Casting Materials – Welding Mastermind

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 flash point of 883 ˚F and burns at a temperature of 4,000 ˚F. This kind of heat turns water and even carbon dioxide into fuel, so not many are willing to try welding magnesium. How do you go about tig welding magnesium? 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. Welding magnesium is generally to repair machine parts and castings. It is a high strength metal with a low weight, making it an ideal metal for aerospace use. Making repairs on these high cost parts is a valuable welding skill, although it requires a different set up for your welding equipment and has its own set of safety protocols. Getting Started with Tig Welding Magnesium When repairing magnesium, it is important to note that while other methods are available that tig welding is the most common way to repair magnesium castings. This is because tig welding was invented to weld on magnesium and aluminum by Northrop Aircraft Corporation in the 1940’s. All other techniques at that time were unable to efficiently and proficiently work on magnesium or aluminum. Welding on magnesium is very similar to welding on aluminum. Often the two separate metals even have a distinctly similar appearance, which can be a little problematic. Being able to identify the two metals from one another is good to know because neither metal will stick to the other, even though there are magnesium alloys with aluminum and aluminum alloys with magnesium. However, there are a few distinct differences between working on magnesium and working with aluminum, including the following: the distance of the arc lengththe amount of soot that welding createsthe weld technique itself While being very similar to the techniques of welding aluminum, these differences are what allows a good weld. Due to magnesium’s high strength to weight ratio, the metal is often used in aerospace applications. This can be a very valuable skill for any welder to have, as repair these parts via welding is often the most cost-effective way to maintain use. Being as how an airplane’s gearbox can cost upwards of $50,000, I would like to point out that there is a significant amount of money to be made knowing how to properly weld on magnesium. However, welding magnesium often comes with its own set of precautions as the metal itself is highly flammable, tough to weld, and can potentially be very dangerous. Before we dive too far into the topic of welding on magnesium, we should first look at what additional safety precautions should be taken for welding on magnesium. 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 identifyBecause 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. Besides just these points of welding on magnesium, here is a link to Osha’s standards for the general industry that you may find useful to peruse and study. This is just simply great information to have and know, as you never know when it could come in handy. I know a Boilermaker whose job entails the safety of his crew and often inspects welds that have been laid in the field who told me that this can be an invaluable resource. Preparing to Weld Magnesium The first step in welding magnesium casting is to make sure that the metal you are working with is magnesium. Since other metals have the tendency to create oxide layers on their surface, it can be a little difficult to identify what metal you are working with. Fortunately, there is a very simple and inexpensive test to do to identify what type of base metal you are working with. Determining Whether the Metal is Magnesium or Aluminum You can use regular old white vinegar in order to determine if the casting you are preparing to work on is either magnesium or aluminum. If you have a bottle of vinegar lying around, you can apply it to the casting to test it. Pour a small amount of vinegar directly onto the metal you will be working with. If the metal is made of magnesium, there will be a visible chemical reaction. The vinegar will fizz almost like when you combined vinegar and baking soda. This fizzing action is because magnesium is a substance that easily reacts with acid, creating hydrogen gas and magnesium ethanoate.If you have poured the vinegar on to the metal, and you have no reaction, then the metal you have is, in fact, aluminum, and you will need to make adjustments by either finding different metal or using a different welding technique. Knowing what metal you are working on is key to having a successful weld. Maybe the customer themselves believe they know what type of metal the piece is made from. However, using the wrong filler metal with either magnesium or aluminum will create a weld that will quite literally break apart in your hands. That is an embarrassment that any tradesman would prefer to live without. So, it is simply best practice to always do a test before assuming or taking someone else’s word for it. Identifying the Alloyed Composite Going a step further, now that you have successfully identified that you are, in fact, working with magnesium. You should determine what the alloyed composition of the magnesium is. The piece in question could be a magnesium/aluminum alloy, magnesium/zinc alloy, magnesium/zirconium, or even pure magnesium. Each one of these individual alloys has different requirements for filler metals, preheat, inter-pass and post-heat treatments. Being able to identify what type of alloy you are working with will determine what type of filler metal is most appropriate for welding. As well as give you the type of electrode you will want to use and voltage setting. The best way to identify what type of alloy you are working with is to get in touch with the manufacturer. The manufacturer should have all the information that you require to begin collecting the required equipment for a proper weld. Unfortunately, if you cannot contact the manufacturer or even the original foundry then you will have to experiment on the piece unless you hopefully have some scrap to experiment on. Even if you are able to identify what type of alloy you are working on, and the required welding specifications for, here is an excellent resource for any welder to have. The American Welding Society handbook is chock-full of the information you need to complete a weld of any kind, and any kind of metal. Being as how the human mind is often forgetful and human error even has a formula, then it is a great reference for any welder to have on hand. Preparing the Casting for Welding Once you have successfully identified if you are working with magnesium, and then what alloy of magnesium you are working with, then you are ready to prepare the piece for welding. A few key points for preparing magnesium for welding: First, identify the area of the piece that is going to require to be welded.Once you have identified the area of the piece that is going to require welding, then you should grind the area out in order to lay beads of filler down. Instead of covering or attempting to fill a crack, grinding is the preferring method. Magnesium filler doesn’t often settle in the crack well enough to appropriate bind the metal.When grinding, it is important to use a very coarse carbide burr. A smaller burr will fill quickly and make grinding a slow and tedious process. You will also want to have a stainless-steel wire brush.Since magnesium is such a porous metal, it often is completely soaked with oil. This can make cleaning a magnesium piece a lengthy and tedious process, sometimes require multiple soakings to get and appropriate welding surface. Use a solution of 24 oz of chromic acid, 40 oz of sodium nitrate, and 1/8 oz of calcium fluoride. Then add enough water to make 1 gallon of solution. Allow the piece to soak for approximately 60 minutes, then rinse in cold, then hot water, and inspect the piece for oil residue. If appropriate, repeat until the piece is thoroughly cleaned of oil.Preheating the piece in an oven prior to cleaning in a solution will help the magnesium sweat out the oil that has saturated the metal. Once you have cleaned and prepare the piece for welding, it is time to get the appropriate setup for your welding equipment. Preparing Your Equipment for Welding Magnesium Although welding aluminum is very similar to welding magnesium, it is different enough to warrant some small adjustments to your equipment. You won’t have to completely redo the setup differently from welding from aluminum, but different enough to change some settings. Unlike aluminum welding, which tends to jump to the electrode, magnesium will run from the electrode. This small but important difference allows you to adjust the length of the arc on your electrode. The distance of the arc is as close to the electrode as possible. It will almost feel too close.When selecting your filler, you do not want to use any old filler wire. Magnesium will oxidize, so if your filler has been exposed to the air for a month or longer, the chances are the filler wire has been completely saturated with oxygen and fully oxidized. This oxidized filler will not allow for a good weld. Take your new filler wire, and prior to welding, clean it with acetone until it as shiny and completely free of oxidation.When selecting an electrode for working on magnesium, zirconium alloyed tungsten electrodes are going to do the job efficiently.Argon is generally accepted as the gases you will want to use for shielding and backup.You will want to have your welding equipment set to ac, just like with aluminum welding settings and adjust as necessary to get the filler wire to puddle appropriately. What to Look for While Welding While you are working on the piece of magnesium, you should pay attention to a few key things. For instance, if while welding, you have difficulty getting the filler wire to puddle appropriately go ahead and stop welding, burr out the section again, and clean with a wire brush. Cleaning with a wire brush is very important to do often with welding with magnesium. Welding magnesium is a dirty welding process. It creates a lot of soot due to the zinc that is in the alloy and makes it very difficult to lay down one bead immediately after one another due to the reduced surface contact. Making sure that the magnesium puddles appropriately during welding is highly important to ensure a good weld. If you do not have a good puddle from the wire filler, often what will happen is that your finished weld will look clean and well done, but underneath the surface, it will be filled with holes that reduce the integrity of your weld. If you must fill a large section of a magnesium weld with several layers of beads to get the appropriate fill, then you want to stop with each bead and clean with a wire brush before continuing to fill. This is a bit tedious, but again it is necessary with magnesium to get a proper weld. Also, this is another reason why it is critical to know what kind of alloy of magnesium you are working with is that each alloy may have been heat treated to improve strength. These heat-treated alloys are susceptible to a reduction in material strength if heated too much. If you happen to work quickly when laying down multiple beads, you run the risk of not allowing enough time to pass to allow the heat created by welding to dissipate or be absorbed by the piece. Knowing what the inter-pass heat tolerance for the alloy is important to avoid this. After Care for Magnesium Welding As is good practice when completing any weld on any metal, a thorough inspection is appropriate to ensure a good weld. If you happen to have access to x-ray equipment that is available to you, then that will work. However, FPI will allow you to visually chase down any cracks that may have occurs during the welding. Once you are satisfied with your weld, clean the piece once more, and then apply a corrosion preventative to the piece and you are finished. In Summary If you are still a little skeptical or perhaps this is your first time with welding on magnesium, then there are several videos on YouTube that will give you a visual play by play. Welding is a skill that often takes several hundred times of practice before being able to weld with confidence. If you happen to have access to a metal scrapyard, you can go purchase scrap magnesium to practice on before attempting to weld for a customer.