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A - Unfortunately, I receive variations of this type of question frequently. Always a heat treatable base alloy, usually one of the 6xxx series base materials, weld of otherwise good integrity, unable to meet the minimum tensile strength requirements of the code.
The most common reason for a weld made in this type of base material, which is free from major discontinuities, not to meet the minimum tensile requirement, is overheating of the base material during the welding process. To understand why this problem can occur, we must first understand the characteristics of the heat treatable aluminum alloys and in this case, the 6xxx series base materials. This series of aluminum alloys is one of the heat treatable series, which acquire their strength through a process of thermal treatments. They are often used in the -T6 condition, which indicates that they have been solution heat treated and artificially aged. The -T6 condition is achieved by heating the base material to a temperature of around 990 deg F. This step in the operation is necessary in order to dissolve the major alloying elements into solution. Quenching, usually in water, follows the heating process in order to trap the alloying elements and produce a supersaturated solution. In the case of the 6xxx series alloys, the major alloying elements are magnesium and silicon, which combine during the thermal treatment to form the compound magnesium silicide. After solution heat treatment, the material is reheated to a lower temperature (around 320 deg F) and held at temperature for a predetermined time. This second thermal treatment is termed artificial aging and is conducted in order to precipitate a portion of the elements or compounds back out of the supersaturated solution to enhance the mechanical properties of the material.
When we consider the controlled heat treatment that has been conducted on these materials prior to welding, in order to obtain the -T6 condition, we can appreciate their response to the arc welding process, which heats the material to the same temperatures used for heat treatment in an uncontrolled manner. The 6061-T6 base materials, as purchased, have a typical tensile strength of 45 ksi before welding. The AWS D1.2 Structural Code has recognized the metallurgical changes that occur to this base material from the exposure to heat during arc welding, and consequently, requires a minimum tensile strength of 24 ksi. The minimum tensile strength specified by the code is based on historical testing using a variety of welding procedures. If we consider the fully annealed typical tensile strength of 6061 as being around18 ksi, we can appreciate the importance of controlling the overall heat input during the arc welding process. There is a direct association between the total welding heat input and mechanical properties of the base material adjacent to the weld (the heat-affected zone) after welding. The higher the total heat input, the lower the tensile strength will fall. Fig 1 provides us with an appreciation of the differences in strength between the as welded and post weld heat-treated condition of some of the heat treatable aluminum alloys. Fig 2 provides a relationship between heat input in Joules per centimeter and hardness profiles, which relate to tensile strength. We are able to see quite clearly in fig 2 that the higher the heat input the more substantial the reduction in strength of the base material adjacent to the weld.
Fig 1. There can be significant differences in the as welded and post weld heat treated strength of the heat treatable aluminum alloys
Fig 2. The higher the heat input during the welding operation the more pronounced the reduction in strength in the heat affected zone.
In order to meet the minimum tensile strength requirements of the code, we need to control closely our welding procedure to prevent overheating of the base material.First, we must consider the size of the test samples for welding. The code provides minimum dimensions for groove weld test plate size. You must comply with this requirement; in fact, if practical, use a larger test sample than specified. This will provide for superior heat sink and lower the possibility of excessive overheating and prolonged time at temperature within the heat-affected zone. Secondly, comply with the preheating and interpass temperature requirements of the code, which for this type of material specifies 250 deg F as the maximum preheat and interpass temperature. Also, observe the holding time at temperature requirement, which is not to exceed 15 minutes. If possible, conduct the certification testing without preheating, or at lower preheating temperatures, and allow the base material to cool to well below the maximum interpass temperature before welding resumes. A major contributor to the overall heat input of a weld is the travel speed during the welding process. For this reason, it is preferable to select a welding sequence and technique which makes use of faster stringer type weld beads as opposed to slower weaving techniques. The above recommendations apply to welding the 6061-T6 base materials with either a 4xxx series or a 5xxx series filler alloy, and regardless of shielding gas type or mixture used.