Aluminum Welding Guide

Heat Treatable And Non-Heat Treatable Alloys

This section presents a discussion about the properties, before and after welding, of heat treatable vs. non-heat treatable aluminum alloys. This is an area of concern for anyone attempting to choose the best base material alloys and tempers and the correct filler materials to join them. It is in this area that manufacturers have difficulty achieving consistent mechanical properties and defect free weldments in production. For purposes of discussion this section will limit the dialog to a comparison of the 6xxx and 5xxx series alloys:

The 6xxx series base metals have low alloy content and are easy for mill product fabricators to form into extrusions, tubing, forgings and other shaped products and then to heat treat to obtain high mechanical properties, making them economical to produce. The 5xxx series base metals have high alloy content and because of their strain hardening and higher flow stress characteristics are more costly to fabricate into shapes. However the 5xxx series base metals are economically rolled into sheet and plate and roll formed into shapes when specific shapes are desired.

The 6xxx series base metals obtain their maximum mechanical properties through heat treatment and aging. The aluminum metal matrix is strengthened by the precipitation of the alloying elements as intermetallic compounds whose size and distribution throughout the matrix is carefully controlled through precise thermal operations. When the 6xxx series base metals are welded, the microstructure in the HAZ is degraded and the mechanical properties are typically reduced by 30 – 50%. Figure 1 on page 7 shows that 6061 and its most common filler metal 4043 both have a typical annealed tensile strength of around 19 ksi. Depending on the heat input during the welding operation, the base metal can be fully annealed for some distance from the weld, especially in areas being weld repaired.

The 5xxx series base metals obtain their maximum mechanical properties through alloying element solid solution strengthening and additional strength is gained from cold working. The welding operation does not affect the solid solution strengthening of the base metal, only the cold working portion of the strength is lost in the heat affected zone transforming it to the annealed condition. Figure 1 on page 7 shows that the typical annealed tensile strength of 5083 base metal is 43 ksi.

Figure 1 on page 7 compares the loss of strength in the heat affected zone of welded 6061-T6 and 5083-H321 wrought base metals.

Figure 2 on page 7 shows the loss of strength in the heat affected zone of the as-welded 6061-T4 and -T6 base metals compared with post-weld aging.

The chart on page 7 shows the basic alloying elements and typical ultimate tensile strengths in the non-welded and as-welded conditions for the most frequently welded 6xxx and 5xxx series base metals. The charts illustrate and are supported by the following important points:

1. The loss of as-welded strength in the 5xxx base metals is significantly less than that of the 6xxx base metals.
2. The 6xxx base metal properties shown are dependent on a minimum of 20% dilution of 6xxx base metal into the 4043 filler metal weld pool. The 5xxx base metals when welded with 5xxx filler metals are not dependent on dilution.
3. The 6xxx base metals have 30% higher thermal conductivity than the 5xxx base metals making it more difficult to produce consistent quality welds in the 6xxx base metals. Therefore 6xxx base metals require higher heat input to achieve penetration and this can result in increased distortion of the welded structure.
4. 6xxx base metals welded with 5xxx filler metals are more solidification crack sensitive than 5xxx base metals welded with 5xxx filler-metals. See page 10.
5. The as-welded mechanical properties of the 6xxx base metals are very sensitive to welding variables such as heat input and joint design whereas the 5xxx base metals are far less sensitive to these variables, making the 5xxx as-welded results much more controllable.
6. As-welded 5xxx base metals welded with 5xxx filler metals have higher ductility, toughness, and crack propagation resistance than as-welded or post-weld heat treated and aged 6xxx base metals welded with 4043.