Welding Difficulties in Tungsten and Molybdenum Materials and Solutions

When a tungsten-molybdenum part fails at a joint, the cause is usually not insufficient weld strength but cracking of the joint during cooling or subsequent heating. Oxygen, carbon and hydrogen ingress, together with grain coarsening, often plant the problem at the moment of welding.

Three Categories of Difficulty

The first is oxidation. Molybdenum is sensitive to oxygen at high temperature, and even a trace entering the molten pool forms oxides that weaken grain boundaries. The second is hot cracking. Molybdenum has limited ductility at room temperature, so the weld and heat-affected zone are subjected to tensile stress as they contract on cooling, and the coarse-grained zone is prone to cracking. The third is grain growth. The welding thermal cycle coarsens grains in the heat-affected zone rapidly, reducing ductility and toughness, and this region often becomes the weak link.

The influence of impurities is often underestimated. Oxygen in molybdenum is present at grain boundaries as oxides, while carbon may form carbides; both change behavior at high temperature. Pre-weld cleaning must remove surface contamination and also avoid carbon- or sulfur-bearing solvents, so that fewer foreign elements enter the molten pool. Molybdenum weldments usually serve in vacuum or under a protective atmosphere, which raises the requirements for joint leak-tightness and cleanliness as well.

Applicable Scope of Several Joining Methods

A note on the applicable standards: electrode products are controlled for composition and tolerance under YS/T 1312-2019 Molybdenum Electrodes; molybdenum rods follow ASTM B387 / GB/T 17792, and molybdenum plates and discs follow ASTM B386 / GB/T 3876. Special requirements are executed according to the technical agreement between both parties.

When selecting a method, first establish whether the joint permits melting of the base metal, along with part size and vacuum conditions, then weigh batch size against tooling cost. Vacuum brazing places high demands on the furnace and fixtures, but it avoids oxidation in air. Diffusion bonding requires a coordinated pressure and temperature schedule, with fixture design accounting for a large share of the work. The differences between the processes are discussed further in the Process Technology section.

Several Steps in Process Control

1. Pre-weld cleaning: remove oil, oxide scale and machining residue, and move the part into welding as soon as possible after machining.
2. Shielding atmosphere: argon purity and flow rate must be matched, and trailing shields and back purging cannot be omitted.
3. Preheating and interpass temperature: preheating thick parts lowers the cooling rate and reduces cracking.
4. Post-weld treatment: stress-relief annealing or localized heat treatment, with temperature and time set by part type.
5. Ambient humidity: humid seasons are unfavorable for molybdenum welding, and dehumidification at the workstation helps.

Filler materials must match the base metal. For molybdenum parts, ordinary carbon steel wire is generally not recommended; for dissimilar joints, nickel-based or molybdenum-based materials may be considered, but the interfacial reaction has to be evaluated. In multi-pass welding, interpass cleaning and temperature control are equally important; if the oxide scale from the previous pass is not removed, slag inclusions easily form in the next pass.

How to Inspect the Joint

Visual inspection looks for cracks, undercut and lack of fusion; internal defects are found by radiography or ultrasonic testing; sealed parts undergo leak testing; critical parts can be sectioned for metallography to examine grains and the interfacial reaction layer. Brazed joints also require attention to filler metal spread and residual flux inclusions. The inspection ratio and acceptance level should be specified in the drawing or technical agreement, so that they are not negotiated after completion. Evaluating the material combination together with the service temperature at the joint design stage reduces later rework. Where options are to be compared for joining complete assemblies, similar service conditions can be found in Industry Solutions.