6028 alloy belongs to Al-Mg-Si series heat-treatable aluminum alloy, with moderate strength and good welding performance, corrosion resistance, mainly used in transportation and structural engineering industries. Such as bridges, cranes, roof structures, transport planes, transport vessels, etc.
In recent years, with the rapid development of the shipbuilding industry at home and abroad, reducing the weight of the hull and increasing the speed of the ship, the search for aluminum alloy materials instead of steel parts has become an important issue for the aluminum processing industry and the shipbuilding industry. 6082 aluminum alloy with medium strength and good corrosion resistance, light weight, is an ideal material for high-speed ship parts.
1 Technical requirements
1.1 Chemical composition (%)
Si 0.7-1.3 Fe ≤ 5.0 Cu ≤ 0.10 Mn 0.4-1.0
Mg 0.6-1.2 Cr≤0.25 Zn≤0.20 Ti≤0.10
1.2 Appearance
The surface can not have cracks, delamination, corrosion, oxide inclusions, peeling, bubbles and mechanical damage, edge flush, no burr.
1.3 Mechanical properties
Tensile strength σb≥310MPa, Yield point σ0.2≥250MPa, Elongation δ≥10%
2 casting process
2.1 Composition Control
Due to the high performance requirements of the product, Mg, Si, Fe and other elements mainly play a reinforcing role, and the reasonable ratio is of great significance to the extrusion process and the performance of the final product. Therefore, it must be strictly controlled and the scope should not be too wide. The high Mn content will affect the alloy casting performance. With the increase of Mn content, the viscosity will increase and the flowability will decrease. Therefore, the Mn content should be controlled in the lower range of the middle deviation. Although Cu has the strengthening effect on the alloy, the high content will damage the alloy. Corrosion resistance, copper content should not be too high.
2.2 casting process
2.2.1 Smelting
6082 alloy is characterized by containing Mn, Mn is a refractory metal, the melting temperature should be controlled at 740-760 °C. Stir the sample evenly twice or more to ensure that the metal is completely melted, the temperature is accurate, and the composition is uniform. After stirring, a sample is taken in the middle of the depth of the molten aluminum and on both the left and right sides of the furnace, and the converter can be obtained after the break is qualified.
2.2.2 Purification and Casting
After the melt was transferred to the static oven, nitrogen and a refining agent were used for dusting and air-refining. The refining temperature was 735-745°C for 15 minutes. After the refining, it was allowed to stand for 30 minutes. Through this process, degassing, slag removal, and purification of the melt.
During the casting, there are two filtering devices between the mold and the furnace mouth, and there is a foam ceramic filter plate (30PPI) filtration at the furnace mouth. Before casting, it is filtered with a 14-mesh glass fiber cloth to fully remove the oxides and slag in the melt.
6082 alloy casting temperature is relatively high (as compared to 6063 normal process), the casting speed is low, the water flow is large, the above process must be strictly controlled, can not go beyond the range, otherwise it will easily lead to casting failure.
3 Homogenization annealing
6082 alloy deformation resistance, extrusion difficulties, high mechanical properties indicators. After homogenization, the alloy structure changes as follows: (1) Intragranular segregation disappears; (2) Ms2Si phase dissolves into α(Al), and the unbalanced eutectic disappears; (3) β(Al9Fe2Si2) phase toward α(Al12Fe3Si2) Phase change, and refinement of iron-containing particles.
Through the above changes, the extrusion performance and profile quality will be greatly improved. The disappearance of intragranular segregation will reduce the inhomogeneity of the metal flow during extrusion and improve the surface finish of the extruded profile; the reduction and refinement of coarse coarse-grained Mg and Si particles and coarse Al-Fe-Si phase particles in the microstructure will be reduced. Profiles tend to crack on the surface and increase the extrusion speed; full solid solution of Mg2Si phase is the primary condition for strengthening the alloy and improving its mechanical properties.
Homogenization temperature: 555-565°C Holding time: 3 hours, cooling rate ≥ 200°C/h
4 extrusion process
4.1 cast rod heating method
Casting rod heating using industrial frequency induction heating, this thermal method is characterized by (1) a short heating time, in about 3 minutes can reach about 500 °C; (2) accurate temperature control, the error does not exceed ± 3 °C.
If the furnace is heated slowly, it will cause precipitation of Mg2Si phase, affecting the strengthening effect.
4.2 Extrusion
Changes in the following factors, the rational development of the extrusion process.
(1) 6082 alloy deformation resistance, casting rod temperature should be biased (480-500 °C);
(2) The mold temperature should also be high;
(3) In order to prevent shrinking or bubbles, oxide scales, impurities involved, the pressure should be left longer;
(4) To make the alloy main strengthening phase Mg2Si completely solid solution, it must be ensured that the quenching temperature is above 500 °C, solid extruded extrusion outlet temperature should be controlled at 500-530 °C;
(5) 6082 alloy quenching high sensitivity. The alloy contains Mn, which promotes the formation of intragranular intermetallic compounds and has an adverse effect on the quenching performance. Quenching requires high cooling intensity and fast cooling rate. It must be quenched by water to rapidly reduce the temperature to less than 50°C.
(6) After sawing the profile, the mounting frame should be protected at certain intervals and must not be discharged too densely.
5 aging system
A reasonable aging system must ensure product performance, but also consider production efficiency and production cost. After repeated tests, we have proven that the aging temperature is 170-180°C and the holding time is 5 hours, which is a better aging system for 6082 profiles.
6 test results
6.1 chemical composition stability and low magnification of the cast rod
6.2 Mechanical properties
The above test results show that due to reasonable selection of melting and casting process and heat treatment system, the casting rod has stable composition and uniform structure. Under the appropriate quenching and aging system, the higher the outlet temperature of the profile, the superior performance is excellent.
7 conclusions
According to the characteristics and performance requirements of 6082 alloy marine aluminum profiles, the above process is reasonable. Wherein the casting rod homogenization treatment, induction heating, profile aging after water quenching is a prerequisite to achieve product performance requirements; strict control of process parameters is the guarantee of product quality. In the casting process, the reasonable matching of the casting temperature, speed, and cooling water flow is the key to guarantee the quality of the casting; in the extrusion process, the control of the extrusion outlet temperature is the key to ensure the product performance. The products produced by this process passed the certification of DNV and met the needs of users.
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