ARC MAGAZINE

Alloys and Fillers

Unalloyed aluminum is seldom used in structural applications because it lacks strength and is extremely ductile. It’s easy to bend a 99% pure aluminum rod without using any hand tools for assistance. However, the fabricated part used for this demonstration is made up of three different aluminum alloys: 3003 for the bulk of the part, 6061 for the threaded bungs and 5052 aluminum for the bottom rail.

Given this variety of alloys in a single part, the choice of filler metal for the weld depends on several service conditions:

1. Strength: Will fillet welds be subjected to shear loads?

2. Weldability: How does the filler metal rate relate to preventing hot cracking?

3. Ductility: Will the weld metal be subject to deformation after welding?

4. Corrosion Resistance: Will the weldment be used in a seawater or freshwater environment?

5. Thermal Conditions: Will the weldment be subjected to sustained temperatures of 150° F or higher?

6. Anodization: Will the weldment be color anodized after fabrication?

To illustrate the decision-making process that goes into the choice of a filler metal, let’s look at two commonly used filler materials that we may have to choose from for the project – ER 4043 and ER 5356 aluminum – and then walk step-by-step through the choice of filler metal most appropriate for the job, based on service conditions (see the table below for some criteria and general guidelines).

Both filler alloys may be a fit in terms of weldability, but 4043 might be a slightly better choice to use on the somewhat crack-sensitive 6061 and 5052 base metal. 5356 will provide a slightly stronger weld, but for the part we’re welding in this demonstration – an automotive valve cover – the fillet welds will not be subject to excessive shear loads in service, so shear strength is not a big concern. Corrosion resistance in salt and fresh water is not a concern for this performance automotive part. However, it would be a factor in marine applications. The piece will not be subject to deformation after welding, so ductility is not a requirement. In this case the part will not be color anodized after welding. If color match after anodizing was important, a filler metal such as 5554 would be selected, as it would color match well and be suitable for the sustained service temperatures anticipated.

The deciding factor in this decision is that the part we’re welding will be used in a vehicle, where it will be subject to sustained thermal conditions well above 150° F. Consumer automobile engines run about 190° F for extended periods of time. Aluminum alloy 5356 is not suitable for use in a sustained high temperature environment. Therefore, of the two, alloy 4043 is the choice for welding this aluminum valve cover and other aluminum auto parts such as oil pans, radiators or cooling parts that are not intended to be color anodized.

If there are any doubts, your best bet is to consult a manufacturer’s filler metal chart to properly match the filler metal to the base metal and get the best understanding of the service conditions.

Clean it Up. Set it Up

As always, the parts must be cleaned free of all lubricants and shop dirt before assembling and welding. In this case, a quick wipe with a clean rag soaked in acetone is sufficient. In some cases, such as with aluminum that has been laying around the shop for a while, you may need to do some brushing with a clean stainless wire brush in order to remove excessive contaminated oxides from the surface of the aluminum. When using a power wire brush, be careful not to burnish the aluminum and embed oxides into the surface.

I used a Lincoln Electric Precision TIG® 375 for the job. Shielding gas was 100% argon at a flow rate of 20 cubic feet per hour. I used a number 20 torch with glass lens collet body and a number 7 nozzle with an intermediate back cap. The following settings were preset on the machine, but current was adjusted remotely as needed:

Five Steps to Success

Aluminum is one of the most challenging materials to weld efficiently. Choosing the proper filler metals will have a direct bearing on the quality of your aluminum alloy welds. You’ll get better results when you address all the items on the following checklist:

1. Determine what aluminum alloys you are welding together.
2. Understand the service conditions under which the welded piece will be used.
3. Use a manufacturer’s filler metal chart to select the proper filler metal for the base metal and the service conditions.
4. Thoroughly clean all parts prior to assembly.
5. Fill craters at the end of the weld to a convex shape to minimize crater cracks.

During any welding process, safety is always the first consideration. When TIG welding, the operator should wear the following:

1. A good-quality welding helmet.

2. Safety glasses underneath the welding helmet. These serve two purposes: to protect the eyes from flying objects and to filter out any additional ultraviolet light that might get behind the helmet – possibly as the result of errant arc strikes.

3. TIG gloves to protect against electric shock and ultraviolet light. Keep in mind that all the metal on the bench is part of the welding circuit.

4. Skin protection from ultraviolet light and burns. Any combination of fire-retardant shirt, welding jacket, sleeves, and other protective layers will help keep your arms and neck covered.