Dissimilar Metal Joining: Challenges and Solutions

## Why Dissimilar Joints Are Difficult When two different metals are fusion-welded, the weld pool is a mixture of both base metals plus the filler. If the resulting composition falls in a brittle region of the relevant phase diagram, the joint will crack during cooling or in service. Three fundamental problems arise: **Brittle intermetallic compounds**: Many metal pairs form intermetallic phases that are hard but extremely brittle. Iron and aluminum form FeAl₃ and Fe₂Al₅ layers at their interface during fusion welding. These compounds have fracture toughness values below 5 MPa√m, so even a 10 μm layer can cause joint failure under thermal cycling. This is why steel-to-aluminum fusion welds almost always fail. **Thermal expansion mismatch**: Austenitic stainless steel (17.3 × 10⁻⁶/°C) joined to carbon steel (11.7 × 10⁻⁶/°C) creates differential contraction stresses during cooling. In high-temperature service, thermal cycling fatigues the joint at the fusion line where these stresses concentrate. **Carbon migration**: When austenitic stainless steel is welded directly to carbon steel and the joint operates above 400 °C, carbon migrates from the carbon steel (higher carbon activity) into the stainless steel. The carbon steel side becomes decarburized and weakened; the stainless side forms chromium carbides and becomes sensitized. This mechanism has caused failures in power plant dissimilar metal welds after years of service. ## Filler Metal Strategies The correct filler metal is the most critical decision in dissimilar welding. The goal is to select a filler whose diluted composition with both base metals remains in a ductile phase field. **Nickel-based fillers** (ERNiCr-3, ENiCrFe-3, Inconel 82/182) are the standard choice for carbon steel to stainless steel joints. Nickel is fully soluble in both iron and chromium, avoids martensite formation in the mixed zone, and resists carbon migration better than stainless fillers. ERNiCr-3 (Inconel 82) has been the workhorse filler for nuclear and fossil power plant dissimilar metal welds for decades. **Austenitic stainless fillers** (ER309L, E309L) are used for carbon steel to stainless steel joints in non-critical, lower-temperature service. The high chromium and nickel content of 309L produces a weld deposit that tolerates significant dilution from the carbon steel side without forming martensite. However, 309L is inferior to nickel fillers for high-temperature service due to carbon migration susceptibility. | Joint Combination | Recommended Filler | Notes | |-------------------|-------------------|-------| | Carbon steel to 304/316 SS | ERNiCr-3 or ER309L | ERNiCr-3 for elevated temp | | Carbon steel to Inconel 625 | ERNiCrMo-3 | Matches Inconel side | | 304 SS to Inconel 718 | ERNiCrMo-3 | Avoids Laves phase | | Copper to steel | ERCuSn-A (phosphor bronze) | Low dilution process | ## Buttering and Cladding For critical dissimilar joints, buttering applies one or more layers of a transition filler onto one base metal before making the final joint. This controls dilution and allows PWHT of the carbon steel side before the stainless or nickel alloy side is joined. In nuclear reactor pressure vessel nozzles, the carbon steel vessel is buttered with Alloy 52/152 (ERNiCrFe-7), stress-relieved at 620 °C, then welded to the stainless steel safe-end. This sequence prevents exposing the nickel alloy weld to the high PWHT temperature that could degrade its stress corrosion cracking resistance. ## Solid-State Joining Methods When fusion welding creates unacceptable intermetallic layers, solid-state processes that avoid melting offer solutions: **Friction welding** (rotary and linear): One part rotates against the stationary other under axial pressure. Frictional heat plasticizes a thin layer at the interface without melting. Aluminum-to-steel friction welds routinely achieve strengths exceeding 200 MPa because the FeAl intermetallic layer is limited to 1–2 μm thickness (vs. 50–100+ μm in fusion welds). Automotive drive shafts commonly use friction-welded aluminum tubes to steel end fittings. **Explosion welding**: A controlled detonation accelerates one plate onto another at velocities of 200–500 m/s, creating a metallurgical bond through a wavy interface that mechanically interlocks the two metals. This process bonds nearly any metal combination: titanium to steel, copper to aluminum, zirconium to steel. Explosion-welded transition joints are used in shipbuilding (steel hull to aluminum superstructure) and cryogenic equipment (stainless to aluminum). **Diffusion bonding**: Surfaces are held in intimate contact under moderate pressure (5–30 MPa) at elevated temperature (0.5–0.7 Tm) for extended time. Atoms diffuse across the interface to form a bond. Used for titanium aerospace structures and copper-steel bimetallic circuits. Requires very flat, clean surfaces and long cycle times. ## Design Considerations Dissimilar metal joints should be located away from high-stress regions and cyclic thermal gradients when possible. Transition pieces (explosion-welded bimetallic strips) allow the dissimilar joint to be made in a controlled factory environment and then same-metal field welds connect each end. This approach is standard in LNG piping (stainless to aluminum) and naval construction (steel to aluminum).