Place a canister of argon gas on the back of the MIG welder and attach the gas connector tube to the top of the can. Leave the gas valve off and the power cord unplugged.
Put a spool of 35 to 45 mm aluminum welding wire into the MIG wire feed. Open the spool holder by removing the screw cap. Put the spool on and replace the cap. Feed the wire end into the wire dispenser wheel mechanism. Keep the wire from bending or collapsing between the spool and the dispenser.
Twist the shroud off the tip of the welding torch. Unscrew the contacting tip with a slotted screwdriver and pull it off. Plug in the power cord and turn on the machine. Depress the trigger on the torch until the aluminum welding wire is at the end of the tip. Turn off the welder, unplug it and replace the contacting tip and shroud.
Hold the broken ends of the aluminum coat hanger together with a C-clamp or a vise. Clear the area of anything flammable including cloth, saw dust and flammable liquids.
Put on protective clothing and gloves to cover all of your skin. Put on a welding mask that has a darkened lens to protect your eyes from the bright flaming arc.
Clip the grounding clamp from the front of the welder onto a sturdy section of the coat hanger. Turn on the power and open the gas valve. Hold the tip of the torch shroud 1/4 inch from the broken pieces at a 45 degree angle and depress the trigger. Move the torch tip slowly and smoothly over the joint of the broken pieces. Weld the two pieces together and allow a bead to form around the repaired section. Let the hanger cool and file the bead down with a metal file.