6063 aluminum alloy belongs to the low-alloyed Al-Mg-Si series heat-treatable aluminum alloy. It has excellent extrusion molding performance, good corrosion resistance and comprehensive mechanical properties. It is also widely used in the automotive industry because of its easy oxidation coloring. With the acceleration of the trend of lightweight automobiles, the application of 6063 aluminum alloy extrusion materials in the automotive industry has also increased further.
The microstructure and properties of extruded materials are affected by the combined effects of extrusion speed, extrusion temperature and extrusion ratio. Among them, the extrusion ratio is mainly determined by the extrusion pressure, production efficiency and production equipment. When the extrusion ratio is small, the alloy deformation is small and the microstructure refinement is not obvious; increasing the extrusion ratio can significantly refine the grains, break up the coarse second phase, obtain a uniform microstructure, and improve the mechanical properties of the alloy.
6061 and 6063 aluminum alloys undergo dynamic recrystallization during the extrusion process. When the extrusion temperature is constant, as the extrusion ratio increases, the grain size decreases, the strengthening phase is finely dispersed, and the tensile strength and elongation of the alloy increase accordingly; however, as the extrusion ratio increases, the extrusion force required for the extrusion process also increases, causing a greater thermal effect, causing the internal temperature of the alloy to rise, and the performance of the product to decrease. This experiment studies the effect of extrusion ratio, especially large extrusion ratio, on the microstructure and mechanical properties of 6063 aluminum alloy.
1 Experimental materials and methods
The experimental material is 6063 aluminum alloy, and the chemical composition is shown in Table 1. The original size of the ingot is Φ55 mm×165 mm, and it is processed into an extrusion billet with a size of Φ50 mm×150 mm after homogenization treatment at 560 ℃ for 6 h. The billet is heated to 470 ℃ and kept warm. The preheating temperature of the extrusion barrel is 420 ℃, and the preheating temperature of the mold is 450 ℃. When the extrusion speed (extrusion rod moving speed) V=5 mm/s remains unchanged, 5 groups of different extrusion ratio tests are carried out, and the extrusion ratios R are 17 (corresponding to the die hole diameter D=12 mm), 25 (D=10 mm), 39 (D=8 mm), 69 (D=6 mm), and 156 (D=4 mm).
Table 1 Chemical compositions of 6063 Al alloy (wt/%)
After sandpaper grinding and mechanical polishing, the metallographic samples were etched with HF reagent with a volume fraction of 40% for about 25 s, and the metallographic structure of the samples was observed on a LEICA-5000 optical microscope. A texture analysis sample with a size of 10 mm×10 mm was cut from the center of the longitudinal section of the extruded rod, and mechanical grinding and etching were performed to remove the surface stress layer. The incomplete pole figures of the three crystal planes {111}, {200}, and {220} of the sample were measured by the X′Pert Pro MRD X-ray diffraction analyzer of PANalytical Company, and the texture data was processed and analyzed by X′Pert Data View and X′Pert Texture software.
The tensile specimen of the cast alloy was taken from the center of the ingot, and the tensile specimen was cut along the extrusion direction after extrusion. The gauge area size was Φ4 mm×28 mm. The tensile test was carried out using a SANS CMT5105 universal material testing machine with a tensile rate of 2 mm/min. The average value of the three standard specimens was calculated as the mechanical property data. The fracture morphology of the tensile specimens was observed using a low-magnification scanning electron microscope (Quanta 2000, FEI, USA).
2 Results and discussion
Figure 1 shows the metallographic microstructure of the as-cast 6063 aluminum alloy before and after homogenization treatment. As shown in Figure 1a, the α-Al grains in the as-cast microstructure vary in size, a large number of reticular β-Al9Fe2Si2 phases gather at the grain boundaries, and a large number of granular Mg2Si phases exist inside the grains. After the ingot was homogenized at 560 ℃ for 6 h, the non-equilibrium eutectic phase between the alloy dendrites gradually dissolved, the alloy elements dissolved into the matrix, the microstructure was uniform, and the average grain size was about 125 μm (Figure 1b).
Before homogenization
After uniformizing treatment at 600°C for 6 hours
Fig.1 Metallographic structure of 6063 aluminum alloy before and after homogenization treatment
Figure 2 shows the appearance of 6063 aluminum alloy bars with different extrusion ratios. As shown in Figure 2, the surface quality of 6063 aluminum alloy bars extruded with different extrusion ratios is good, especially when the extrusion ratio is increased to 156 (corresponding to the bar extrusion outlet speed of 48 m/min), there are still no extrusion defects such as cracks and peeling on the surface of the bar, indicating that 6063 aluminum alloy also has good hot extrusion forming performance under high speed and large extrusion ratio.
Fig.2 Appearance of 6063 aluminum alloy rods with different extrusion ratios
Figure 3 shows the metallographic microstructure of the longitudinal section of the 6063 aluminum alloy bar with different extrusion ratios. The grain structure of the bar with different extrusion ratios shows different degrees of elongation or refinement. When the extrusion ratio is 17, the original grains are elongated along the extrusion direction, accompanied by the formation of a small number of recrystallized grains, but the grains are still relatively coarse, with an average grain size of about 85 μm (Figure 3a); when the extrusion ratio is 25, the grains are pulled more slender, the number of recrystallized grains increases, and the average grain size decreases to about 71 μm (Figure 3b); when the extrusion ratio is 39, except for a small number of deformed grains, the microstructure is basically composed of equiaxed recrystallized grains of uneven size, with an average grain size of about 60 μm (Figure 3c); when the extrusion ratio is 69, the dynamic recrystallization process is basically completed, the coarse original grains have been completely transformed into uniformly structured recrystallized grains, and the average grain size is refined to about 41 μm (Figure 3d); when the extrusion ratio is 156, with the full progress of the dynamic recrystallization process, the microstructure is more uniform, and the grain size is greatly refined to about 32 μm (Figure 3e). With the increase of extrusion ratio, the dynamic recrystallization process proceeds more fully, the alloy microstructure becomes more uniform, and the grain size is significantly refined (Figure 3f).
Fig.3 Metallographic structure and grain size of longitudinal section of 6063 aluminum alloy rods with different extrusion ratios
Figure 4 shows the inverse pole figures of 6063 aluminum alloy bars with different extrusion ratios along the extrusion direction. It can be seen that the microstructures of alloy bars with different extrusion ratios all produce obvious preferential orientation. When the extrusion ratio is 17, a weaker <115>+<100> texture is formed (Figure 4a); when the extrusion ratio is 39, the texture components are mainly the stronger <100> texture and a small amount of weak <115> texture (Figure 4b); when the extrusion ratio is 156, the texture components are the <100> texture with significantly increased strength, while the <115> texture disappears (Figure 4c). Studies have shown that face-centered cubic metals mainly form <111> and <100> wire textures during extrusion and drawing. Once the texture is formed, the room temperature mechanical properties of the alloy show obvious anisotropy. The texture strength increases with the increase of the extrusion ratio, indicating that the number of grains in a certain crystal direction parallel to the extrusion direction in the alloy gradually increases, and the longitudinal tensile strength of the alloy increases. The strengthening mechanisms of 6063 aluminum alloy hot extrusion materials include fine grain strengthening, dislocation strengthening, texture strengthening, etc. Within the range of process parameters used in this experimental study, increasing the extrusion ratio has a promoting effect on the above strengthening mechanisms.
Fig.4 Reverse pole diagram of 6063 aluminum alloy rods with different extrusion ratios along the extrusion direction
Figure 5 is a histogram of the tensile properties of 6063 aluminum alloy after deformation at different extrusion ratios. The tensile strength of the cast alloy is 170 MPa and the elongation is 10.4%. The tensile strength and elongation of the alloy after extrusion are significantly improved, and the tensile strength and elongation gradually increase with the increase of the extrusion ratio. When the extrusion ratio is 156, the tensile strength and elongation of the alloy reach the maximum value, which are 228 MPa and 26.9%, respectively, which is about 34% higher than the tensile strength of the cast alloy and about 158% higher than the elongation. The tensile strength of 6063 aluminum alloy obtained by a large extrusion ratio is close to the tensile strength value (240 MPa) obtained by 4-pass equal channel angular extrusion (ECAP), which is much higher than the tensile strength value (171.1 MPa) obtained by 1-pass ECAP extrusion of 6063 aluminum alloy. It can be seen that a large extrusion ratio can improve the mechanical properties of the alloy to a certain extent.
The enhancement of the mechanical properties of the alloy by extrusion ratio mainly comes from grain refinement strengthening. As the extrusion ratio increases, the grains are refined and the dislocation density increases. More grain boundaries per unit area can effectively hinder the movement of dislocations, combined with the mutual movement and entanglement of dislocations, thereby improving the strength of the alloy. The finer the grains, the more tortuous the grain boundaries, and the plastic deformation can be dispersed in more grains, which is not conducive to the formation of cracks, let alone the propagation of cracks. More energy can be absorbed during the fracture process, thereby improving the plasticity of the alloy.
Fig.5 Tensile properties of 6063 aluminum alloy after casting and extrusion
The tensile fracture morphology of the alloy after deformation with different extrusion ratios is shown in Figure 6. No dimples were found in the fracture morphology of the as-cast sample (Figure 6a), and the fracture was mainly composed of flat areas and tearing edges, indicating that the tensile fracture mechanism of the as-cast alloy was mainly brittle fracture. The fracture morphology of the alloy after extrusion has changed significantly, and the fracture is composed of a large number of equiaxed dimples, indicating that the fracture mechanism of the alloy after extrusion has changed from brittle fracture to ductile fracture. When the extrusion ratio is small, the dimples are shallow and the dimple size is large, and the distribution is uneven; as the extrusion ratio increases, the number of dimples increases, the dimple size is smaller and the distribution is uniform (Figure 6b~f), which means that the alloy has better plasticity, which is consistent with the mechanical properties test results above.
3 Conclusion
In this experiment, the effects of different extrusion ratios on the microstructure and properties of 6063 aluminum alloy were analyzed under the condition that the billet size, ingot heating temperature and extrusion speed remained unchanged. The conclusions are as follows:
1) Dynamic recrystallization occurs in 6063 aluminum alloy during hot extrusion. With the increase of extrusion ratio, the grains are continuously refined, and the grains elongated along the extrusion direction are transformed into equiaxed recrystallized grains, and the strength of <100> wire texture is continuously increased.
2) Due to the effect of fine grain strengthening, the mechanical properties of the alloy are improved with the increase of extrusion ratio. Within the range of test parameters, when the extrusion ratio is 156, the tensile strength and elongation of the alloy reach the maximum values of 228 MPa and 26.9%, respectively.
Fig.6 Tensile fracture morphologies of 6063 aluminum alloy after casting and extrusion
3) The fracture morphology of the as-cast specimen is composed of flat areas and tear edges. After extrusion, the fracture is composed of a large number of equiaxed dimples, and the fracture mechanism is transformed from brittle fracture to ductile fracture.
Post time: Nov-30-2024
