Analysis of the causes affecting the strength of 6063 door and window aluminum profiles

6063 aluminum alloy profiles are ideal structural and decorative materials for doors, windows, and curtain walls. With the development of the door, window, and curtain wall industries, higher requirements have been placed on the mechanical properties of 6063 aluminum alloy profiles. High-strength profiles can reduce the design wall thickness and structural weight, making them more popular in the market.
6063 aluminum alloy series AI-Mg-Si series can be heat-treated and strengthened aluminum alloy. It has high plasticity and can be extruded at high speed into profiles with complex cross-sections and different wall thicknesses. The quenching temperature is wide and the quenching sensitivity is low. It can achieve online air-cooling forced quenching and has medium strength after artificial aging. The extruded profiles have a smooth surface and are easily anodized and colored. They can also be used to produce profiles such as electrophoresis, spray coating, fluorocarbon spray paint, wood grain, and broken bridge insulation. Therefore, they have a monopoly position in building profiles and are playing an important role in the rapid development of our country’s economy. , especially in the rapid development of the real estate market, it has been widely used.

In Al-Mg-Si alloys, the main strengthening phase is Mg2Si. When the alloy is quenched, the more Mg2Si that is solidly dissolved in the matrix, the higher the strength of the profile after aging, and vice versa.

In actual production, aluminum alloy extruded profiles often have low or substandard hardness. There are many factors that affect the hardness and strength of profiles (see fishbone diagram). The following is based on the actual production of our factory, analyzing and summarizing the factors that affect the strength of 6063 aluminum alloy profiles and the measures taken from the aspects of process, equipment, operation, etc.

1. Control of chemical composition of 6063 aluminum alloy

The national standard GB/T 3190-2008 and the company’s internal control standards stipulate the chemical composition of 6063 aluminum alloy as shown in Table 1.

The main elements of this alloy are Mg and Si. They form the metallic compound Mg:Si in the alloy, which is the main strengthening phase of the alloy. The ratio of Mg to Si in Mg2Si is 1.73. When Mg:Si>1.73, there is still excess Mg, which will significantly reduce the solubility of the Mg2Si phase in solid aluminum. Due to the influence of excess Mg, the strengthening effect of the Mg2Si phase during heat treatment is significantly reduced. Thereby affecting the mechanical properties of the profile.

The increase in Si content in the alloy can improve the casting performance and welding performance. When Mg:Si<1.73, there is excess Si in the alloy, which can form compounds with other impurities such as Fe and Mn in aluminum to increase the strengthening effect. Therefore, when the strength requirements are high, excess Si is often controlled in the alloy. But as the Mg2Si and excess Si content in the alloy increases. The corrosion resistance of 6063 aluminum alloy profiles will be reduced, and excess Si will affect the oxidation coloring effect. The proportion of Mg and Si is mainly selected according to the requirements and uses of the alloy. “Peace” company’s 6063 cast rod Mg:Si=about 1.38-1.40, of which Si is excess, which is beneficial to enhancing mechanical properties.

2. Homogenization annealing of cast rods

In order to improve or eliminate the composition inhomogeneity (i.e. composition segregation) formed during the smelting process, eliminate the internal stress generated by the cast rod during the casting process, and improve the internal structure of the cast rod as needed, the cast rod needs to be homogenized. annealing.

Uneven composition within the casting not only affects the mechanical properties of the profile. It will also affect the surface treatment quality of the profile. The casting stress inside the cast rod will cause cracks. Different cooling methods after homogenization annealing are used to effectively control the size and distribution of the Mg2Si phase. In order to achieve the purpose of controlling intensity. Usually kept at 570~580℃ for 6~8 hours. In order to make the Mg2Si phase evenly distributed in the entire metal matrix as fine particles, the cast rod should be cooled at a high cooling rate after coming out of the furnace. It can be water-cooled or forced air-cooled. Do not cool it with the furnace.

Most of our company’s cast rods are purchased from outside, and most of them are used to produce construction profiles. Therefore, only the Φ254 cast rods used in the 3000T extruder require homogenization annealing treatment, and the cast rods with other diameters do not have homogenization annealing. deal with. We can adopt corresponding countermeasures: use a box-type chain heating furnace to extend the tail area (inlet area), and set up separate temperature control systems in the tail area and middle area of the furnace. The temperature in the temperature control zone is set to the homogenization temperature, and the homogenization and heating are placed in the aluminum rod heating furnace and completed simultaneously with the furnace. It makes up for part of the extrusion process changes and damage to the extrusion die caused by the non-homogeneous treatment of the aluminum rods. At the same time, it controls the temperature of the aluminum rods at the exit of the heating furnace to ensure the quenching temperature of the extruded profiles, thereby ensuring the strength of the profiles.

3. Cast rod heating temperature and air cooling quenching

During hot extrusion, the cast rod must be heated to make the metal have higher plasticity, reduce deformation resistance, and create conditions for the profile to obtain better mechanical properties during aging. In order to obtain higher strength. A higher extrusion temperature should be selected, but good surface quality of the extruded profile must be ensured. Generally, the temperature of the cast rod is controlled between 440 and 500°C. We have actually measured that the outlet temperature of the extruded profile is controlled between 500 and 525°C.

6063 has low quenching sensitivity. As long as the extrusion profile temperature is between 500-525℃. Forced cooling with a fan as soon as possible can prevent the precipitation of the strengthening phase in the alloy and maintain the ability of subsequent aging strengthening. However, two points should be paid attention to in actual production: first, the number and capacity of fans must be large enough to ensure that the cooling rate is higher than 60°C/min; second, forced cooling must be started as soon as possible after the profile is released from the mold, especially for thicker walls (δ >2.5mm) profiles must ensure the cooling speed so that the profiles can be “hardened”.

If the temperature of the cast rod is low, or the cooling intensity of the outlet fan is not enough or the fan is not turned on, the quenching effect of the profile will be very poor. Even if the artificial aging temperature is very high (205°C) and the holding time is long (8 hours), the mechanical properties of the profile will not meet the requirements. , or even the result that the hardness cannot be achieved.

4. Framing and aging of aluminum profiles

The framing of profiles requires ventilation, and those with large wall thickness differences cannot be mixed. Curtain wall profiles and door and window profiles must be framed separately. Insulated profiles and non-insulated profiles must be framed separately. Profiles with different alloy states cannot be mixed.

The purpose of aging is to obtain mechanical properties that meet standards or user needs. The main performance indicators include: tensile strength σb, yield strength σ0.2, elongation δ, and Webster hardness HW. These indicators are actually proxy characteristics for the ability of aluminum alloy profiles to withstand various stresses during use.

When formulating aging process parameters, several factors must be comprehensively considered: First, the selection of internal control standards for the composition of 6063 alloy, which will cause the content of the strengthening phase Mg2Si to be different, which will have an impact on the aging process; second, production scheduling and ” “Parking time” control; third, the aging furnace types are different. Mainly due to different aging furnace circulation methods, furnace temperature uniformity, sealing performance and furnace processing capacity, the selection of process parameters will also be different.

In actual production, for different aging furnaces, when the internal control standards for the composition of 6063-T5 alloy are relatively fixed, insulated profiles are generally kept at 175±5°C for 2 hours, and non-insulated profiles are kept at 195±5°C for 2 hours. aging process. The temperature control failed and the circulation fan was broken. Profiles with abnormal strength indicators due to thermocouple damage, poor sealing, over-aging, etc. must be controlled by rework, repair or scrapping to prevent them from flowing into the next process and ensure that the strength indicators of the finished profiles meet or meet user needs.

5 Conclusion

The failure of the final hardness of 6063 aluminum alloy extruded profiles is an irreparable loss. During the production process of aluminum profiles, the extrusion process must be strictly followed to ensure the casting rod temperature, air-cooling quenching strength, air-cooling speed, aging temperature and holding time, etc. Craftsmanship. In the actual production process, process control and inspection must be strengthened. The key is to control the extrusion outlet temperature, online air-cooling quenching, aging process, etc., and strictly follow various procedures to obtain ideal strength performance indicators.