Manufacturing Processes - Different Fusion Welding Processes
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Fusion welding belongs to the general manufacturing processes category of joining. One of the original welding techniques used was gas welding the best known of which is called Oxy- Acetylene welding.
The basic principle behind all welding processes is to produce a molten pool in the locality of the joint being formed and to add a filler material (if required) as the materials from either side of the joint run into one another forming a re-enforced joint that is completely penetrated and fuses the mating parts together. A joint that is not fully penetrated by the molten pool would be weaker than the parent material.
The primary techniques used for fusion welding are:
- Gas welding, where the melt pool is supplemented by a filler rod and the combustion gases prevent oxidation of the joint during forming.
- Metal Arc, sometimes known as electric arc welding or manual metal arc (MMA). The welding rod is consumed as the metal core joins the melt pool. The rod is coated in a flux that melts at a slower rate than the metal core which aids in directing the arc and the gas shield it produces in the process as it melts. The flux also leaves a protective slag layer over the weld which can be chipped off once the weld has cooled. Stating the obvious both components need to be electrically conductive so that the electric arc that forms between the material being joined and the filler rod can be formed.
- Metal Inert Gas (MIG). This is a metal arc gas shielded process where a continuous wire electrode is fed to the joint through an inert gas shield. Both rod and gas are delivered through the welding torch nozzle. The gas is usually argon and is directed by a cowl at the end of the torch.
- Tungsten Inert Gas (TIG). This can be likened to gas welding in technique but using an electric arc to create the molten pool and an insert gas to protect the material being welded. The tungsten rod is not consumed in this case, a separate filler rod is fed into the molten pool, much like gas welding, to supplement the pool and ensure penetration and strength of the joint.
- Hot Gas Welding, used for welding thermoplastics. A force is usually required to facilitate mixing of the softened material.
General considerations that need to be taken into account with all welding processes are that for a successful joint that reaches or exceeds the strength of the material being welded are that full penetration of the molten pool must be achieved. In order to do that the critical parameters that need to met are:
- the angle of approach to the joint to be welded
- the distance of the torch from the joint being formed
- the rate of feed of the filler material
- the rate of advancement of the torch along the joint being welded
- an appropriate level of heat energy for the size of the joint being formed
- the use of suitable fluxes and or gas protection for the material being welded
- joints should be scrupulously clean
- the material in the joints should be in close contact
- where possible tack weld along the joint to be formed i.e. small tacks equispaced along the joint which helps prevent the joint deforming during the final weld
Each process has different specific requirements which need to be considered, for example the TIG welding process will use different types of tungsten rod for different materials. Typically you use a thoriated rod for stainless steels and a zirconiated rod for aluminium. When welding aluminium you also need to set the welder to an alternating current in order to use the oxide lift function, assuming your welder affords that facility.
Setting up a welding facility in terms of equipment is relatively low, the exception being TIG welding rigs and for some specialist welding techniques such as induction welding.
All welding techniques produce a heat affected zone (HAZ) in the location of the joint, this is unavoidable. There can also be the presence of inclusions and voids in some welds that can reduce the strength of the joint. In cases where joint strength are important it may be necessary to invest in Xray examination of the joints after welding and to ensure that the operatives have completed a certified welder course that has taught proper techniques.
It is possible to teach yourself welding by referring to appropriate reference books or you can opt for a welding course to learn the basic principles of welding. In most cases to become proficient at welding beyond knowledge of the basic requirements means practice, practice and more practice.
The latter option of taking a welding course is the recommendation because a properly constructed welding course will include all the health and safety requirements associated with fusion welding. Never forget that dealing with gases, electricity and molten metals can be extremely dangerous so familiarisation with all health and safety precautions should always be a serious consideration.
Tungsten Inert Gas Welding
Fusion Welding Using Oxy-Acetylene
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