ARC MAGAZINE

Will a check valve stop a flashback?
Andrew Mayfair, Canfield, Ohio

First, let’s establish exactly what a flashback is. A flashback is an uncontrolled flame that recesses back into your oxy-fuel gas equipment, upstream of the location where the gas is designed to mix. In order to have a flashback, you must have oxygen and fuel mixed together. Pure oxygen won’t flash back and pure fuel won’t flash back. You need both gases combined in a specific ratio range to have a flashback. Check valves are designed to allow gases only to pass in one direction. If your check valves are working properly, you won’t have premixed gases upstream of your equipment’s designed gas mixing chamber. A check valve will not stop a flashback. However, properly functioning check valves will stop back-flow of fuel gas and/or oxygen, thus preventing the conditions required for a flashback to occur.

How do you choose the proper shielding gas for MIG welding?
Ernesto Velez, Adjuntas, Puerto Rico

There are many factors that determine gas for MIG. The first one is type of material you’re welding. The next one is thickness of material and joint design. Also, consider what mode of transfer you’re going to use — short arc, globular, spray, pulse spray? Are we looking for mechanical properties? Some gases will take away mechanical properties versus facilitating mechanical properties. Another factor is the desired profile of the weld. If you require a clean, non-spattery weld, you generally want to use spray or pulse mode. Also, the cost of gas may be a deciding factor. What can you afford? The more you mix gases, the more it will cost. Generally, a very large bottle of tri-mix is going to be about $75. A bottle of CO2 is about $9. Generally, for short arc, 75 argon/25 CO2 is the gas preferred for your typical garage welding on mild steel. That’s generally a good mixture to get good bead and less spatter, and it’ll take you in the range of the short arc mode that you need. If you’re welding thicker materials, you need to go to a 90/10. So for ¼-inch or more, you’ll want to a 90 argon/10 CO2 mix with a spray arc, because you won’t get the penetration from the short arc. The most common gas is 75/25 for short-circuiting on sheet metal. CO2 is cheaper, but it will give you a lot of spatter in the weld.

What’s the best way to hold the filler rod when doing GTAW?
Joe Ludlow, Smithville, Oklahoma

That all depends on the position you’re welding in, and what type of joint you’re welding — horizontal, vertical, overhead, flat, fillet welds, groove welds, pipe. All these things will have a bearing on how you hold the filler rod. That’s a lot of ground to cover, but in simple terms, let’s say you’re doing a basic horizontal weld on a table. The filler should be no higher than 15 degrees from the weld surface. At the same time, the filler should be 90 degrees from the TIG torch.

A friend and I are restoring a 1949 Oldsmobile convertible. Some of the components that need repair are pot metal. Some of these parts are very rare and expensive or irreplaceable — and will be re-chromed when/if they are repaired. What is pot metal and how can we repair it? We tried TIG, but it melted as soon as the arc started. Should we try lead or solder? Any information or advice would greatly appreciated.
Neil Waddell, Lake Mills, Wisconsin

Pot metal is basically a cheap alloy with a low melting point. Generally it’s zinc, lead, copper, tin, and sometimes some other elements as well. It’s also known as monkey metal, or die-cast zinc. For fillers, you can generally use Super Alloy 1, which you can purchase from Muggy Weld®. TIG welding is the best option, with low amps and AC polarity. Generally, you’re going to use 1/16-inch or 3/32-inch filler. It oxidizes very rapidly, so it really needs a lot of coverage. The low amperage is important, because too much heat will disintegrate everything due to the low melting points of the alloys. It’s also very important to keep the joint clean as you go. Start by getting it heated up and melted, add the filler, stop, wire brush it, clean it, and then repeat the process. You’re essentially welding in layers, stacking and cleaning each time. The cleaning is very important because the joint will get very dirty due to the low melting point of the alloys involved. Every time you clean, the quality of the joint gets better, because you’re able to get more filler into it.

From the welding experts at
Lincoln Electric