Before you weld
- November 1, 2016
What you do before the arc is started is just as important as the actual weld
When discussing welding, most of the themes brought up is travel speed, wire feed, arc-on time, and arc manipulation. All this discussion is to get the best possible weld and the most efficient use of the welding machine.
The best welding machine using the recommended welding parameters will still not be able to produce a solid weld if the joint being welded has not been properly prepped or cleaned.
“Metal prep ensures you have the highest probability of success right from the start,” says Thomas Pfaller, technical services engineer for AlcoTec Wire Corp. “If you have a bad design, you can have the best welder in the world and you’re fighting uphill because you’re fighting a design issue. It’s the same thing with metal prep. If you’re not doing the proper prior metal preparation you’re going to be fighting uphill right from the start.”
Weld prep is not simply just rubbing a brush over the area to be welded. In many instances it will incorporate more work to make the welding easier and more effective. “Weld metal prep includes bevel angles, root openings,” says Tre’ Heflin-King, welding engineer–CWI applications engineering at Hobart. “It’s all a part of the engineer’s overall plan for that joint and so that it will have the expected engineered outcome.”
Like neglecting administrative tasks because they are not exciting, many welders will gloss over weld prep or ignore it altogether. “I would say that one of the most neglected parts is weld prep in general,” says Pfaller. “The people doing it are those in the nuclear area and people building airplanes, things like that. Those are the guys that are doing all the steps. It’s the people that aren’t doing any of the steps that are the ones that we see the most of. It’s not that they’re cutting corners on one step, it’s that they’re cutting all the corners.”
Properly fitting the metal to be welded together is the first step in metal prep. “Usually after selecting the proper base material and having a joint design, most of the steps are generally the same across all metals,” says Heflin-King. “But basically you will bevel the plate or bevel the part to achieve the desired joint design. And there may be some tacking involved so that you can hold it in place to weld it.”
Beveling the joints increases the surface area of the joint so the weld has more to hold on and will be stronger. Poor fit-up can potentially lead to joint failure. “If you have poor fit-up you may have burn through where the parts didn’t fit properly,” says Scott Stanley, national marketing manager for Lincoln Electric Co. of Canada.
The welding engineer that designed the weld will specify the bevel angles and other parameters to complete a successful weld. These are more stringent in some applications than in others. “I know for nuclear it would be way more stringent verses if you were just doing some decorative hanging then I probably wouldn’t even have criteria for that. Just make sure you tack it and it does what you need it to do,” says Heflin-King.
Keeping it clean
It goes without saying that the metal to be welded should be ground down to the base metal, with all paint and coatings removed. The simplest form of cleaning base metal for weld prep is scraping the joint with a stainless steel brush. “When preparing metal for welding, rust should be removed in most cases to ensure a successful weld,” says Stanley. A wire brush removing rust from mild steel works well in non-critical, decorative, weld applications, but for crucial joints, more needs to be done. It may be common knowledge, but it is often overlooked and “you can’t use the same brush that you use on mild carbon steel for the stainless because it will pull over that carbon and then cause the stainless to actually rust,” adds Heflin-King.
Not all metals have the same cleaning procedure. “From an aluminum standpoint there’s three big steps. The first one is degreasing,” explains Pfaller. “That’s really a good first step for any material whether it’s stainless or mild steel, things like that. But particularly for aluminum, degreasing is big deal because any oil can result in porosity.” A lot of industrial degreasers will work well in this step. “The important thing is to stay away from brake cleaners that have tri-chlorinated solvents in them which OSHA, here in the States prohibits in any operation you’re welding because it forms a poisonous gas when it gets exposed to welding,” he says. Regular dish soap works well and in particular, like in the Dawn dish commercial showing oil covered ducks and other animals being cleaned by that blue liquid, “the blue Dawn dish soap works extremely well. Again, you think about it in terms of dish soap cutting the grease in your dishes is the same thing that it’s going to do on your weldments,” says Pfaller. However, using dish soap and water has some major drawbacks when welding critical parts in aluminum since this metal is very susceptible to porosity as a result of hydrogen being a large part of water. “So any time you get water involved in the welding process you run into the potential hazard of introducing porosity into your weld,” he advises. “It’s very detrimental.”
Eliminate the oxide layer
The next step is removing the oxide layer on the aluminum. “Aluminum has an oxide layer that forms on its surface similar to rust in steel. But in aluminum it’s not this ugly orange color,” explains Pfaller. “So in an effort to ensure that oxide layer hasn’t become excessive, we scrap it off. So use some manual process, whether it’s filing with a very aggressive file or some machining process, just to scrape the very top layer of the aluminum off to ensure that that oxide layer hasn’t gotten too thick.” This step is critical especially when parts are left exposed before welding. “One other thing that we see with aluminum is what’s called the hydrated oxide,” he said. “Some companies will leave big part assemblies, like trailers for example, sitting outside in the rain and they’ll get big milky water stains on them. That’s a hydrated oxide.” These stains are very detrimental to welding and need to be removed prior to welding to ensure that you’re getting good quality welds.
Scrapping the oxides off aluminum is crucial to a good weld. “You really want to make sure you’re scrapping that top layer off really cleanly,” continues Pfaller. “Aluminum melts at about 1,200°F. So if you imagine having a skin on your part that melts at roughly 3,700°F, whereas everything underneath it melts at about 1, 200°F, that’s a multiple of three differentials between the two. And that’s where you can really run into issues.”
Brush it off
The last process is brushing the weld area with a dedicated stainless steel brush, “to make sure there’s no other debris left,” says Pfaller.
“As you can imagine, if you did those in a different order, if you don’t degrease it, there’s a chance that things could get left behind. If you wire brush before you degrease you could be just scrubbing that grease into the base material.”
Part of the reason that weld prep gets ignored is that it is time consuming and difficult to automate.
“I would say it’s mostly a manual process. Some parts of it can be automated. I know in more sophisticated applications, like with laser welding, a lot of that stuff is automated,” says Heflin-King. “They can make sure they have proper gap spacing and fit up.” There are machines that automatically bevel pipe ends for pipe and tube welding, but these are specialized processes. The cleaning is still largely manual. For aluminum welding, some shops will use an acid bath. “So rather than doing all the metal prep manually, like degreasing and scrubbing manually, they have a big cage and they drop parts in and they’re scrubbed that way,” explains Pfaller.
Welding prep for automated or robotic welding is very crucial since a manual welder will be able to identify problems while welding, most automated systems will not and the result will be in many parts that will fail inspection. “Part preparation is a crucial part of many welding tasks. Particularly with automated welding processes like automated or orbital welders. Consistent and precise fit-up of the parts can help ensure that welds are completed as expected,” advises Stanley. SMT