shinoda, Butt Welding Between Dissimilar Metals by Friction Stirring, Proceedings of the Fifth International Conference on Friction Stir Welding, Sept 14-16, 2004 (Metz, France), TWl, paper on CD... 

Design (avoidance of dissimilar metals, galvanic couples, improper materials, high fluid velocities in inappropriate places, caulking or seal welding of areas prone to crevice corrosion, roof design, etc.)... 

Where permanent types are used, the parts themselves can be joined and narrow gaps filled with weld or brazing metal. Where separation is wider, a strip of metal can be welded or brazed at both ends across the gap. Bonds connecting one vibrating part to another part that may or may not vibrate should be of a flexible material that will not fail under vibration. Corrosion because of the joining of dissimilar metals may cause the electrical resistance across the bond to increase. This is especially noticeable in humid or corrosive atmospheres. The types of metal for the bond and its fastenings must therefore be selected with care. Grounds and bonds should... 

Corrosion control in recirculating water systems is always complicated by the presence of dissimilar metals in contact, crevices, and areas where an oxygen defidenqt exists (differential aeration), e.g. welds and joints. Unfortunately, protective paint schemes almost always break down in these areas and localized corrosion follows. 

An assembly of dissimilar metals in a design should be preselected on a well-balanced utilitarian basis with compatible affinity. Figure 9.7 shows how the excess insulation compound squeezed out of the joint, augmented with sealing compound if necessary, should be formed into sealing fillets. Welds and other points of high corrosion incidence in proximity should be included within the fillets. 

Possibility of galvanic corrosion when resistance welding some dissimilar metals. 

Source Adapted from Intermediate temperature joining of dissimilar metals, F. M. Hosking, J. J. Stephens, and J. A. Rejent, Welding Journal, 1999, April, pp. 127s-136s. (With permission.)... 

FlOiei Evaluation of techniques for assessing corrosion cracking In dissimilar metal welds Dr D.R. Tice AEA Technology... 

Ultrasonic Testing of Austenitic and Dissimilar Metal Welds,... 

Stainless steel develops a passive protective layer (<5-nm thick) of chromium oxide [1118-57-3] which must be maintained or permitted to rebuild after it is removed by product flow or cleaning. The passive layer may be removed by electric current flow across the surface as a result of dissinulat metals being in contact. The creation of an electrolytic cell with subsequent current flow and corrosion has to be avoided in constmction. Corrosion may occur in welds, between dissimilar materials, at points under stress, and in places where the passive layer is removed it may be caused by food material, residues, cleaning solutions, and bmshes on material surfaces (see CORROSION AND CORROSION CONTROL). 

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