in Aluminum Welded Structures|
Strategies minimize nonuniform dimensional changes caused
by heat stress
BY TONY ANDERSON
Welding distortion can be defined as the nonuniform 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 principles
behind this reaction are fundamentally the same. However, when welding aluminum, compared to carbon steels, the effects of some of the main contributing factors for distortion are increased. Aluminum has high thermal conductivity, a property that substantially affects weldability. The thermal conductivity of aluminum is about 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 arc welding aluminum, high localized heating to the material in and around the weld area is applied. 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 ‹ 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 it changes its dimension almost twice as much as steel for the same temperature change.
Fig. 1 - Welding sequences can help minimize thermal distortion. Prior welds on small sections create a locking effect.
If a nonrestrained piece of metal in a furnace is evenly heated to a prescribed temperature and then allowed 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 this kind of uniform heating and cooling is applied to an unrestrained structure, the heating and cooling process should promote no distortion of the structure. Unfortunately, when arc welding, nonuniform localized heating is usually applied to the structure being welded. 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 nonuniform heating, resulting in nonuniform 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 causes of thermal distortion problems in welding.
Fig. 2 - Presetting components by an offset angle to compensate for weld shrinkage.
Ways to Reduce Distortion
Probably the most common cause of excessive distortion is from over welding. In order to reduce distortion, heating and shrinkage forces should be kept to a minimum. 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. Do not produce fillet welds that are larger than specified on engineering drawings.
Provide welders with fillet weld gauges so they are able to measure their welds to ensure they are not producing welds that are much larger than specified. With butt joints, always control edge preparation, fitup, and excessive weld buildup 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.
Try to use intermittent fillet welds where possible. Often adequate strength requirements can be maintained and the volume of welding reduced by 70%, if the design allows, by using intermittent fillet welds over continuous welding.
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 automated techniques are often capable of depositing accurate amounts of weld metal at extremely high speeds. Fortunately, with modern arc welding processes, it is often possible to use high welding speeds, which help fight distortion.
Use welding sequences or backstep welding to minimize distortion ‹ Fig. 1. 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 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 multipass welding. An even better method to control distortion is to weld both sides of a double-sided weld simultaneously.
Preset components so 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 you during the manufacturing process. Through experimentation, determine the correct amount of offset required to compensate for weld shrinkage. Then, only control of the size of the weld is needed to produce consistently aligned welded components ‹ Fig. 2.
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. Prebending can be combined with this technique by inserting spacers at suitable positions between the assemblies before clamping and welding ‹ Fig. 3.
Fig. 3 - Restraints and back-toback assembly can reduce distortion, and can be combined with prebending.
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 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, 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, there are some options available 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 welding and thereby reduce distortion. 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 ‹ and 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 those effects of the welding process that promote distortion.©
TONY ANDERSON (email@example.com) is Technical Services Manager for AlcoTec Wire Corp., Traverse City, Mich. He is Chairman of the AWS D10H Subcommittee on Aluminum Piping and D8G Subcommittee on Automotive Arc Welding ‹ Aluminum, Vice Chairman of the AWS D1G
Subcommittee 7 on Aluminum Structures, and Chairman of the Aluminum Association Advisory Committee for Welding and Joining.