How to MIG weld thick structural plates
Joining of thick material by using a welding process, always has been a major concern in fabrication industries. Here are some important points that are needed to know about the structural welding of thicker pates using Metal Inert Gas (MIG) process. Below listed are the primary concerns in thick plate MIG welding.
Short-Circuiting
MIG Welder Chart
Bevel Edge Joint
Pre-heating
Pre-cleaning
Shielding
Trained Welding associates
Multi-pass Structural Welding
You know there are four common transfer modes in MIG welding viz; Spray transfer, globular transfer, Short circuiting, and Pulse transfer. But here we will discuss only Short Circuiting transfer mode.
1. Short-Circuiting
A commonly used technique in MIG welding process, where an electrode (which is a filler material), hits the parent metal. Due to which, a short circuit initiates an arc and arc makes the filler and parent metal to melt. This method is coolest among the other transfer modes as it offers welding in all positions as compared to other methods where only flat (horizontal) position is possible. The downside of this method is the occurrence of least amount of penetration.
2. MIG welder Chart
A chart pasted on the MIG welder provides you the relevant information about voltage and thickness of joint. With the help of this chart you can select the required voltage and amperage according to the thickness. The charts are available for 110-115 Volts and 230 Volts with the relevant metal thickness. It is noticeable the maximum thickness which is mentioned in the chart should be welded in single pass. Furthermore if you want to weld thicker than this mentioned thickness, then do it in multiple passes.
3. Bevel Edge
If you have to weld thick plates with butt joint then it is helpful to some extent. However making a bevel joint will be quiet successful which is a common practice in pressure welding industries. Bevel joints are prepared by machining process, to join thick plates in order to get sound welds (having good mechanical properties)
4. Pre-Heating
Well the phenomenon of pre-heating is an essential part for thicker plates to avoid high thermal gradient. Pre-heating to about 65-150 degree Celsius is required to ensure that metal is hot enough for perfect fusion. It offers good penetration and sound welds are obtained.
5. Pre-Cleaning
Pre-cleaning of parent metal, removes scales and debris to produce inclusion free weld. It also prevents other weld defects to occur.
6. Shielding
Proper shielding is another important part to produce defects free weld. Normally a combination of 75 % argon and 25 % carbon dioxide is used as shielding. This combination has been a successful practice and these days, it is frequently used by many welding industries worldwide. However other combination of gases are also used for MIG welding of other materials. In carbon steel welding, Carbon dioxide shielding plays a vital role in achieving good mechanical properties. Shielding with hundred percent carbon dioxide gives good penetration results. See Why Carbon Dioxide is used in GMA welding of carbon steels?
7. Properly trained welder
Welding associates (welder and helper) must be properly trained which is the foremost requirement of weld quality assurance. So a skillful welder is required for the welding operation.
He must know how to configure the welding equipment and tackle the problems accordingly.
He should have good experience on MIG welding.
He must be aware of all welding imperfections which may happen during process.
8. Multi-pass Welding in Structural plates
First assemble the plates according to required shape by tacking. Commence welding from the initial point for root pass. Run it in ‘side to side manner’ as you have to deal with the sides of plates. All you have to do, is to make sure the complete penetration and fusion of each weld pass. On the other hand too little stick out can cause the MIG gun to hit with the plate. Extend the electrode to some suitable length. Pre-heating and cleaning are major keys for sound welding. Try to make beads in such a way that they have a continuous weld look (proper blending). In other words beads must melt and fuse with each other. But if beads looks separately from each other, there will be lack of fusion. So in this case, you need to overlap these beads such that they have complete fusion with one another. See How to prevent Lack of Fusion in weldment?
Conclusions
Short circuit transfer mode is the recommended method for structural MIG welding of thick plates.
Bevel edge joint gives successful results for multi-pass MIG welding of thicker plates.
Pre-heating and Pre-cleaning is an essential part, in achieving quality weld results. For structural multi pass MIG welding, suitable composition of shielding gases is required.
Welding personnel must have good experience and skills to tackle the problems. He must keep an eye on the common weld defect which is the “lack of fusion” in this case.
