Several welding procedures can be used for primarily the same operation. Sometimes, a weld can be made just by heating the metal alone.
Other times, it requires heat in combination with either pressure or a filler metal through a process like MCAW welding. But what is MCAW welding?
MCAW is short for Metal Cored Arc Welding. In this process, welders create an arc between the base metal and a continuously fed consumable filler metal. A DC electric arc produces heat that melts both the metals in its immediate vicinity, fusing them to create a weld joint.
There are four basic metal transfer methods in welding:
1. Short circuit transfer
Short circuit transfer uses a lower voltage when causing a short circuit between the wire and the base metal.
The heat produced by the short circuit, which melts the two metals, creating a weld pool. The second time the wire touches the pool, the fusion begins.
Processes that utilize shielding gases where the ratio of argon is 75% to 85% use the short circuit method. Short circuit transfer produces quite a bit of spatter, but it is still safe to use for all welds and on all thicknesses of steel.
2. Globular transfer
In this technique, the weld metal is transferred across the arc in large droplets. The droplets are sometimes more massive than the electrode itself.
This mode generates the most spatter, which limits its usage to only vertical and overhead welds.
The Globular Transfer method is not known for producing a very smooth, durable bead. It is typically only used on carbon steel with 100% CO2 as the shielding gas.
3. Spray transfer
Similar to Globular Transfer, weld metal in the Spray Transfer method travels across an arc.
The droplets in this technique are minute and resemble that of water spouting out of a hose. Spray transfer uses high voltage and amperage to produce an arc that is on at all times.
Welders use the spray transfer method for most MCAW procedures. It produces vast weld puddles and minimal splatter.
The large weld puddle is why its application is limited for use on very thick metals in flat or horizontal positions. It uses different combinations of shield gases with various metals.
However, in all combinations, the minimum argon level must be 80%.
Current levels in this process must be higher than the transition current. The transition current levels vary depending on the diameter of the electrode, the shield gas combination, and the distance from the tip to the workpiece.
4. Pulse-spray transfer
In this technique, a combination of low background currents and high spray transfer current is used to transfer one drop of metal at a time. It combines the benefits of both spray transfer and globular transfer.
A lower background current allows the weld puddle to cool down before the high transfer current is pulsed through the metals. This method reduces the spatter produced by the globular transfer method and gives improved sidewall fusion.
It also minimizes the weld puddle, making it suitable for use on more extensive positions.
Essentially, all four of the transfer methods mentioned above are used in Metal Inert Gas (MIG) and gas metal arc welding (GMAW) procedures. Even though in terms of technicalities, both MIG and MCAW are pretty similar, not all of these procedures are suitable for MCAW.
From the four mentioned above, MCAW mostly uses either the spray transfer method or the pulsed spray transfer method, which is also known as the dip transfer method. Here is a look at the difference between MIG and MCAW.
Difference between MIG and MCAW
MIG is also an arc welding procedure in which a solid wire electrode feeds metal into the weld pool via a welding gun. The only major difference between the two processes is that you can also use alloy mixtures as metals for filler metals.
Advantages of MCAW
In metal cored arc welding processes, the slag levels are low as compared to solid wire welding processes. The lack of slag makes welding easier, and mechanization allows for higher productivity.
An investigation carried out on various welding parameters, including the welding current, the welding speed and the voltage on the depth of penetration, the reinforcement, and the weld bead width were studied. The automatic metal-cored arc welding process was used for this study.
The results show that MCAW provides better control over the finished product provided the right combination of metals, gas, and mode of transfer are employed.
MCAW also allows you to use a wider combination of shield gases, which provides more flexibility to the welder. Selecting the right shield gas is critical to producing a quality weld, especially in MCAW.
Importance of shielding gas
The entire arc must be covered with shielding gas. The gas will protect the weld pool from the atmosphere, especially oxygen and nitrogen, ensuring a smooth weld.
The absence of the shielding gas can cause multiple problems in the weld.
porosity is weld metal contamination that comes about as a result of gas trapped within the weld. The trapped gas then forms either spherical or elongated holes within the weld.
Porosity is not only caused by the lack of shielding gas. Sometimes, a rusty or tarnished base metal can result in a porous weld.
Make sure you prep the surface before you start performing the weld procedure. A porous weld can be rejected depending upon industry standards and the level of porosity.
Inefficient production rate
The combination of the shield gas directly affects arc stability and production efficiency in some cases. For the spray transfer method, in particular, shield gases with high argon rates allow for higher productivity rates.
Because the shield gas also directly influences the wire feed rate, choosing the wrong gas can disrupt the entire weld procedure.
In addition to ensuring a smooth weld, shield gases have multiple other functions as well:
- It forms the arc plasma
- It stabilizes the arc on the metal surface
- It also guarantees the smooth transfer of metal droplets from the wire to the metal pool.
Choosing the right shield gas
Several factors can affect the process of selecting the right shield gas.
- Filler metal deposit rate and efficiency
- Spatter control
- Bead profile
- Post weld cleaning
- Bead penetration
- Weld positions
- Welding fume generations rates
- Welding process technicalities
You must consider all these factors when using the shield gas. Using gas with higher reactivity rates like CO2 for out of position welds can be a problem.
CO2 produces weld puddles faster. Increased weld puddle fluidity makes it harder to create a smooth weld, especially in vertical positions.
In such cases, you should choose a gas with slower wire feed rates for optimal results.
Similarly, using a shielding gas with high reaction rates may create rapid fume generation. When choosing a shielding gas, keep in mind the filler metal, the base metal, and the operating parameters to minimize the production of harmful welding fumes.
Using a CO2 shield blend produces convex-shaped weld beads, which can lead to over welding. Over welding can increase welding costs by 50%.
Increased costs significantly decrease production efficiency. If you’re facing this problem, then you should consider changing your shield blend to an argon-concentrated one.
Argon allows for better bead control and produces a flat bead face that reduces the chances of over beading.
Commonly used gases
Argon, helium, carbon dioxide, and oxygen are some of the most commonly used gases in MIG and MCAW processes. A combination of these is used for some special metals. A few examples include:
- Argon +2 to 5% oxygen
- Argon +5 to 25% CO2
Non-ferrous (e.g., Aluminum, copper or nickel alloys):
- Argon / helium
Argon based gases are less reactive to the atmosphere as compared to CO2. They generate lower spatter levels when used as a shielding gas in dip transfer mode.
However, with argon, there is an increased risk of lack of fusion, and since it is a colder gas, it can cool down the weld metal pretty quickly. In such situations, a blend containing CO2 proves ideal.
CO2 also has its disadvantages and cannot be used in pulsed or spray transfer methods due to its high reactivity. For both these methods, argon-based gases containing either oxygen or CO2 are employed.
They have lower back plasma forces, making them an ideal choice.
Like MIG welding, success in MCAW depends upon the selection of the right consumables, power source, polarity of the power source, shielding gas, and application technique. The advantages of MCAW outweigh those of MIG, making it the better choice.
What is the main difference between MIG and MCAW?
The main difference between the two processes is that MCAW uses a greater range of gases as shielding gases.
What are some of the most commonly used gases in MIG and MCAW welding?
In most MIG and MCAW welding processes, you’ll see the use of Argon, Carbon Dioxide, Oxygen, Helium, or a combination of the four.