[China Aluminum Network] In the aluminum production enterprises, the mold cost accounts for about 35% of the extrusion production costs. The quality of the mold and whether the mold can be properly used and maintained directly determine whether the company can produce normal and qualified profiles. The working conditions of the extrusion die in the production of the profile extrusion are very bad, both need to withstand severe friction and wear under high temperature and high pressure, and also need to bear cyclical load. This requires that the mold has high thermal stability, thermal fatigue resistance, thermal wear resistance and sufficient toughness. In order to meet the above requirements, high-quality 4Cr5MoSiV1 (U.S. Grade H13) alloy steels are currently widely used in the country, and vacuum heat treatment quenching and other methods are used to make molds to meet the requirements of aluminum production.
However, in actual production, there are still some molds that fail to reach the predetermined output during extrusion, and even severely squeeze less than 20 rods or less than twice on the machine to scrap ahead of schedule, resulting in the use of expensive tool steel. The mold is far from achieving its expected benefits. This phenomenon is currently prevalent in many domestic aluminum profiles manufacturers. To study its causes, we need to start from the following aspects.
First, the aluminum profiles themselves are ever-changing, and aluminum extrusion industry has developed to today, aluminum alloys have important advantages such as light weight, good strength, and so on. At present, many industries have adopted aluminum profiles instead of the original materials. Due to the special profile of the profile, the mold has a special cross-section and is difficult to design and fabricate. If you still use the conventional extrusion method is often difficult to achieve the rated output of the mold, you must use a special process, strict control of the production process parameters in order to normal production. And some molds due to their own profile section special or the quality of the mold itself, resulting in the mold can not be squeezed to the rated output, which requires the sales staff to contact the technical department and the mold factory to fully communicate. At the same time, the mold design and production department needs to continuously optimize the mold design technology, improve the mold making precision, and improve the mold quality.
Second, select the appropriate extrusion machine for production. Before the extrusion production, it is necessary to fully calculate the section of the profile, and determine the tonnage of the extruder according to the complexity of the profile section, the wall thickness and the extrusion coefficient λ. In general, λ>7-10. When λ>8-45, the service life of the die is longer and the profile production process is smoother. When λ>70-80, it is difficult to squeeze the profile, and the die life is generally shorter. The more complex the product structure, the more likely it is that the local rigidity of the mold is insufficient, the metal flow in the mold cavity is difficult to be uniform, and local stress concentration is accompanied. In the production of profiles, plugs and boring cars or twisted waves are easily formed, and the molds are easily deformed by elasticity. In severe cases, plastic deformation occurs and the molds are directly scrapped.
Third, a reasonable choice of billet and heating temperature. It is necessary to strictly control the alloy composition of the extruded ingot. At present, general enterprises require that the grain size of the ingot be up to a standard, so as to enhance plasticity and reduce anisotropy. When there are pores in the ingot, loose tissue, or central cracks, the sudden release of gas during the extrusion process is similar to "spraying guns," causing the local working belt of the mold to suddenly deload and load, resulting in a local huge impact load, which has a great impact on the mold. Big. Conditional companies can homogenize ingots, forced cooling after 550 ~ 570C for 8 hours, extrusion pressure can be reduced by 7-10%, extrusion speed can be increased by 15%.
Fourth, optimize the extrusion process. To scientifically extend the life of molds and use molds reasonably for production is an aspect that cannot be overlooked. As the working conditions of the extrusion die are extremely harsh, reasonable measures must be taken in the extrusion process to ensure the organizational properties of the die. (1) Take the appropriate extrusion speed. In the extrusion process, when the extrusion speed is too fast, the metal flow will be difficult to make uniform, the aluminum metal flow and the friction between the inner wall of the mold cavity will intensify, causing the wear of the mold working belt to accelerate, and the mold temperature actually higher. If the residual heat generated by the metal deformation cannot be taken away in time, the mold may fail due to local overheating. If the extrusion speed is appropriate, the above adverse effects can be avoided. The extrusion speed should generally be controlled below 25 mm/s. (2) Reasonably select the extrusion temperature. The extrusion temperature is determined by the mold heating temperature, the temperature of the ingot and the temperature of the aluminum rod. The low temperature of the aluminum bar can easily cause the extrusion force to rise or the boring car phenomenon to occur. The mold is prone to local minute traces of elastic deformation, or cracks occur at stress concentrated sites and cause early die scrap. Excessively high temperature of the aluminum rod will soften the metal structure and cause adhesion to the surface of the mold working belt and even block the mold (in severe cases, the mold will collapse under high pressure). The reasonable heating temperature of the ununiform ingot will be 460-520°C and homogenized. The reasonable heating temperature of the ingot is 430-480°C.
Fifth, the use of extrusion mold must be a reasonable surface nitriding process. Surface nitriding treatment can make the mold greatly improve the surface hardness of the mold under the premise of maintaining sufficient toughness, so as to reduce the thermal wear generated when the mold is used. It should be noted that the surface nitriding can not be completed in a single time. During the service of the mold, it must be carried out 3-4 times of repeated nitriding treatment. Generally, the thickness of the nitriding layer should be about 0.15mm. A more suitable nitriding process is to perform the previous nitriding after the mold is inspected. At this point, because the nitrided layer is still unstable, it should be nitrided again after extruding 5-10 rods. After the second nitriding, 40-80 rods can be extruded. After the third nitriding, no more than 100-120 rods are suitable. Before nitriding, the working belt must be polished, and the mold cavity should be cleaned. Residues of alkali slag or foreign particles must not be left. In general, the nitridation of the mold is not more than 4-5 times, because at this time, if the nitrided layer is not pulled by the working belt, the nitrided layer structure is relatively stable after repeated nitriding and extrusion. It should be noted that when the previous nitriding process is carried out through an appropriate production process, nitriding can be performed, and the number of times of nitriding can not be too frequent, otherwise, the work belt is liable to delamination.