ã€China Aluminum Industry Network】Friction stir processing (FSP) is a new and effective processing technology developed on the basis of friction stir welding (FSW), which can be used for microstructure modification and new material preparation. During the processing, high-speed rotary stirring heads are used to mix and crush the material in the processing zone, and severe plastic deformation and heat cycle effect occur to achieve refinement, densification, and homogenization of the microstructure. FSP can break the coarse dendrite structure and the second phase, dissolve the precipitated phase, eliminate the defects of the as-cast state, and significantly improve the performance of the metal material. Aluminum-iron alloy has many excellent properties such as light weight, good heat resistance and corrosion resistance, and it has a wide application prospect in the aerospace field. In ordinary cast aluminum-iron alloys, the solid solubility of iron in aluminum is very low, and intermetallic compounds such as Al3Fe are mainly generated. Al3Fe is acicular or flaky, severely fractures the matrix, and becomes a source of stress concentration, significantly reducing the mechanical properties of aluminum-iron alloys. Controlling and improving the morphology, size, and distribution of the iron-containing phase can make the aluminum-iron alloy a practical structural material and improve the performance and practical application value of the alloy. Therefore, the search for effective processing and refinement becomes the key to solving the problem. At present, the strong plastic deformation methods such as high-voltage torsion and equal-diameter bending can significantly refine the microstructure of Al3Fe intermetallic compounds, increase the solid solubility of iron atoms in the aluminum matrix, and improve the mechanical properties of the alloy. However, these methods are complicated in the processing steps, and the resulting sample has a small size, and thus is limited in practical use. FSP can effectively refine the alloy structure and is suitable for the continuous processing of large-area bulk materials. It is a promising material refinement method. Therefore, in this paper, FSP is used to carry out 3-pass reciprocating machining of aluminum-iron alloy prepared by common casting method, and the changes of microstructure and properties of aluminum-iron alloy after 3-pass processing are studied.
The experimental use of 99.9% industrial pure aluminum and Al-20Fe master alloy as raw materials, preparation of iron containing 3% (mass fraction) Al-3% Fe alloy. The alloy was smelted with graphite crucible in a box type resistance furnace, degassed and refined, and cast into a 100 mm×80 mm×5 mm slab sample at 820° C. in a copper mold. The FSP experiment was conducted on a modified X5032 vertical lift table milling machine. The material of the stirring head is W18Cr4V, the diameter of the shoulder is 16mm, the diameter of the stirring needle is 5mm, and the height is 3.8mm. The stirring head rotation speed was 1180 r/min and the welding speed was 47.5 mm/min. Three pass reciprocating FSPs were performed on the as-cast alloy.
A large number of acicular Al3Fe phases exist in the as-cast alloy and the size is about 20-50 μm. After the friction stir processing, the needle-like Al3Fe phase was broken into a granular shape with a length of less than 1 [mu]m, and dispersed evenly in the aluminum matrix. The as-cast microstructure transforms into a low dislocation density recrystallized grain with a small iron-containing metastable phase in the matrix. After the friction stir processing, the microhardness of the processing zone is lower than that of the as-cast zone, but the distribution is more uniform. The tensile strength of the alloy in the processing zone slightly decreased, and the elongation significantly increased. Before and after the friction stir processing, the fracture fracture of the alloy exhibits the micro-cellular polymer ductile fracture characteristics. Prior to processing, the dimples were parabolic tear dimples with dimple sizes smaller and shallower, while processed dimples were equiaxed.
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