Joining Processes for Molybdenum and Dissimilar Metals

When molybdenum is welded to stainless steel and cracks appear near the joint after cooling, the problem is often not the welding technique. The linear expansion coefficients of the two materials differ several-fold, and as the temperature falls the interface is pulled into a highly stressed state.

Where the Mismatch Comes From

Molybdenum has a linear expansion coefficient on the order of 5×10⁻⁶/K near room temperature, austenitic stainless steel is several times that, and copper and nickel-based alloys lie between the two. On heating both expand together; on cooling the amount of contraction differs, so the joint interface is subjected to shear and tension. The larger the dimensions and the more severe the temperature difference, the more pronounced the stress. Molybdenum has limited ductility at room temperature, and cracks form readily where stress concentrates. Design points for dissimilar material combinations are discussed for specific operating conditions in Industry Solutions.

The choice of brazing filler metal depends on the service temperature and the joint requirements. Silver-based filler metals wet well but have limited high-temperature strength; nickel-based filler metals withstand higher temperatures but also require higher brazing temperatures, and their effect on the base metal grain structure must be assessed.

Interfacial Reactions Cannot Be Ignored

Molybdenum forms intermetallic compounds with certain metals at high temperature; such phases are usually hard and brittle, and as their thickness grows the joint toughness falls. Copper and molybdenum have low mutual solubility, so the interface is relatively stable; molybdenum and nickel react noticeably at high temperature, so temperature and time must be controlled. Elements in the filler metal may also react with molybdenum, so selection should consider not only melting point but also wettability and reaction products.

Metallizing or plating the molybdenum surface first can improve wettability, but the bond between the coating and the molybdenum, and the diffusion of the coating at high temperature, both require verification; room-temperature appearance alone is not enough.

Choosing Among Three Joining Routes

Supplementary note on the basis: electrode products are controlled for composition and tolerances per YS/T 1312-2019 Molybdenum Electrodes; molybdenum rods per ASTM B387 / GB/T 17792; molybdenum plates and discs per ASTM B386 / GB/T 3876; special requirements per the technical agreement between the two parties.

Brazing can relieve the mismatch through an interlayer, for example by choosing a more ductile filler metal or placing a transition metal at the interface. Diffusion welding treats temperature, pressure and time as three variables, and the thermal expansion of the tooling must also be taken into account. When bolts and pressure plates are used for a mechanical connection, the brittleness of molybdenum and the difficulty of thread machining must be considered, and a sleeve is usually fitted.

Avoid forced correction during assembly. Molybdenum parts develop microcracks easily under clamping and press-fitting; the tightening sequence and torque must be uniform, and gasket materials must match the service temperature.

Coordinating Structure and Inspection

1. Reduce rigid constraint: use a flexible transition section or place the joint in a low-stress zone.
2. Control heating and cooling rates: slow cooling lowers transient stress.
3. Provide an interlayer: absorb part of the mismatch through plastic deformation.
4. Post-weld inspection: leak test, ultrasonic or radiographic examination; for critical parts, examine the interface metallographically.

For locations that must be dismantled repeatedly, a mechanical connection with elastic elements adapts to thermal cycling better than rigid clamping. Joint geometry should be as symmetrical as possible so that contraction stress is distributed evenly across both sides and concentration on one side is reduced. Verify part dimensions and coaxiality before assembly.

The actual service life of the joint also depends on service temperature and the number of cycles; passing inspection at room temperature does not mean it is reliable under high-temperature conditions, so thermal cycling tests should be added where necessary. Similar cases for related application combinations can be found in the Applications section.