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critical on thin panels because of the large inuence of weld size on shrinkage forces that cause buckling. Control of part t-up includes attention to the quality and handling of plates and stieners to prevent damage and distortion of the components that can occur even prior to welding. e accuracy of thermal cutting operations also is critical to the dimensional accuracy of component t-up. Dont over weld. Over welding contributes to distortion by increas- ing thermally induced stresses and shrinkages. Doubling a llet weld leg size increases weld volume and heat input by a factor of four, thereby increasing all types of distortion including buckling. Figure 2 shows that the applied weld load for llet welding of stieners var- ies approximately linearly with the heat input and the square of the weld leg length (weld size). Size control applies to tack welding oper- ations as well since over-sized tack welds can cause distortion and also lead to over-sized welds. Minimize welding heat input. Increased heat input increases shrinkage and all types of distortion for all types of weld joints. Control of heat input is particularly important for thin panels because, as shown in Figure 2, the applied weld load that drives buckling increases with the heat input. erefore selection of weld- ing processes and procedures that reduce heat input as well as the careful control of welding parameters in production can have major benets in reducing buckling distortion. Maximize restraint. Restraining the part to limit movement and add resistance to shrinkage forces can help reduce all types of distortion and is critical to helping control buckling of thin panels. Panels should be welded on a solid foundation that provides support for the entire panel and permits free edges to be restrained from out-of-plane move- ment. Clamps, jigs, xtures, and weights can be used to hold panels down to the solid foundation at the edges as well as between stieners. Egg crate construction. The egg crate construction technique involves tack welding or welding the longitudinal and trans- verse stiffeners together to form a frame structure that is then tack welded to the plate. The technique is useful as a distortion control technique because the stiffener assembly is rigid, and partitions the plate into small areas that have a higher resis- tance to buckling. Establish assembly and welding sequences. Operational sequences should be established and followed. is includes sequences for t- ting and tacking operations as well as assembly and welding. For large stiened panels where egg-crate construction is not employed, the recommended practice is to tack weld all longitudinal stieners in place prior to welding the rst stiener. en weld the stiener nearest the center of the panel rst, followed by welding stieners on alter- nating sides working from the center of the plate toward the edges. Control each production operation. A key ingredient in distortion control is to have specic procedures for each step of the production operation and to control the use of these procedures. is includes gathering data on the accuracy of production parts and implement- ing a continuous improvement program to correct deciencies and make advancements. Special techniques. Transient thermal tensioning has been shown to be an eective means of reducing or eliminating buckling distortion. is process uses heat sources that move with the welding process to create stress patterns that counteract those produced by the weld. e use of mechanized, automatic, or robotic welding equip- ment can help produce consistently small-sized welds. And a given weld size made at high speed will tend to distort less than a weld made at slow speeds, because the heat input is generally less at higher speeds. Also, very low heat input welding processes, includ- ing laser beam welding, may have major advantages in reducing heat input and distortion for thin panels. MTHarvey Castner is senior project manager at Columbus, Ohio-based Edison Welding Institute. Randy Dull is engineering technology leader at Edison Welding Institute. . X . / PAGE VERTICAL April 2011www.sname.org/sname/mt (mt notes) Welding Distortion continued