As can be seen in below picture, one side of the weld is clean but other side is having black surface. This black soot which is generated after welding has many reasons (described below)

Excess soot can be caused by:

1.) too long of an arc length, either by operator stick out, or excess voltage.
2.) Not enough weld energy, increase wire feed speed, or drop down a size in wire to increase current density.
3.) Incorrect direction of travel, you’ll always want to use a push for aluminum
4.) Incorrect Gun angle, next time you are playing around with some scrap, vary your gun angle, you can actually move the soot from one side of the weld to the other just by changing gun angle.
5.) Travel speed too fast, which doesn’t allow sufficient breakup and removal of the outer oxide layer.
6.) Insufficient gas flow coverage, do you have a way to check flow at the nozzle? Never trust what the flowmeter or regulator reads, the only way to know for sure is to check the flow at the torch

Check list before you start welding:

What gas are you using? What flow rate? Do you push to shield ahead of the MIG weld? Do you pre flow to make sure the starts are shielded? Any leaks in your gas delivery system?

Keep reading, Happy welding

Thank you,

KP Bhatt