What is Distortion in Welding?
Distortion is the general term for the component’s differences from the intended shape after heat input and cooling to room temperature. Distortion is being initiated by plastic deformations due to local and timely differencing heat expansion and shrinkage. Shrinkage is seen as the contraction of the, for example, heated area of a component by welding during cooling.
For prevention or reduction of distortions and residual stresses a distinction is made between preventive applied measures (during engineering and partly during manufacturing) and subsequently applied measures.
For economic manufacturing, it is always beneficial to already consider the initiation of distortions and residual stresses at the engineering phase. However, the quality of manufacturing is important.
The quality of welds depends, among others, on:
1. the base metal
2. plate thickness
3. the welding process
4. the welding fixtures and
5. the craftsmanship of the welder.
From this respective, continuous cooperation between engineering and manufacturing should exist.
Control of Distortion in Aluminium Welding
In assembling and joining parts of a structure or of built-up members, and in welding reinforcing parts to members, the WPS and sequence shall be such as will minimize distortion and shrinkage. Joints expected to have significant shrinkage should usually be welded before joints expected to have lesser shrinkage.
Insofar as practicable, all welds shall be deposited in a sequence that will balance the applied heat of welding while the welding progresses. When required by the contract documents, the Contractor shall prepare a welding sequence specification for a member or structure which, in conjunction with the WPS and overall fabrication methods, will produce members or structures meeting the quality requirements specified.
2. Direction of Welding.
The direction of the general progression in welding on a member shall be from points where the parts are relatively fixed in position with respect to each other toward points where they have greater relative freedom of movement.
3. Crack Prevention.
In making welds under conditions of severe external shrinkage restraint, the welding shall be completed or completed to a point that will ensure freedom from cracking, before the joint is allowed to cool completely.
Members distorted by welding shall be straightened at ambient temperature by mechanical means or by carefully supervised application of a controlled amount of localized heat in conjunction with mechanical means. If localized heating is to be applied in any straightening operation, the complete procedure shall be filed with and approved by the Engineer. This procedure shall adhere to the maximum temperature values in the below Table and the following: Except for those stresses resulting from the mechanical straightening method used in conjunction with the application of heat, the part to be heated for straightening shall be substantially free of stress and from external forces. Maximum holding times for the forming and straightening of aluminum alloys at various temperatures are given in the below table.
5. Heat input
The heat input can be controlled/minimized by the choice of an appropriate welding process. The weld settings also are of influence. For example, during manual metal arc welding the choice of electrode type, the electrode diameter and the welding current are to be considered.
Along with the choice of welding process also the preheating as heat-input should be considered. It is specifically being applied, among others, to reduce the extent and the enlargement of the residual stresses and therefore to reduce the distortion. I.e. Preheating is beneficial according to the following reasons:
- Reducing stress peaks in thick-walled components
- Prevention of hardening and stress cracking of high strength steel and alloyed steel by reducing the cooling speed.
- improvement of the fusion of base materials with high thermal heat conductivity ( aluminium, copper)
- reducing the heat input and the HAZ
- welding under low temperature / cold weather conditions
- Reducing the residual stresses of base materials with limited ductility (for example steel casting)
It is recommended to apply a uniform heating on both sides of the weld joint. The required preheating is to be determined in dependency of the chemical analysis, the heat input, the heat conductivity, material thickness and the type of joint.
Also the design of the weld joints and the welding sequences are important for the heat input. For example, in case of thin sheet metal constructions with long welds it useful to apply intermittent welding in order to prevent unnecessary distortion.
6. Pre-setting /pre-bending
One possibility in order to guarantee a free distortion after welding is for example applying a pre-setting or pre-bending of the component in the opposite direction of the angle shrinkage /distortion to be expected. This can be applied to as well as V butt welds as for fillet welds. Pre-setting of sheet metals in the opposite direction of the expected angle deformation of butt welds.
6. Clamping / pre-clamping
The earlier mentioned measures of pre-setting or pre-bending counteract the expected distortion. Due to the free shrinkage high stresses are not being generated. Another possibility for preventing angle distortion is by means of clamping or pre-clamping. In doing so the components are placed into a fixture in a way that they can move in the direction of the plane (free transverse shrinking of the weld!) but lifting out of this plane (angle distortion) is being prevented by holding-down clamps. Such fixtures however have to be deformation rigid and be able to take huge forces.
This can either be achieved by clamping ) a component or by mounting several components to each other e.g. back-to-back assembly.