Preventing Distortion
There are three main ways to prevent and control distortion – prevention by design, modelling and prediction, and more advanced methods such as low-stress no distortion welding (LSND).
Prevention by design includes joint design and preparation, welding procedure selection, welding sequence selection, the use of jigs, clamps and strongbacks, and material selection. Weld position can also help prevent distortion by using intermittent welding or welding along a neutral axis. This requires pre-setting and fitting-up, with the required angles determined by experiment or experience. When selecting a welding procedure, it is worth noting that concentrated heat sources, such as with TIG welding, produce less distortion, while fewer passes and a shorter welding time also reduce cumulative shrinkage effects. With regard to clamping, the use of flexible clamps can help maintain the root gap and back-to-back clamping balances shrinkage forces in mirror image fabrications. Bolted or tongue and groove clamps are particularly suited to thin sheet metal welding, making them useful for aluminium welding. Pre-bending can also be used to counteract shrinkage during welding.
Modelling and prediction involves weld sequence optimisation, distortion prediction and thermal-mechanical modelling. Modelling and prediction uses thermal-mechanical models to account for transient temperature profiles, using temperatures to drive thermal expansion and reaction against constraints and calculating the accumulated distortion and residual stress. This allows for the prediction of distortion, the effects of post-weld heat treatments and the optimal weld sequence.
Low-Stress No Distortion welding is usually applied to thin materials and uses a variety of methods. One technique is to cool immediately after deposition, while another method involves heating and cooling different zones. Thermal tensioning and mechanical restraint can also be incorporated into LSND.