The weldability of aluminum varies greatly with the alloy elements.
Aluminum is highly sensitive to thermal fractures, so high welding speeds and low heat input methods are often used in welding.
With a suitable joint form, a more compatible filler alloy can reduce the occurrence of thermal cracking.
Aluminum should be cleaned of the surface prior to soldering to remove oxides, oil and loose impurities.
Surface cleaning is very important because excessive hydrogen can cause foaming when the aluminum alloy is welded, and excessive oxygen can form scum.

Thermite welding as following :