Dissimilar Metal Welding (DMW): Need & Causes
Dissimilar Metals Welding (DMW) finds application in shipbuilding, transport, power plants, aerospace, and construction. Here, different parts of the structures are made from separate metals / alloys as they have to possess diverse properties.
Furthermore, it is economical to utilize inferior materials in areas of a framework where service conditions are less harsh. Of course, you have to ensure the safety of the structure. What better way than to fasten such diverse materials with welding?
Transition zones between the base metals in DMW can be a problem area. Intermetallic compounds in this zone can be hard, brittle, and crack prone. This endangers the very integrity of the structure and can lead to catastrophic failures.
Of particular note is carbon migration wherein carbon moves from one part of the weld interface to another creating a non-uniform weld open to failure.
Primary causes of problems in DME are mutual insolubility of the base metals and large differences in their melting temperatures, coefficients of thermal expansion, thermal conductivities, intermetallic chemistry, and electrochemical potentials.
Better DMW Practices
Composition and properties of weld metal and the choice of the welding process influence the success of DMW. The weld joint’s microstructural stability, mechanical-physical properties, and resistance to oxidation and corrosion determine its service life.
Deciding the type of weld needed for a certain task is the first step. Meticulous control over heat input, filler material, process parameters, and post weld heat treatment (PWHT) usually addresses all issues with DMW.
Superior DMW practices include:
- Transition Materials Soluble in Both Base Materials
- Composite Inserts
- High Temperatures
Nickel is the transition metal when welding steel to copper. Research has also proved its efficacy in diluting chromium and carbon compounds formed when welding a wide array of disparate metals.
These compounds are hard, tough, and brittle. Moreover, you cannot easily detect their effect as it unfolds at the microstructural level. When using nickel as the transition metal, employ the surfacing or buttering technique.
Surfacing involves separately welding several layers of the nickel to each base material. To start with, butter nickel on the lower melting point material. This creates a thermal buffer and protects it from heat stresses. Buttering also cuts down dilution and related consequences.
If you are joining metals with widely divergent melting points, use a transition metal that has a melting point between those of the two metals.
Composite Inserts usually establish the weld joint without heating. The outer part of the composite is of a more-welding-compatible material such as nickel that simplifies welding. The inner part is of stainless steel that provides strength and durability.
With such arrangement, you establish the weld joint rapidly and without complications. You can then remove the outer layer with carbon gouging to extend the weld deep into the inner material.
Most welding processes for joining with composite inserts are non-fusion ones in order to avoid or minimize the problem of intermetallic compounds and Heat Affected Zone (HAZ). These include:
- Cold Welding: employs immense pressure to join metals without applying heat. It usually joins metals used in extra-atmospheric applications such as spacecrafts and satellites
Such welding gives best results when you rid the metal surfaces of oxide layers before welding and maintain their hygiene after welding
- Explosion Welding: routinely joins incompatible metals not usually connected via conventional welding processes
It uses controlled explosive detonation as the energy source to establish the bond with minimal heating. This minimizes the HAZ and unwanted thermal compounds
The process has made rapid strides since its development in the 1950s. Today, we can join plates of up to 30m2 and achieve joints between totally incompatible materials such as stainless steel and zirconium
- Friction Welding: employs pressure and heat created from friction of the two base materials to establish the joint. It involves minimal heating of the base metal. Plus the process throws the heated metal away from the joint
It joins steel/copper, steel/brass, aluminum/ceramic, steel/aluminum etc. The process is typically used to fabricate valves for internal combustion engines, printing rollers, drill pipes, cross members, and hydraulic components
- High Frequency Resistance Welding: combines high frequency electromagnetic energy from electric current and intense pressure. Focusing energy on the required location avoids HAZ and intermetallic compounds
- Percussion Welding: is a type of resistance welding that utilizes very high welding currents (~100kVA) for very short durations (~10ms). The high welding current creates an arc while an electromagnet provides the joining force
Inherently, the process is suited for welding metals with high thermal and electrical conductivity. And it conveniently joins parts with large differences in thickness and cross section
It is typically suited to join combinations of tungsten, copper, silver, nickel, molybdenum, and their alloys. The process also efficiently joins parts made by powder metallurgy
- Diffusion Welding: heats the base metals to 50-70% of their melting points and then fuses them together with pressure. It is usually used for manufacturing aerospace components
- Ultrasonic Welding: connects extremely thin parts by utilizing heat generated from the high frequency motion of the base parts
- Electron Beam Welding: is a fusion welding process that utilizes the kinetic energy of a beam of high-velocity electrons to join metals. The process typically uses electrons moving at 30-70% of the speed of light
- Arc Welding: is of course the most common welding process in the world with nearly 40% share in the global welding product market. A fusion welding process, it joins parts by melting them at their intersection
Employ high temperatures if you are anyway adding large amounts of heat for DMW and are using nickel as the filler material. This is particularly true for steels as the heat transforms it to an austenitic phase.
With its Face Centered Cubic (FCC) structure, austenitic steel possess better weldability, ductility, cold-workability, plasticity, and toughness. You can weld and shape it easily and rapidly. And once austenitic steel sets in, it is superbly strong with higher ultimate tensile strength.
To avoid brittle compounds in the HAZ, make light weld passes till you cover the base metal thoroughly. This slashes the mixing and dilution of the base metals and minimizes the quantity of intermetallic compounds.
Preheating metals / alloys with higher thermal conductivity is a good practice when welding metals with large differences in thermal conductivities. This directs more heat towards the metal with low thermal conductivity.
Thicker base metals require more preheating because you have to heat larger amount of mass. Preheating also lowers the post-weld cooling rate and resultant thermal stresses. Do not preheat excessively lest this soften the metals and introduce unwanted metallurgical modifications.
When dealing with metals whose coefficients of thermal expansion differ significantly, use a filler metal with a thermal expansion coefficient roughly midway between those of the two metals to minimize thermal stresses.
You can also apply heat treatments such as stress relief annealing before or after the welding operation. Annealing is only a half measure, for it does not relieve stresses completely. Then again, you have to guard against overheating of one of the base metal / alloy.
Carbon migration is inevitable. In order to minimize the failures it can cause:
- closely monitor weld quality and choose joint configurations appropriately
- use carbide formers
- if possible, connect metals with comparable coefficients of thermal expansion
- utilize graded transition joints that evenly distribute the difference in thermal expansion coefficients
- prevent vibration loads and bending stresses where possible
- use stabilizer buttering procedures
- minimize post weld heat treatments
Although the list of preventive measures is fairly elaborate, it is by no means comprehensive. Because there are umpteen material combinations. Research in this case is a never ending process.
^ Shipbuilding Regularly Employs DMW – Image Courtesy of CreativeNature R.Zwerver at ShutterStock.com