TWI Industrial Member Report Summary 902/2008
By R Saraswat
Distortion of welded fabrications primarily occurs due to forces produced by differential contraction of different locations as material solidifies and cools from the non-uniform heating produced by welding. Fabrications of thin material are more prone to distortion than thicker fabrications because there is less stiffnesss against out of plane bending moments and lower resistance to buckling. Industry sectors like shipbuilding, automotive and aerospace are particularly affected since their components are fabricated from thin sheets and plates.
In a thin sheet compressive forces are produced in the sheet adjacent to the weld as a result of the mismatch in longitudinal contraction between the weld metal and the parent sheet. If the compressive forces are too high they can cause the sheet to buckle. The aim of low stress no distortion (LSND) techniques is to reduce plastic strains that are generated by contraction and phase transformations in and near to the weld and thereby reduce the tendency for buckling, and for out of plane distortion that can be produced as a result of differential through-wall strains in V prep butt welds.
There are several LSND techniques available. These include thermal tensioning, auxillary cooling, and mechanical restraining. The present work is about the auxiliary cooling LSND technique, where atomised water is sprayed on the weld area through a nozzle trailing the welding arc. In order to use the technique effectively there is a need to determine the LSND parameters that minimise distortion. In this work the optimum LSND parameters are investigated for a butt joint between aluminium alloy plates using a FE model to simulate the thermo-mechanical conditions of the welding. It extends earlier CRP work (Bagshaw et al, 2003), (Bagshaw, 2004) on LSND welds joining steel plates.
The development of a moving heat and cooling source model to simulate the LSND welding is quite time-consuming. The commercial FE based solvers are usually quite generic and not well suited for weld modelling, particularly where structures have multiple welds. A major part of this work was the development of graphical user interface (GUI) software to simplify the setting up of a single pass weld model and speed up the solution time. This can scale up very well for LSND modelling in complex structures with a lot of welds.
- Simulate LSND welding of a butt joint in an aluminium alloy using a FE model for the prediction of parameters optimised to minimise distortion.
- Develop software to simplify the process of setting up a weld model
- Use the software to simulate LSND welding in a plate with multiple stiffeners.