Welding is the act of joining two or more pieces of metal by melting and joining them with a filler metal.
The welded metal is melted by means of an electrical current that arcs between the welding gun and the workpiece. In the image, the gun is what is being held by the operator, and the arc is clearly visible. The heat produced by the arc melts both the base metal and the filler metal. The area consisting of melted base and filler metal is referred to as the weld bead. When the bead solidifies, the metal has been joined together.
WHY USE WELDING?
Welding can be used for a multitude of different manufacturing applications and is used for the quality and speed in which it can join metal. There are several different types of welding, all with advantages, disadvantages, and specific areas of use. For example TIG or GTAW (gas tungsten arc welding) can be used for applications requiring high quality and control over the weld bead, while stick welding is used for its versatility, portability, and speed. The type of welding that will be used in this instructional document is MIG or GMAW (Gas metal arc welding). MIG is one of the most widely used forms of welding in industry and is considered one of the easier forms of welding to learn. Anyone with access to a welding machine can take advantage of this manufacturing process.
Step 1: Equipment
MIG welding uses filler metal that will continuously feed through the welding gun (torch) as it is melted. The filler metal (or electrode) takes the form of a long thin wire and carries the current that arcs to the base metal. When the trigger is pulled on the gun, the wire electrode will feed out of the tip. Pulling the trigger also activated the electricity, supplying the current.
Without shielding the molten metal in the weld pool is subject to atmospheric gases, causing defects. In order to prevent this a form of inert gas is used to shield the area directly around the weld bead; this system is built directly into the welding machine. It is not necessary to know the specifics of this system, but it is important to know that shielding gas is involved and where the storage tank is located.
The figure shows the basic components in the MIG welding system.
1. The welding torch/gun
2. The workpiece or base metal
3. The electrical power supply
4. The feed unit
5. The electrode (filler metal) source
6. The inert shielding gas source
Step 2: Safety
Due to the use of electricity and high temperatures, there are several pieces of safety equipment and procedures required. The safety equipment needed includes gloves, a face shield, and an apron. The gloves will protect your hands from the electrical current as well as the molten metal. NOTE: Do not pick up the workpiece with your hands immediately after welding, even with gloves, due to the extremely high temperature of the metal. The face shield is required to shield the eyes from the arc, and the apron is to shield against the possible splatter of molten metal.
Along with personal protective equipment, it is important that the proper safety measures are taken when setting up the equipment. It is important to make sure that the compressed gas cylinder is upright and secure during use in order to prevent personal harm. You must also ground the workpiece in order to prevent the chance of electrocution.
Step 3: Welding basics (techniques)
In order to lay down a quality weld, there must be several variables taken into account. These include distance from the workpiece, speed, angle, and motion of the torch.
Largely the technique will improve with practice, and certain applications may require different techniques, but here are some basic tips to get started.
- Before welding, cut the wire electrode back so there is about one quarter to one half inch protruding from the gun.
- As discussed in the safety section, the workpiece must be grounded to protect from electrocution.
- Begin by pointing the torch at the end of the weld path, with the gun at a 45 degree angle and with the electrode about one quarter inch from the workpiece.
- Pull the trigger to initiate the arc, and and either "push" or "pull" the torch along the workpiece. (dependent on the direction the torch is tilted)
- Adjust the speed and distance from the weld in order to produce the desired weld bead size. If the weld is too small, there may not be sufficient fusion between the base metals. If the weld is too large, you will be using unnecessary filler metal.
In order to explain how to use MIG welding we will look at the procedure for welding several different types of joints. These joints are:
• Square joint
• Lap joint
• Tee joint
These are joints that will be used in a variety of applications and will be used to examine the technique required for MIG welding.
Step 4: Turn on the gas
Step 5: Turn on the machine and adjust settings
Step 6: Square joint (set up)
NOTE: Thicker materials may require a groove to be cut in the seam in order to ensure the weld penetrates the material entirely.
Step 7: Square joint (tack welds)
Step 11: Tee joint (set up)
The setup may require more extensive fixturing than the last two joints, especially if the workpiece does not stand up on its own. Fixturing is also important in tee joints due to the thermal expansion and contraction of the metal. As the metal solidifies, it will contract and pull the upright piece down to make a smaller angle with the horizontal piece. This can be prevented by fixturing the pieces, with something like a clasp, to prevent the pieces from moving.