To avoid cold cracking, consider pre-heating the base material to slow down the cooling process. You may also utilize low-hydrogen welding consumables to reduce the amount of hydrogen that is dispersed into the weld. Finally, use caution not to overheat the metal when joining together aluminum components because it can cause thermal stress which could lead to cracking.
Aluminum is a lightweight metal used in manufacturing airframes because of its strength and resistance to corrosion. However, like other metals, it can become brittle when exposed to heat or cold. If you try to bend or hammer an aluminum plate then this will create a lot of stress on the surface which could lead to cracking.
The best way to prevent cracking is by keeping the temperature of the aluminum below its recrystallization temperature. For example, if you want to join two aluminum plates together then you should not let them cool below 200°C (392°F) or else they might crack.
If the temperature drops too low then you should use some type of additive to keep the joint fluid for longer so more time is available to lower the temperature further. For example, sodium silicate is commonly used as an additive for glassing metal surfaces because it forms a protective layer that prevents oxidation and helps retain moisture inside the joint.
Another method is by utilizing friction during heating.
Existing flaws or notches exacerbate fracture development. Heat treatment with a low temperature soak followed by quick heating to high temperatures, grinding or peening the weld toes, and utilizing a two-layer welding process to improve the HAZ grain structure are all things that can assist avoid reheat cracking.
Reheat cracking is the most common form of welding failure. It usually occurs during the first heat of a multi-pass weld, but it can also happen during subsequent heats if the weld is overheat. The cause of this type of crack is thermal stress. During the first pass of the weld, the metal is at a lower temperature, so it's more flexible. But as the weld progresses further away from complete fusion, there is a tendency for the metal to try to return to its original state, which means it will try to cool off faster than the surrounding material, creating a stress point. This problem can be avoided by ensuring that the initial heat up period is kept short, especially if multiple passes are required.
There are several ways to prevent this type of crack from forming. The first thing to remember is that metals that yield or bend before they fail under load are typically stronger than those that break in half. So any part of the joint that is being welded together should have some degree of flexibility. If one side of the joint is going to be welded completely, then it should be annealed first to bring out its natural springiness.
Hot cracking occurs as a result of weld metal embrittlement or the partly melted zone (PMZ) cooling between the liquidus and solidus temperatures or just below the solidus (Fig. 4). Hot cracking can also occur during welding if the filler material is softer than the base metal (e.g., aluminum filler metal in steel). This type of cracking is called "filler crack" or "filler hole."
The most common site for weld hot cracking is at the surface of the weld where heat from the welding process tends to accumulate (Fig. 5). If the weld metal is cooled rapidly after being heated to a high temperature, small crystals will form along grain boundaries and these crystals will cause the metal to become brittle when it re-solidifies.
The second most common site for weld hot cracking is within the body of the weld where there is less chance of heat accumulation. Here, cold flow of the metal into cracks can occur as the metal cools down below its recrystallization temperature.
Finally, hot cracking can also occur within the filler material if this component is more susceptible to thermal stress than the base metal.
Hot cracking, also known as solidification cracking, occurs in aluminum welds when significant levels of heat stress and solidification shrinkage are present when the weld is solidifying to varying degrees. As the weld cools, the difference in temperature between the core and surrounding material causes the core to contract more than the surrounding material, resulting in a stress pattern that can cause hot cracks when the weld reaches its full size.
The severity of this problem depends on how large the weld is compared to the surrounding material and how long it takes for the weld to fully cool. Large, poorly cooled welds are most likely to crack when they cool down. Cracks may appear anywhere from immediately after welding to many years later. However, cracks tend to form near the bottom of the weld where the metal is being pushed up by the weight of additional layers added to the joint as it is built up over time.
There are three main factors that determine how quickly a weld will cool: the thickness of the metal being welded; the type of metal being welded; and how well the weld is insulated from the environment. Thin sheets of aluminum will cool faster than thick ones because more surface area means more heat loss. Welding aluminum alloys instead of pure aluminum speeds up the cooling process because the alloys have additives that protect them from corrosion when heated like other metals would.