Energy saving in welding - a greener approach

TWI Bulletin, May/June 1991

In 1986 she joined the CEGB to investigate the microstructural properties of materials and components for steam turbine applications. Jackie is a Chartered Engineer, a Member of the Institute of Metals and a Member of the Institute of Physics.

Jim Foster joined the Manufacturing Systems Department in 1986 after a number of years in industry. His past experience in manufacturing engineering has covered the manufacture of computer systems, motor vehicle brakes and clutches and radio communications equipment.

Since moving to TWI he has worked on the application of vision to control of the TIG process, the design and construction of TWI's 10kW laser beam manipulation system and the introduction of integrated manufacture to the constructional steelwork industry. Latterly his work has concentrated on factory communication systems.

Though welding technology is highly developed, few of its theorists or practitioners have paid more than scant attention to the economic or ecological costs of respective welding techniques. Today the story is changing. Jackie Maguire of the Energy Efficiency Office and Jim Foster of TWI examine the energy considerations of some of the key processes.

Until recently, the development of welding technology has been concerned with the quality of welds, rather than energy wastage or the comparative energy efficiency of various processes. Productivity and welding costs were well down the priorities list when the main consideration was being able to produce a weld at all.

Only in the past 20 years have alternative technical solutions become available for which production and economic comparisons can be made. Here some of these successful developments are discussed (including friction welding, high power electron beam welding and laser welding), with a view to establishing and comparing their total energy consumptions.

Energy consumption

This approach can lead to confusion when processes are compared for which energy consumption has been expressed differently. Also, existing approaches do not reflect total energy requirements accurately. So to calculate the energy used in each process, it is necessary to define the energy consuming stages.

Weld production consumes energy in three distinct ways:

Primary - the joint is heated to produce a satisfactory weld.
Secondary - services and equipment are operated which:

• are necessary for the process to happen, such as power supply units, wire feed motors and vacuum pumps;
• prepare and maintain workpieces or fillers, such as machines for edge preparation or drying ovens;
• ensure operator safety, like fume extractors;
• manipulate welding equipment - a robot or gantry;
• ensure satisfactory metallurgical conditions (preheating and post-heating)

Ancillary - production of all materials and equipment used in the welding process. Consumption of energy in this way is not considered in this article. Table 1 compares the energy consumption of several welding processes. Further details of the calculation are given in Table 2.

TABLE 1 Comparison of overall welding performance

TABLE 2 Build-up of overall energy consumption

* Includes transmission loss    † Includes pumpdown    ** Hydraulic powerpack    (F) Factored from standard data for 25mm material

Potential for savings

Arc welding

In arc welding the bevel angles of plate edges in butt joints have an important effect on the amount of parent metal to be removed and additional weld metal required. For example, an increase in bevel angle from 30° to 35° requires an additional 20% of weld metal to fill the joint with attendant energy consumption.

Similarly, considerable amounts of weld metal and energy are wasted when a fillet weld is over-welded. If, for example, a fillet weld specified as 6mm leg length is increased to 8mm during fabrication - not uncommon - over 75% excess weld metal, and also energy, is used. There is therefore considerable scope for saving energy by designing fabrications to have the minimum welding required and ensuring that this specification is adhered to in manufacture.

Further saving can be achieved by using a larger electrode and increasing the current accordingly. By increasing electrode diameter from 4 to 5 mm, and appropriately increasing the current, a 33% increase in deposition rate can be achieved. This in a typical case can produce an energy saving of approximately 25%.

The best deposition rate is achieved when welding in a flat position, because a larger pool of molten weld metal can be maintained. It can therefore be worthwhile to manipulate a workpiece so that it can be worked on more effectively. Depending on the shape and size of the component involved, the overall rate of welding can be doubled or even trebled by this means. Productivity can also be increased in submerged - arc welding by replacing the single filler wire by two smaller diameter wires sharing the same current. The increase in current density in the wires can produce a 20% increase in deposition rate, with corresponding energy savings.

Electron beam welding

The two main users of electrical energy in electron beam welding are the high voltage supply - where a major loss of efficiency occurs when a motor generator is used for control - and the pumping system. At present, many applications require a full vacuum for satisfactory results, though clearly less energy is required for partial-vacuum or non-vacuum working where this produces acceptable results. To extract air from the welding chamber, most engineers prefer the tried and tested diffusion pump, in spite of its relative electrical inefficiency.

Wherever possible, it would pay to use non-vacuum methods. This, however, does not give the same deep, narrow welds as vacuum or partial-vacuum methods and is at present used only in the US automobile industry, for high speed, low penetration applications. The situation will, it is hoped, change as a result of a programme at TWI which aims to increase the depth of penetration of the process so that it can be used in a much wider range of industries. The energy saving compared with existing arc welding methods will also be considerable if, as is anticipated, a penetration of 100mm at about 400mm a minute proves to be possible.

Laser welding

The efficiency with which a laser beam may be generated is low, and there is a lengthy warm-up period of 10-15 minutes. It is also necessary to keep such systems on 'stand-by' mode when not in use since the electrical disturbance of switching on and off is liable to cause damage. While carbon monoxide lasers offer theoretical benefits over currently evolved carbon dioxide systems, it seems likely that the overall efficiency of lasers will remain low in the next decade. Nevertheless the technical benefits of laser welding make the process very desirable, and there is scope for recovering energy in the form of warm water which could be used for heating buildings, for example. Attention is also being given to maximising beam usage by more efficient work practices and computer control.

Efforts are also being concentrated on improving the efficiency of beam transmission by:

• reducing the number of mirrors involved in focusing the beam;
• optimising the lens type to produce a smaller, more intense focal spot;
• protecting the beam from scatter-producing airborne contaminants by enclosing it and pressurising it with filtered air.

Resistance welding

Magnetically impelled arc butt (MIAB) welding

Overall efficiencies in MIAB systems are better than conventional arc welding processes, as welding times are short and all heat is generated at the workpieces. The greatest potential for energy saving lies in increased efficiency in converting the electrical input to electrical arc and magnetic energy. Further improvements may come from exploring the effects of a vertical rather than the customary horizontal welding position, particularly to deal with low alloy aluminium and other low strength alloys. Automatic flash removal can also lead to significant time and energy savings.

Friction welding

Japan has been ahead of Europe in appreciating the comparative advantages of friction welding over power beam systems and arc techniques for certain applications. Further savings in energy and time could be achieved by using a pulsing action during the friction force stage, or by reducing the rotational speed in stages in continuous drive friction welding. Linear and orbital motions also offer scope for wider applications.

Conclusions

While laser and electron beam methods promise energy savings through improved productivity and are therefore likely to increase their currently small proportion of the market, there is an urgent need for measures to improve power source efficiency, to make effective use of energy extracted and to minimise idling losses.

Investment would be justified in the forge welding techniques of friction and MIAB welding, which offer considerable potential for energy efficiency across a broad range of industrial applications.

There is a need for more detailed and accurate data to be collected to assist in the selection of appropriate processes and techniques, and an in-depth survey of current (and planned) welding practices in UK industry. Techniques for minimising idling losses could be of great benefit if they were researched, developed and promoted, as could the use of energy-efficient joint designs and more economical workshop practices. In electron beam welding, there is scope for development of non-vacuum systems, and the use of waste heat in laser applications is not yet fully exploited.

Where there are technical problems to be resolved, these could be addressed through a co-ordinated programme of research spread over the appropriate centres, and also by industry-led initiatives stemming from commercial pressures and perceived opportunities.

A comprehensive training programme backed by working examples could change attitudes towards energy efficiency at all levels of industry. Such a programme could embrace seminars, exhibitions, training on and off site and published articles. Opportunities will need to be made available to test specific solutions in the workplace so that the benefits of energy saving can be fully appreciated across the industry.

Companies interested in pursuing the opportunities for energy efficiency in welding are invited to contact Jim Foster at TWI.