Molybdenum is a formidable choice for high-temperature applications, due to its high-temperature strength, high hardness, high melting point, high electrical and thermal conductivity, high corrosion resistance and relatively low coefficient of thermal expansion. Moybdenum is commonly called out for use in aerospace and defense projects involving missiles, propulsion units and engines, and in applications involving nuclear fuel processing — really any application requiring great strength at high temperatures.
Molybdenum can withstand many acids, including sulfuric, hydrochloric, hydrofluoric, and most organic acids, as well as many molten materials, such as metals, alloys, glasses, and compounds. In fact, molybdenum is often added to stainless steel to improve its corrosion resistance. It is corroded by alkalis in the presence of oxidants.
Electron Beam Welding Molybdenum
Molybdenum has a very high melting point of 4,753°F (2,623°C) and is very sensitive to gas impurities in the welding process. Hence, Electron Beam welding, which typically requires a vacuum, is a natural fit for welding applications involving molybdenum and its alloys.
Welds in molybdenum can be unacceptably brittle, due to grain coarsening and increased Ductile-to-Brittle Transition Temperature (DBTT) in the weld and in the Heat Affect Zone (HAZ). At service temperatures, these brittle welds can fail, rendering the part useless. Electron beam welding can concentrate a tremendous amount of power into a very small area, resulting in a tight HAZ, which is useful in overcoming these particular difficulties. This, combined with the gas free environment of an EB welder’s vacuum chamber, result in pure, strong welds.
Pre-Weld Cleaning and Decontamination
When welding molybdenum parts, it is important to ensure that all joint areas are thoroughly cleaned. This means removing any and all hydrocarbon contamination.
To clean molybdenum, first it must be degreased and then washed in acid. There are two recommended ways to chemically clean molybdenum: either an alkaline-acid or dual-acid wash. The choice depends primarily on the level of contamination and the accessibility of the contaminated area. An example of a formulation that would remove both light and heavy oxides is about 50% (by volume) nitric acid plus about 10% (by volume) hydrofluoric acid. All chemical washes are followed by a rinse in distilled water.
Here are some common sense guidelines that should be followed when preparing molybdenum parts for EB welding:
- It is best to use clean cloth, like cheese cloth or paper towels, when cleaning molybdenum surfaces with solvents.
- Do not use compressed shop air to blow debris off the joint, as it often contaminates the weld area. Instead, use a bottled gas, such as argon or nitrogen.
- It is important to clean parts and joints using solvents prior to using the wire brush. This allows the solvents to be far more effective by preventing the embedment of hydrocarbons and other contaminates into the part of joint that could be introduced by the brush.
- Since old or dirty stainless steel brushes risk contaminating the joint areas, it is critical to always use new or recently cleaned stainless steel brushes when cleaning.
- If you happen to weld an etched surface, be sure to clean it with your stainless steel wire brush it before further processing the part. By-products from etching may drastically weaken joints by altering the chemical composition of the weld pool.
- Clean all wire brushes and scraping/filing/cutting tools frequently.
Because of how the narrow HAZ is on the EB welder, molybdenum does not need to be pre-heated before welding, as the risk of embrittlement is already reduced.
The parameters of electron beam welding include high energy density, deep penetration, narrow welding seam, and small HAZ. For molybdenum specifically, these advantages make EB welding an ideal process. However, EBW does demand extreme precision in fixturing the joints. If there are spaces or gaps in the joint, less energy is directed into the weld and the joint is consequently weaker. This makes weld fixturing pivotal in effectively welding with molybdenum.
Electron beam welding is capable of welding the same joints as other welding methods. However, because of the precision of the process and the fact that EB welding doesn’t require a filler, joints adhere to slightly different and generally tighter tolerances and standards.
- Butt Joints require:
- A fit-up tolerance of 15% of the material thickness
- Less than 25% of the material thickness for misalignment and out-of-flatness of parts
- That sheared edges are straight and square, should they be present at all
- Lap Joints (burn-through or seam welds) require:
- That there is not a significant space between the pieces that will be welded, to maximize weld penetration and speed
- Fillet Joints require:
- Square edges and a good fit-up