11.5.3 Basic Operation of GMAW
a. Preweld cleaning, equipment check and labor protection
Preweld cleaning: Chemical cleaning methods vary with the materials. 2. Mechanical cleaning:
Mechanical cleaning includes sanding, scraping and sand blasting, and it is used to clean oxidation film
on the metal surface.
Equipment check: Firstly, check if there are obvious damage marks on the exterior of the welding
machine, and if there is any lack or damage of any components of the welding machine. Get known of
the maintenance history and lifespan of the welding machine, the welding environment and welding
process. Then, Check if the category, connection, grounding and capacity of the welding machine, and
the welding process used are correct. After making sure there is no problem with the welding machine,
check other equipment.
Labor protection: Operators should take proper labor protection such as masks, protective gloves,
protective footwear, and canvas overalls before welding, and should wear protective goggles or welding
helmet during operation. Wear rubber overshoes when welding in wet places or in rainy day. Meanwhile,
pay attention to avoiding the damage from dust, electric shock, scalding, fire and radiation.
b. Welding parameter selecting
The parameters for MIG mainly include welding current, welding voltage, welding speed, stick-out, angle
of welding wire, wire diameter, welding position, polarity, type and flow of the shield gas and so on.
Current and welding voltage
Generally, operators choose proper wire diameter according to the thickness of the workpiece, and then
decide the welding current, mode of metal transfer and arc voltage.
1) Welding speed
The welding speed of single-pass welding refers to the relative moving speed of the torch moving along
the center line of the weld joint. Other things being equal, the penetration will increase when reducing the
welding speed, and the depth and width of the molten pool will reduce when increasing the welding
speed.
2) Stick-out
The longer the stick-out is, the greater the resistance heat will be, and accordingly the higher the melting
sped of the wire will be. If the stick-out is too long, the filling metal will be too much. If the stick-out is too
short, the contact tip will be burned easily. Therefore, the proper stick-out should be about 10 times as
long as the wire diameter.
3) Position of welding wire
The angle and position of the axis of welding wire relative to the center line of weld bead will affect the
shape of weld bead and the penetration. In the plane of the axis of welding wire and the center line of
weld bead, the angle formed by the axis of welding wire and the vertical line of the center line of weld
bead is called traveling angle.
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