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Distortion in aluminum weldments
Q: In the past I have fabricated some fairly complex steel structures that have, on occasion, presented problems with distortion. I am now considering moving to aluminum as a replacement material for some of these structures. My questions are, what amount of distortion can I expect to see with aluminum? And how can I prevent it?
A: We need to consider the principal reasons for distortion in any arc-welded structure, and some of the characteristics of aluminum specifically. Welding distortion can be defined as "the non-uniform expansion and contraction of weld metal and adjacent base metal during the heating and cooling cycle of the welding process". Distortion is a consideration when arc welding all materials, and the principals behind this reaction are fundamentally the same.
If we evenly heat a non-restrained piece of metal in a furnace to a prescribed temperature and then allow it to cool to its original temperature, it will first expand (to a degree based on its coefficient of expansion) and then contract as cooled to its original size. If we apply this kind of uniform heating and cooling to an unrestrained structure, the heating and cooling process should promote no distortion of the structure. Unfortunately, when arc welding, we are usually applying non-uniform localized heating to the structure which we are welding. This heating is limited to the area of the weld and its close vicinity. Also, the heating and cooling is conducted under varying amounts of restraint during the welding process. The part of the welded component outside of the weld area that is not heated, or heated to a much lower temperature, acts as a restraint on the portion that is heated to the higher temperatures and undergoes higher expansion. The non-uniform heating, resulting in non-uniform expansion and contraction, along with weld metal and base metal shrinkage, and the partial restraint from the less affected parts of the structure are the primary cause of thermal distortion problems that occur in welding.
Theoretically, when welding aluminum compared to carbon steels, the effects of some of the main contributing factors for distortion may be somewhat increased. Aluminum has high thermal conductivity; this being a property that may affect distortion and can substantially affect weldability. The thermal conductivity of aluminum is around five times that of low-carbon steel. Aluminum also has high solidification shrinkage, around 6% by volume, and also a high coefficient of thermal expansion. When we arc weld aluminum, we apply high localized heating to the material in and around the weld area. There is a direct relationship between the amount of temperature change and the change in dimension of a material when heated. This change is based on the coefficient of expansion. This is the measure of the linear increase per unit length based on the change in temperature of the material. Aluminum has one of the highest coefficient of expansion ratios, and changes in dimension almost twice that of steel for the same temperature change. However, it is not uncommon to apply higher material thickness to a comparable aluminum structure when compared to steel. This is a design consideration that may be used to provide the necessary rigidity and/or required strength. Because aluminum is approximately 1/3 the weight of steel, we could, in fact, double the original design thickness for our aluminum structure and still have only 2/3 the weight of the original structure made of steel. The significance of such an increase in material thickness would be a substantial reduction in the potential for distortion.
What methods can we employ to reduce distortion?
The methods used for the control of distortion when welding aluminum are the same as other materials; however dependent on material thickness and structure design, we may need to give greater consideration to the following:
- Probably the most common cause of excessive distortion is from over welding. In order to reduce distortion, we should try to keep the heating and shrinkage forces to a minimum. We should design the weldment to contain only the amount of welding necessary to fulfill its service requirements. The correct sizing of fillet welds to match the service requirement of the joint can help to reduce distortion. We should not produce fillet welds that are larger than specified on engineering drawings. We should provide welders with fillet weld gauges so they are able to measure their welds to ensure that they are not producing welds that are much larger than that specified. With butt joints we should control edge preparation, fit-up and excessive weld build-up on the surface in order to minimize the amount of weld metal deposited and thereby reduce heating and shrinkage.
- When welding thicker material, a double-V-groove joint requires about half the weld metal of a single-V-groove joint and is an effective method of reducing distortion. Changing to a J-groove or a U-groove preparation can also assist by reducing weld metal requirements in the joint.
- We may consider the use of intermittent fillet welds, where possible. We can often maintain adequate strength requirements and reduce the volume of welding by 70% by using intermittent fillet welds over continuous welding, if the design allows.
- Balance welding around and position welds near to the neutral axis of the welded structure. The neutral axis is the center of gravity of the cross section of the part. Placing similarly sized welds on either side of this natural centerline can balance one shrinkage force against another. Placing the weld close to the neutral axis of the structure may reduce distortion by providing less leverage for shrinkage stresses to move the structure out of alignment.
- Reduce the number of weld beads, if possible. Few passes with a large electrode are preferable to many passes with a small electrode. The additional applications of heat can cause more angular distortion in multipass single fillet welds and multipass single-V-groove welds.
- Carefully select the welding process to be used. Use a process that can provide the highest welding speeds and is able to make the weld in the least amount of weld passes. Make use of automated welding, whenever possible, as these techniques are often capable of depositing accurate amounts of weld metal at extremely high speeds. Fortunately, with modern arc welding processes we are often able to use high welding speeds which can help us when fighting distortion.
- Use welding sequences or backstep welding to minimize distortion. The backstep technique allows for the general welding progression to be in one direction but enables us to deposit each smaller section of weld in the opposite direction. This provides us the ability to use prior welds as a locking effect for successive weld deposits.
- Whenever possible, weld from the center outward on a joint or structure. Wherever possible, alternate sides for successive passes on double-sided multi-pass welding. An even better method to control distortion is to weld both sides of a double-sided weld simultaneously.
- Preset components so that they will move during welding to the desired shape or position after weld shrinkage. This is a method of using the shrinkage stresses to work for us during the manufacturing process. Through experimentation we can determine the correct amount of offset required to compensate for weld shrinkage. We then need only to control the size of the weld in order to produce consistently aligned welded components.
- Consider the use of restraints such as clamps, jigs and fixtures and back-to-back assembly. Locking the weldment in place with clamps fixed to a solid base plate to hold the weldment in position and prevent movement during welding is a common method of combating distortion. Another method is to place two weldments back-to-back and clamp them tightly together. The welding is completed on both assemblies and allowed to cool before the clamps are removed. Pre-bending can be combined with this technique by inserting spacers at suitable positions between the assemblies before clamping and welding.
- Consider the use of aluminum extrusions. Aluminum can be easily acquired in standard and customized extruded configurations. Many manufacturers are taking advantage of extruded aluminum sections to reduce the amount of welding in their fabricated components. Extruded aluminum offers a perfect opportunity to reduce welding (potential for distortion), assist with assembly and often improve aesthetics.
Weld distortion is caused by localized expansion and contraction of metal as it is heated and cooled during the welding process. Constraint from the unheated surrounding metal produces permanent changes in the internal tension stresses that are generated. If these stresses are high enough and cannot be adequately resisted by the structure, distortion results. A large number of factors determine what stress levels are developed, their orientation, and whether they will cause unacceptable distortion. These factors include the size and the shape of the welds and where they are located in the structure being welded, the rate of heat input during the welding process, the size and material thickness of the components being welded, the assembly sequence, the welding sequence and others. Ideally to avoid distortion, there should be as little welding as possible in a structure, and especially where thin gauge metal is involved. With aluminum we have some options that are available to us at the design stage that may help to eliminate excessive welding. The use of castings, extrusions, forgings and bent or roll-formed shapes can often help to minimize the amount of welding and thereby reduce distortion.
One method of understanding and planning for distortion prevention is the use of specialized computer software (see fig1 and fig2). Computer software has been developed as a tool to understand and predict distortions caused by the welding processes. This software is presented as being able to predict residual stresses and distortions after welding thus allowing welding engineers the opportunity to optimize their process (weld sequence and/or clamping condition).
Many complex aluminum structures are welded every day without excessive distortion problems. This is often achieved through the combined effort of designers and manufacturers. The designers need to carefully consider options that are available to help reduce the amount of welding within the structure. Also, to position those welds that are necessary in areas that least promote distortion. The manufacturer needs to develop, employ and control the necessary equipment (welding process, fixturing, etc.) and techniques (welding sequences and balancing methods) to reduce the effects of the welding process that promote distortion.
Ark blue, which shows little if any distortion, to red, which indicates maximum distortion within the structure.
Pictures courtesy of SYSWELDÒ a product of the ESI Group.