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How to optimize aluminum extrusion and heat treatment processes

July 24, 2023
1. Ingot heating
For extrusion production, extrusion temperature is the most basic and most critical process factor. Extrusion temperature has a great impact on product quality, production efficiency, die life, energy consumption and so on.
The most important problem of extrusion is the control of metal temperature, from the beginning of the ingot heating to the quenching of the extruded profile to ensure that the soluble phase structure does not precipitate from the solid solution or the dispersion of small particles.
The heating temperature of 6063 alloy ingot is generally set in the temperature range of Mg2Si precipitation, and the heating time has an important impact on the precipitation of Mg2Si, and the possible precipitation time can be greatly reduced by rapid heating. Generally speaking, the heating temperature of 6063 alloy ingot can be set as follows: unhomogenized ingot: 460-520℃; Homogenized ingot: 430-480℃.
The extrusion temperature is adjusted according to different products and unit pressure during operation. During the extrusion process, the temperature in the deformation zone of the ingot changes. With the completion of the extrusion process, the temperature in the deformation zone gradually increases, and it increases with the increase of the extrusion speed. Therefore, in order to prevent extrusion cracks, the extrusion speed should be gradually reduced with the progress of the extrusion process and the increase of the temperature in the deformation zone.
2. Control the extrusion speed of aluminum
The extrusion speed must be carefully controlled during the extrusion process. Extrusion speed has important influence on thermal effect of deformation, uniformity of deformation, recrystallization and solution process, mechanical properties and surface quality of products.
Extrusion speed is too fast, the surface of the product will appear pitting, cracks and other tendencies. At the same time, excessive extrusion speed increases the non-uniformity of metal deformation. The outflow rate during extrusion depends on the type of alloy and the geometry, size and surface condition of the profile.
6063 alloy extrusion speed (metal outflow speed) can be selected for 20-100 m/min.
With the progress of modern technology, the extrusion speed can be controlled by program or simulated program, and new technologies such as isothermal extrusion process and CADEX have also been developed. By automatically adjusting the extrusion speed to keep the temperature in the deformation zone in a constant range, the purpose of rapid extrusion without cracking can be achieved.
In order to improve production efficiency, many measures can be taken in the process. When induction heating is used, there is a temperature gradient of 40-60℃(gradient heating) along the length of the ingot. During extrusion, the high temperature end faces the extrusion die and the low temperature end faces the extrusion pad to balance part of the deformation heat. There are also water-cooled mold extrusion, that is, water forced cooling at the back end of the mold, and the test has proved that the extrusion speed can be increased by 30%-50%.
In recent years, nitrogen or liquid nitrogen has been used abroad to cool the die (extrusion die) to increase the extrusion speed, increase the die life and improve the surface quality of the profile. In the extrusion process, nitrogen is led to the exit of the extrusion die, which can make the cooled products shrink rapidly, cool the extrusion die and the deformation zone metal, so that the deformation heat is taken away, and the exit of the mold is controlled by the atmosphere of nitrogen, reducing the oxidation of aluminum, reducing the bonding and accumulation of aluminum oxide, so the cooling of nitrogen improves the surface quality of the products, and can greatly improve the extrusion speed. CADEX is a recently developed extrusion process in which the extrusion temperature, extrusion speed and extrusion pressure in the extrusion process form a closed-loop system to maximize extrusion speed and production efficiency while ensuring the best performance.
3. On-machine quenching
The 6063-T5 quenching is to retain Mg2Si dissolved in the base metal at high temperatures after rapid cooling to room temperature. The cooling rate is usually proportional to the content of the strengthening phase. 6063 alloy can be strengthened with a minimum cooling rate of 38 ° C/min, so it is suitable for air cooling quenching. Changing the number of fan and fan revolutions can change the cooling intensity, so that the temperature of the product is reduced to less than 60 ° C before the tension is straightened.
4. Tension straightening
After the profile is out of the die hole, it is generally drawn by a tractor. When the tractor works, it gives the extruded product a certain traction tension and moves synchronously with the product outflow speed. The purpose of using the tractor is to reduce the length of the multi-line extrusion and scratch, but also to prevent the profile from twisting and bending after the die hole, which brings trouble to the tension straightening. Tension straightening can not only eliminate the longitudinal shape of the product, but also reduce its residual stress, improve the strength characteristics and maintain its good surface.
5. Aluminum artificial aging
Aging treatment requires uniform temperature, temperature difference does not exceed ±3-5℃. The artificial aging temperature of 6063 alloy is generally 200℃. The aging holding time is 1-2 hours. In order to improve mechanical properties, there is also the use of 180-190℃ aging for 3-4 hours, but at this time the production efficiency will be reduced.
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