Welding Magnesium is not something many welders I know are willing to attempt, even though it is possible. Magnesium tends to catch fire if heated too much and God help you put out a magnesium fire once it gets started.
Magnesium has a flashpoint of 883 ˚F and burns at a temperature of 4,000 ˚F. This kind of heat turns to water and even carbon dioxide into fuel, so not many are willing to try welding magnesium.
Magnesium Welding – Tig Welding Magnesium Castings
Welding magnesium can be a bit tricky compared to other metals, but it can be done. First, being able to identify if the magnesium you are attempting to weld is fully aluminum or an alloy is a good place to start. Then once you are sure of what you are working with, having the proper set-up and materials is key to a successful weld.
Building up worn metal, replacing a broken lug with weld beads, repairing pits, inclusions, and other casting defects are all more common than joining pieces of magnesium together. Magnesium is used quite a bit in aircraft components because of its high strength-to-weight ratio.
Magnesium castings are lightweight and strong, but they do crack and break in service due to all kinds of factors like thermal cycles, vibration, and just plain human factors like a mechanic over-torquing a stud. An aircraft gearbox made from magnesium might cost upwards of $50,000 dollars so being able to make a weld repair is often an economical way to repair magnesium casting.
One of the biggest hurdles in welding magnesium castings like this is that they are often oil soaked. The oil that lubricates the gears, shafts, and bearings stay in contact with the inside of the gearbox during many heating and cooling cycles and so the pores on the metal soak up oil like a sponge.
You can actually see it weep out of the metal by heating the metal with a torch. Another problem is the high zinc content. Magnesium alloys that have a zinc content of 5-6% are crack sensitive and don’t weld nearly as well as the ones that have about 10% aluminum content along with lower zinc.
AZ101 magnesium filler rod is considered one the most forgiving fillers and the reason why is that it fits the description I just gave…. about 10% aluminum with a maximum zinc content of 1.25 %.
Like Aluminum, Magnesium comes in different alloys and some alloys can be strengthened by heat treatment, and some cant. And just like Aluminum alloys that have been heat-treated to improve properties…Getting magnesium casting too hot during the welding process can remove strength.
Additional Safety Precautions for Tig Welding Magnesium
Magnesium can be a treacherous and difficult metal to work on. The reasons being for this are that:
- Magnesium is a flammable metal, once it catches, the metal burns at 4,000 ˚F, which is hot enough to break apart to molecular bonds of H2O releasing the oxygen and immediately turning it into fuel for the fire, the same goes for CO2. Often in order to put out a magnesium fire the only option is to starve it of oxygen via either sand, cement powder, or Class D Osha recommended extinguishers.
- Magnesium is a porous metal, meaning that its microscopic structure is filled with minute pores. Due to magnesium casting being used largely in machine parts and its inherent porousness, a magnesium casting will often absorb large amounts of oil. This increases the risk of a fire, and it is difficult to get a good contact with the metal in order to establish a solid weld.
- Magnesium, like aluminum, can come in a variety of different alloys, sometimes they are zinc alloys. Also, both metals tend to develop an oxide layer on its surface, making the two metals difficult at times to identify
- Because magnesium is flammable, the shavings from grinding and cleaning the metal can easily burst into flame. After grinding and preparing the metal for welding, it is good practice to go ahead and wipe down the station and sweep the area, so any errant arcs or sparks won’t cause a fire.
- Magnesium alloys, like aluminum, that have been heat treated in order to improve strength are sensitive to getting too hot. If you get these alloys too hot, it will reduce the strength of the material.
Tips for welding magnesium castings
- Clean as good as you can with acetone and a carbide burr…. A coarse burr that won’t load up.
- Often, filler rod choice is governed by a written procedure according to the composition of the magnesium alloy base metal… But if you have the option, use az101 filler rod
- Preheat thick parts to around 300f
- Use same tig machine settings as for aluminum, except that a 50/50 argon/helium mix makes a huge difference.
- Light up and increase amperage until it barely puddles. If it does not puddle clean, stop, burr, wire brush, repeat as many times as necessary to get it to puddle clean
- Oil gets into the pores of this stuff and it takes a while sometimes to get enough out to weld clean
- Use a much closer arc length than for aluminum…like so close that you think it’s too close.
- Weld small beads and clean in between with your small 90 air grinder and a carbide burr and then a stainless wire brush.
Magnesium and magnesium alloys can be soldered. However, soldering is limited to the filling of small surface defects in castings, small dents in sheet metal, and other minor treatments of surfaces.
Soldering should not be used in stress areas or to join magnesium to other metals because of low strengths and brittle joints obtained.
Magnesium alloy surfaces must be cleaned to a bright metallic luster before soldering to ensure good fusing between the solder and magnesium. This cleaning can be accomplished by filing, wire brushing, or with an aluminum oxide cloth.
The area to be soldered should be heated to just above the melting point of the solder. A small quantity of solder is applied and rubbed vigorously over the area to obtain a uniform tinned surface. A stiff wire brush or sharp steel tool assists in establishing a bond.
After the bond is established, filler metal may be added to the extent desired. Flux is not necessary.