There are several different aluminum alloy families, each with their specific uses and applications. One of the most common and versatile is the 6xxx series alloys.
These alloys see service in a myriad of applications that range from dump trucks to beverage carts. As technical consultants, the engineers at AlcoTec Wire receive numerous calls about 6xxx series alloys with questions about filler selection, crack prevention and procedure qualification, to name a few. This article takes a look at some key topics to help better explain how to successfully weld with these alloys.
Filler selection for 6061-T6
When selecting an aluminum filler alloy for a given base alloy, there are usually a few acceptable options. For example, 6061-T6 can be welded with four common filler alloys as well as several specialty alloys.
To know with certainty what filler alloy should be used for the base material requires an understanding of the welded component application, expected performance in service and what variables associated with weld performance are most important. Filler alloys for arc welding aluminum are evaluated against the following attributes:
- Ease of welding – This is the relative freedom from weld chemistry-related cracking.
- Strength of welded joint – In the case of two different filler alloys, both may meet the tensile strength of the base material, but have significantly different shear strength performance.
- Ductility – A consideration if forming operations are to be used during fabrication as well as a design consideration for service if fatigue and/or shock loading are of importance.
- Corrosion resistance – A consideration for some environmental conditions and typically based on exposure to fresh and salt water.
- Sustained temperature services – The reaction of some filler alloys at sustained elevated temperature (above 150° F). This may promote premature component failure due to stress corrosion cracking.
- Color match – Base alloy and filler alloy color match after anodizing can be of major concern in some cosmetic applications.
- Post-weld heat treatment – The ability of the filler alloy to respond to post-weld heat treatment associated with filler alloy chemistry and joint design.
The following three examples demonstrate the complexity of choosing a filler alloy.
Situation 1: Welding on 6061-T6 tubing for an outdoor table that is to be clear-coat anodized post-welding.
In this situation, a filler alloy is needed that will provide the best color match after anodizing. For this application, using 5356 would be the best option. Although filler alloys 4043, 4047 or 4643 are often shown as being suitable for this base material, the weld would become dark gray in color during post-weld anodizing (see Figure 1).
Situation 2: Using 6061-T6 bracket as a welded handle for a cooking pot that will be operating consistently above 150°F.
Filler alloys suitable for elevated temperature service include 5554, 4043 or 4047. Using other 5xxx series alloys would introduce the possibility of stress corrosion cracking and premature failure of the welded component.
Situation 3: Using 6061-T6 to fabricate a structural frame that will be post-weld solution heat treated and artificially aged to restore strength and return the structure to the -T6 temper.
In this application, the most suitable filler alloy would be 4643. This is a heat-treatable filler alloy that will respond to the heat treatment after welding and typically provide weld strengths comparable to that of the base material.
Crack prevention: 6xxx aluminum alloys
The 6xxx base alloys are, by their nature, very prone to cracking. This is a result of the chemistry falling very close to the peak of the solidification crack sensitivity curve. The magnesium silicide (Mg2Si) chemistry of the 6xxx alloys is a primary reason there are no filler wires in this alloy family.
A simple test of this can be done by autogenously welding (without filler) on 6061-T6 and observing the cracking problems (see Figure 2). The addition of filler alloys generally results in a more acceptable chemistry. When utilizing a 5xxx filler alloy, the chemistry will be flooded with excess magnesium. On the other hand, using a 4xxx filler alloy will result in an abundance of silicon in the weld. In either case, the chemistry of the weld is moved from the peak of the crack sensitivity curve.
Another cracking situation that is sometimes encountered is a consequence of bead contour. Fillet welds with concave faces can produce throat cracking. This cracking results in the weld not being able to withstand the thermal stress induced during the arc welding process (see Figure 3).
GMAW procedure qualification: 6061-T6
There are two common problems that welders encounter when qualifying welding procedures on the 6xxx series alloys to the AWS D1.2 Structural Welding Code-Aluminum. The first of these is when the bend samples pass and the tensile samples fail. The second is when the tensile samples pass and the bend samples fail. One type of test passing and the other failing indicates that weld discontinuities (e.g., lack of fusion or porosity) may not be the source of the failure.
Heat input during arc welding produces softening of the 6xxx base metal. It is not uncommon to have a welding procedure that results in excessive heating of the 6xxx. When this occurs, the bends typically pass while the tensile samples will not meet the strength requirements of the D1.2 code. Another indicator that overheating is the source of weld failure is finding tensile fractures in the heat-affected zone near the weld and not in the weld metal itself with discontinuities present.
There are a few methods of keeping a weldment from becoming overheated. Primarily, it’s important to maintain the preheating and interpass temperature requirements of the code. This type of material specifies 250° F as the maximum preheat and interpass temperature. Carefully monitoring this helps prevent overheating of the weld joint. In the case of these values, for optimum results, it is actually best to avoid preheating entirely and to keep interpass temperatures to well below the maximum allowed.
There are a few other welding procedural techniques that can also help reduce the likelihood of overheating a weld assembly. For instance, increasing travel speed can greatly reduce the overall heat input. Another option is to utilize pulsed GMAW [MIG] technology or a modified pulsed spray transfer process such as ESAB’s SuperPulse process, which can also control heat input. These are all methods that can be used to prevent significantly reduced weld strength through overheating.
In the case of a weld that is failing the bend tests and passing the tensile tests, there could be an issue with the testing procedure. In the event that there are no visible discontinuities in the fractured bend samples, it is important to go back and review the testing parameters listed in the code. For testing aluminum welds, a wrap-around style bend tester is recommended. Because of the way aluminum transmits stresses, the plunger-style bend tests can be problematic.
Shavings when welding with 5356
Conventional 5xxx wires have a well-deserved reputation for generating aluminum wire shavings that build up in the drive rolls, liner and contact tip. As the shavings build up, they hinder feeding performance, create excessive consumable wear, cause more frequent burn backs and increase the need for routine maintenance.
Even worse, the problem creates a vicious circle: As aluminum particles flake off and embed in the gun liner, they score the wire and create more shavings. Over-tensioning the drive rolls generates shavings as does wire-to-wire scuffing.
To address this issue, AlcoTec’s NT wire process uses patent-pending manufacturing technology to eliminate the microfines and surface abrasion generated during wire drawing (see Figure 4). This new technology minimizes or eliminates the shaving problem, thus greatly improving wire feedability and arc stability to produce a clean, high-quality aluminum welding wire that will significantly reduce clogged gun liners. This results in less wear and tear on contact tips and liners that don’t have to be replaced as often as a result of wire shaving.
In addition to reduced contact tip and liner replacement costs, NT wire also reduces downtime, ultimately extending the life of the welding gun and increasing throughput.
Clearly, the items to take into consideration when welding different aluminum alloy families are vast. As each manufacturer or fabricator has specific needs for various applications, it’s helpful to know that a myriad of resources exist to field questions about filler selection, crack prevention and procedure qualification, among others.
As an example, aluminum filler alloy selection charts have been developed to assist fabricators with the most appropriate choice of filler alloy, and they are available at www.alcotec.com. For fabricators unsure of the most suitable filler alloy, AlcoTec can also be contacted directly.