Ian Scott, BSc, Msc, Grad WeldI, is a Senior Research Engineer in the Gas Shielded Section of the Arc Welding Department, and Rick Leggatt, MA, PhD, CEng, MWeldI, MIMechE, is Head of the Stress Analysis Section in the Fracture Department.
A new technique for eliminating distortion and residual stress problems caused by welding in thin material has been pioneered at the Beijing Aeronautical Manufacturing Technology Research Institute in China and has been further developed at The Welding Institute. A controlled background temperature distribution is created in the workpiece to counteract the shrinkage of the weld. This article gives an overview of the technique and demonstrates the dramatic reduction in distortion which can be obtained.
Buckling distortion due to welds made using conventional fusion welding processes has presented fabricators with problems for many years. This is particularly so when fabrication involves the use of thin section sheet materials typically in the range 0.5-8mm in thickness. These sheet materials are not inherently stiff enough to resist the contraction forces imposed by welding. This feature has imposed limitations on the design of components made in sheet materials and has restricted the application of fusion welding techniques in sheet metal fabrication.
The low stress non-distortion (LSND) welding technique * developed jointly by Beijing Aeronautical Manufacturing Technology Research Institute, China (BAMTRI), and The Welding Institute (TWI), actively controls welding distortion to ensure dimensionally consistent fabrication with low residual stress levels while still using (Conventional fusion welding processes.
* Patents applied for
The development of the technique has been undertaken by Dr Q Guan and his team from BAMTRI and has been further developed at The Welding Institute. Dr Guan has spent a year at The Welding Institute as part of a scientific and technical exchange programme with The People's Republic of China organised by The Royal Society.
Low stress non-distortion welding (LSND)
Low stress non-distortion welding is a method for in-process control of welding stresses and distortion during the manufacture of thin walled structural elements. The process involves creating an initially uneven temperature distribution across the workpiece as shown in Fig. 1.
Restraining forces are applied to the workpieces in order to resist the out-of-plane buckling which is caused by both the heating and welding operations.
A cross sectional view of the equipment is shown in Fig. 2. The technique ensures that welds are produced with low residual stress levels and complete freedom from weld distortion. It can be applied to any weldable material and is applicable to both butt welds and fillet T type welds.
- cooled backing bar
- heating elements
- welding heat source
- Workpieces
- LSND clamp
- Mandrel
The incorporation of the LSND technique into normal manufacturing operations ensures that distortion free weldments are produced. It allows the fabricator to meet strict dimensional requirements without having to undertake costly reworking operations.
Equipment for LSND welding
In order to carry out LSND welding an appropriate temperature distribution must be set up across the weld area in addition to the special clamping arrangements. This is achieved by one of two methods. The first is to incorporate the equipment into a welding fixture. This involves modifying commercial systems by introducing thermostatically controlled heating and cooling elements beneath the appropriate region on either side of the joint. Equipment for LSND welding has been constructed and is shown in Fig. 3. The temperature distribution required for LSND welding is established in a short period of time (typically three minutes) but this period is dependent upon material thickness and thermal properties as well as the rate at which heat can be applied to the material.
A second method is envisaged which will be more applicable to components with long welds or to fabrications which cannot readily be put into a welding fixture. This equipment will either use a mobile LSND apparatus moving ahead of the welding heat source, or will allow the component to be fed through the LSND fixture. When welding takes place, the correct temperature distribution will have been established and distortion free weldments will be produced.
Applications for LSND welding techniques
The LSND technique can be applied to any material which can be fusion welded. The benefits of the technique are greatest in those materials which distort the most, such as titanium and aluminium alloys and austenitic stainless steels. It is of particular benefit when welding large thin section sheet metal components which must otherwise be subject to straightening or correction after welding.
Areas of potential application for LSND welding include:
- Rocket and missile fuel tanks and casing
- Hull structures in shipbuilding and hovercraft
- Gas turbine ducting
- Thin walled nuclear containments
- Carriage and container fabrication
- Stiffened panels and beam fabrication
- General sheet metal fabrication
Using LSND welding techniques
One of the principal benefits of LSND welding is that it allows well established welding processes to be used with reduced risk of distortion. The technique can be used with TIG, MMA, plasma and MIG processes. There is also the possibility that the LSND technique could permit the use of submerged-arc welding in thinner section materials than is usually practised. Because of the way in which the temperature distribution is established, there is little change in welding parameters from those used during conventionally jigged welding. The temperatures involved during the preheating and cooling operations are not sufficiently high to affect adversely the metallurgical properties of heat sensitive alloys.
Results
Photographs of aluminium and stainless steel specimens after TIG welding arc shown in Fig. 4 and 5. These photographs show that LSND welding produces distortion free workplaces, compared with the severely buckled specimens resulting from conventional welding using the same parameters. The investigation carried out at TWI on butt welds in aluminium and austenitic stainless steel has demonstrated the reduction in distortion that is achieved by using this technique. The residual stress distribution across the weld area is considerably reduced compared with welds produced under conventional welding conditions. The peak tensile stress is typically reduced by more than two thirds of its usual value as shown in Fig. 6. This will reduce the tendency for components to distort in service. In connection with modern welding equipment, it will enable thinner section materials to be welded with freedom from distortion. It should be possible to weld successfully materials in which weld metal cracking may occur due to weld shrinkage. Because of the 'active' control of stress, the forces which lead to solidification cracking will be considerably reduced.
Industrial application of LSND welding
The LSND technique represents a new approach to distortion and stress control on thin section structural elements. The technique should be considered at the design stage in order to permit weight reduction resulting from the use of thinner materials, and time saving by the elimination of post-weld correction of distortion. The introduction of this technique into a new generation of welding fixtures, either static or mobile, will allow the designer greater freedom of choice in materials selection, design and location of welded joints.
