What Is Welding?
Welding is a process that joins two or more materials together by melting parts together and then allowing them to cool, resulting in fusion. We can prevent entire workpieces from melting by precisely distributing heat resulting in local melting. The two (or multiple) materials joined together are referred to as the base metals and the filler material is the material added to the joint from the process. This is commonly achieved by arc welding, which there are many types of.
The combination of the base metal and filler material during the process results in the weld metal. The weld metal commonly has a different composition compared to the base and filler material. Preventing contamination during the process is pivotal as it can result in detrimental failure during service. During the process, a shielding gas (most commonly Argon) is used to prevent oxygen and water vapour entering the weld pool which could result in a low quality of the weld.
Welding is a process used throughout various industries and is a core element of fabricating products. Core equipment includes a welder (MIG or TIG welders are popular), electrodes, feeds, welding table and safety equipment.
There are various types of energy utilised for the process including, gas flame (chemical), electrical (arc), which we will discuss here, a laser, electron beam and friction.
The most popular method is arc welding, which creates a circuit including the word pieces and the equipment. During the process an arc is generated between the workpiece and electrode, which can either be consumable or non-consumable. The arc generates enough heat to melt the parts and join them together as discussed above.
What is Arc welding?
As stated above, the energy source for arc welding is electricity. So what does this mean? Electricity melts the two materials together, which requires an electrical arc to generate enough heat.
The electrical arc is generated between the electrode (metal stick) and the base material, melting the materials at the point of contact. Depending on the type of arc welding, either an AC or DC current can be used. Additionally, arc welding can use consumable and non-consumable electrodes.
What is an Arc and how is it generated?
An electrical arc occurs when a current flows through the air between two conductors, a product of an electrical breakdown of gas. For an electrical arc to form, an electrical potential difference must exist between two points.
An electrical potential difference means two points have a different amount of electrons present resulting in them having different charges. The difference generates a voltage.
Forces attempt to overcome the disbalance of charges, and in doing so, dielectric breakdown occurs. A channel is created between the two electrodes of ionised air, creating a plasma column that reduces the electrical resistance.
Current flows through the plasma column creating an electrical arc.
The potential difference between the two points must be sufficient for the air gap present. A smaller air gap requires a lower potential difference and a larger air gap requires a greater potential difference. If the air gap is too large, the arc will be put out.
Power Supply
As electrical power is a necessity for arc welding, various power supplies can be used. There are two popular classifications of power supplies for arc welding. These are constant voltage and constant current.
In arc welding, voltage related to the length of the arc, and the current is related to the amount of heat generated.
The arc welding process will determine which power supply is required. TIG (Tungsten Inert Gas) welding, and other manual welding processes tend to use a constant current power supply. Automated arc welding processes, such as MIG (Metal Inert Gas), favour constant voltage power supplies.
Why do the different types of welding processes favour different power supplies? In Manual processes, it is difficult to hold the electrode steadily in place. The knock-on effect is, the arc length will vary through the operation. Therefore, a constant current power supply is suitable because the voltage varies with arc length.
Automated arc welding processes keep the arc distance constant, making a constant voltage power supply suitable. A constant arc distance is achieved by short bursts of increased current when the electrode and workpiece come too close. The increased current generated more heat, melting more of the electrode tip, restoring the desired gap.
Types of Arc Welding Processes
Arc welding processes are categorised into consumables and non-consumable electrode methods.
Consumable Electrode Methods
MIG welding (also called GMAW) is a popular automatic welding process. The process uses a MIG welder, which continuously feeds a consumable wire, having the dual purpose of an electrode and filler material. A MIG welder is a unit that contains the gun and allows you to control the feed speed by using a dial. MIG welders are compact, making that easy to transport.
GMAW is often used in the automotive industry and for DIY projects as it has high versatility and speed. However, it is not convenient for outdoor use because a shielding gas must be maintained around the weld site during the process.
Other consumable electrode processes are stick welding (SMAW), Flux-cored arc welding (FCAW) and Submerged arc welding (SAW).
Non-consumable Electrode Methods
The most popular non-consumable electrode welding process is TIG welding (also called GTAW), which is a manual process. GTAW involves manually adding the filler material, unlike MIG welding. The electrode (made of tungsten) and filler material are two different parts.
Due to the manual element, GTAW is more difficult to master and conducted at a low speed. However, TIG welding produces high-quality welds, therefore the process is used when quality welds are required e.g. on a bike frame.
Other non-consumable electrode methods include plasma arc welding.
Heat Affected Zone (HAV)
As welding processes generate so much heat (1,650°C), it is not just the welded area that is affected. The surrounding material is affected by the heat, in turn affecting its mechanical properties.
The affected surrounding area is known as the Heat Affected Zone (HAZ).
The heat-affected zone is a ring-shaped area surrounding the weld, exposed to the high temperatures from the process. The zone is easily distinguished due to ‘colouring’ of the material (see image below). The material in the heat affect zone has its heat treatment properties altered.
Stresses produced from nonuniform heating and cooling, combined with the temperature of the process, are what cause the changes. The effects from the process on the surrounding material can potentially be detrimental due to structural changes. The structural changes can affect properties such as fatigue resistance, distortion and surface cracking. As a result, this could lead to premature failure.
Summary
We have covered the basics of the crucial welding process to help build your fundamental knowledge and the different types. The key materials during this process can now be identified and we understand what the Heat Affected Zone is and how it can potentially cause detrimental results if not controlled.
I was not exposed to welding during university, and I have found this to be a gap in my knowledge during my graduate role. I hope I have taught you the fundamentals of welding that you will need to know.
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