by Bob Moffatt
One of the most often underestimated steps in making a sound weld that is aesthetically appealing is base metal prep. Many people have the mindset that the welding process is so hot, that the temperatures will burn all the impurities out.
That is just simply not true. We should always clean the base metal to the best of our abilities. Maybe not for small projects around the house and for other odd jobs that aren’t code work. But, it is defiantly a good habit to get into.
Each type of base metal will have a different cleaning method that works best for it. For instance, due to the iron content in steel, it has a tendency to rust or heavily oxidize when exposed to the elements. Additionally, due to the manufacturing process, hot rolled steel has a mill scale on the outside surface that occurs naturally. Wire wheels are not strong enough to remove the mill scale and zerc wheels typically just polish the surface and don’t really remove it. Your best option is to use a hard rock wheel to remove the contaminants on the surface, as well as the mill scale. In the video, Bob puts all three cleaning methods to the test and demonstrates the results with each type of cleaning process.
When it comes to stainless, you can just wipe it down with a good degreaser and follow up with a clean water rinse. However, there may be times when you want to buff the area to be welded with a zerc wheel. In our test, there wasn’t much change in the weldability or the outcome between the two methods. One thing to be careful with, is not to use wire brushes, wire wheels, grinding wheels etc. on stainless that have ever been used on steel. Using these items on steel and then on stainless could impregnate the stainless with iron oxides and it could induce rusting in our stainless.
Moving on to the aluminum, we note that aluminum has a layer of oxide on the surface that melts at around 3,700°F. This is a big consideration when dealing with aluminum welding because the aluminum under that thin layer of oxide melts at around 1,200°F. That is why we us A/C TIG welding for aluminum. The ability to use A/C allows us the ability to use the DC+ side of the cycle to get cleaning action, which helps to break up and lift the layer of oxide and follow it up with DC- to get penetration into the piece. Although the oxide is very thin, it is also very hard. As crazy as it may seem, the best thing to clean off the oxide layer is a stainless wire brush. Some people thing a grinder would be the best but since aluminum is so soft, you can actually cause contamination in the welding area with a grinding wheel as it will push the top layers of contaminants further into the material. Also keep in mind that you only want to use discs and attachments approved for aluminum on aluminum. Using a grinding wheel made for steel could clog up the wheel and throw it off balance. Additionally, if you have used the wheel on steel previously, you could cross contaminate your aluminum and impregnate it with iron oxide, which will cause rusting on your materials.
As you can see, base metal prep is very important to the overall outcome of your welding. I have always told people that the key to a good weld is 80% prep work, the other 20% is skill. Doing a good pre-clean and prepping your materials correctly will make your life easier in the long run. Cleaner materials will weld better, and the bead appearance will be much better than its contaminated counterpart.