Steel and aluminum have different chemical and physical properties, such as melting point, thermal expansion coefficient, elastic modulus, etc., when welding steel and aluminum with hot processing welding technology will face many problems, that is, aluminum and steel easy to form a very hard and brittle IMP phase (intermetallic phase), and the greater the welding heat input, the more IMP phase generated. This brittle phase severely destroys the joint's static and dynamic strength, as well as deteriorates the joint's plasticity. Their main physical differences are showed below:
Iron can melts a portion of aluminum in the solid state, but when the aluminum content exceeds 12%, the crystal structure changes radically, forming FeAL(mesh), Fe3Al(mesh) mixtures that are very hard (250-520hv) and brittle. Further increase, if aluminum content in the iron Fe2Al (factor), Fe2Al5 (eta) and FeAl3 (theta) mixture is formed, which offer higher hardness (600-1100 HV) and higher brittleness. This brittle material results from the diffusion of iron in aluminum or aluminum in iron. When the electrochemical potential of two different materials is different, molecular diffusion occurs to compensate for the potential difference. The greater the potential difference (E~ 1.22v for iron and aluminum), the greater the diffusion trend.
However, when the thickness of IMP brittle phase of the welded joint is less than 10 m, its brittleness becomes less important and obvious. At this point, the performance of the workpiece mainly depends on the ductility of the base material. Corrosion is another major problem because the electrochemical potential of the two materials is quite different, leading to electrolysis (equivalent to a battery).Aluminum, on the other hand, has a low potential and a negative electrode, which corrodes with electrolysis. To sum up, the welding of steel and aluminum needs to meet two requirements:
IMP phase thickness at the joint < 10 m
To prevent corrosion of base material after welding
To achieve these two requirements, a low heat input process is required, followed by special wire or weld corrosion protection treatment.
CMT (Cold Metal Transfer) technology is developed on the basis of short-circuit transition, and its heat input is much lower than normal GMAW welding. The process is: the arc burns, the wire pushes forward until the droplet shortens, at which point the wire feed speed reverses, the wire pulls back, and the current and voltage are almost zero. After the next circuit is formed, the arc rekindles, and the droplet transition begins again before the wire is reattached. The average frequency of this feed/pull back motion is up to 70Hz.
The success is based on galvanized steel and aluminum,the welding experiment is as follows: aluminum thickness range is 0.8 3 mm, filling material for aluminum silicon material, by melting aluminum and zinc formed on the surface of the brazing steel weld joint. Basic experiment is completed in 1 mm on the joint of steel and aluminum. The following table is the average intensity of the test.
It is inevitable that there is some loss of strength in the heat-affected zone of CMT process. When welding heat-treated aluminum alloy (6000 series), the heat-affected zone strength will lose 30-40% due to precipitation crystallization into a mixed crystal structure. Therefore, the heat-affected zone of this joint is the weakest part, and the lowest tensile strength is about 60% of that of the aluminum-based material. For naturally hardened aluminum alloys (5000 series), the strength of the heat affected zone is also reduced due to recrystallization. The strength reduction is related to the heat input in the pretreatment and welding process. The fracture occurs mainly in the heat affected zone.
Experimental data show that welding of steel to aluminum is possible, although steel be galvanized and a special low energy welding process are the prerequisite for success. The welding joints show excellent tensile strength, corrosion resistance and fatigue resistance, and also prove that the value of IMP brittle phase is less than 2.5 m, which is the key to prevent the brittle fracture of steel and aluminum joint.