Chapter # 4                                                                                        PROCESSES

4.2   Welding

Three different metals that are subjected to welding in the manufacture of  Busbar Systems

Steel                           Aluminium                           Copper

Welding technology is a very specialized field.  There are several welding techniques that are very sophisticated and customized for various industries.  Only the most commonly used welding processes that are relevant to the manufacture of the Busbar Systems, and adequate for the purposes, are described.  Specialized processes such as friction welding, explosion welding, electroslag welding, ultrasonic welding, electron beam welding, laser beam welding, and other methods of welding are not discussed.

There are guidelines for effecting the welding of Steel, Aluminium, and Copper.  These include the type of edge preparation, root gap, preheating temperature, the diameter of filler rods, gas nozzle diameter, type of gas, gas flow, welding electrical parameters, etc.  Every industry must develop its welding manual depending upon the nature of the job and ambient conditions.  Inputs from the welding machine manufacturer and welder certification agency will help develop in-house procedures.

Welding jigs & fixtures are industry and product-specific.  While some universal fixtures and manipulators are available in the market, most industries develop their fixtures based on their specific needs.  Robotic welding can be employed for repetitive jobs and all welding parameters can be frozen. A variety of welding is still carried out manually in manufacturers’ premises.  Welding at the site is almost always manual.  Despite all the written inputs, the quality of manual welding depends on the skill & experience of the welder.

The certification of welders is a well-defined and regulated activity.

4.2.1   Steel Welding

Steel is used in the Busbar Systems for the manufacture of indoor & outdoor Support-Steel structures and enclosures of non-segregated & segregated phase bus.  Depending upon the thickness of metal, two types of welding processes are generally adopted:  Manual Metal Arc (MMA) and metal active gas (MAG), also known as  welding.

Steel sections, used as Support-Steel structures, are heavy, with a thickness of 6 mm (1/4”) or more. Steel sections, used for the manufacture of enclosures of Busduct are much thinner, with section thicknesses varying from 3mm (1/8”) to less.  (Stainless Steel has been used as an enclosure material but such requirements are extremely rare). 

Steel structures are often galvanized after fabrication.  The quality of the weld is important to ensure good galvanization.  (It is necessary to design the support structure so that welding and subsequent galvanizing can be carried out most efficiently.  In a good design, molten Zinc should flow uniformly over the surface and not get trapped in the crevices during the dipping process.)  A good weld will ensure that there are no surface defects and there is no possibility of chemical entrapment during pickling operation, before galvanizing

MAG (Metal Active Gas ) welding is commonly used for welding thin sections of sheets.  Welding of thin sheets requires skill as it not only expands the steel but also distorts it.

 

4.2.2   Aluminium Welding

Welding of Aluminium sheets & bars is now an established practice in the Bus industry.  Aluminium laminates are also manufactured by welding the solid tabs to either end of pre-fabricated laminates. 

Aluminium welding is extensively adopted for the manufacture of conductors and enclosures for isolated phase bus, segregated & non-segregated phase bus, gas-insulated bus, high voltage open bus in Switchyards, and high current open bus DC connections in metal extraction plants.  Many of these products require significant amount of welding at site.

Tungsten inert gas (TIG) welding and metal inert gas (MIG) welding are adopted for the welding of Aluminium.  TIG welding is generally limited to thin sections up to 3 mm (1/8^th”), while MIG welding is carried out for thicker sections. Because of the low melting point of Aluminium, pre-heating is not required for MIG welding unless the welding is being carried out under low ambient temperature and the sections are large.

4.2.3   Copper Welding

Copper welding in the manufacture of Busbar Systems is now limited to welding tabs to meet the requirements of terminations & orientations.  A bolted joint is an alternative solution.  However, every bolted joint may require plating, fixings, boots, and accessibility to reach the location, all of which will be far more expensive than welding.

Copper laminates were fabricated like Aluminium laminates.  Solid tabs were welded to a stack of laminates.  With the development of resistance weld technology, which gives much better results, this method of making laminates has now become obsolete.  However, site welding of laminates to terminals in arc furnaces (and similar applications), is still being carried out.

TIG welding and MIG welding are adopted for the welding of Copper.  TIG welding is generally limited to thin sections while MIG welding is carried out on thicker sections.  Because of the high melting point and very high thermal conductivity, to obtain a satisfactory weld, it may be necessary to pre-heat the conductor, before welding.  The preheating temperature will depend upon the thickness of the conductor.  It is recommended to pre-heat the conductor of thickness over 3 mm.  For fusion to take place, there must be an adequate weld pool.  Pre-heating will lead to a change in the mechanical properties, especially on cold worked conductors.

Welding of Copper is limited to flat & round bars and round tubes.  The welding procedures for these are well established and documented.  Welding of Copper at the site is rare and not recommended.  Welding of Copper is far more difficult than welding of Aluminium.  The main reason being the higher melting point and thermal conductivity of Copper.

Continued..........