A method for improving the service life of hollow aluminum profile extrusion dies

Abstract

The present invention provides a method for improving the service life of a hollow aluminum profile extrusion die. After the extrusion die is extruded in the 1st or 2nd batch, it is removed from the extruder, and the metallic aluminum in the extrusion die is removed by hot alkali etching and washing. After manual inspection and determination that there are no obvious macroscopic cracks, the working surface of the extrusion die is polished, and after nitriding treatment, it is reinstalled on the extruder. Before the 2nd or 3rd batch of extrusion, an empty die heavy load is applied to the extrusion die through the extrusion rod, wherein the loading temperature is 500°C ± 20°C, the load magnitude is 1.2 - 1.6 times the extrusion breakthrough pressure, and the loading time is 0.5 - 1.0 seconds. The operation of the present invention is simple, without any additional cost, but can effectively reduce the cracking probability of the die bridge of the extrusion die, improve the service life of the die, and save the production cost.

Description

Technical Field

[0001] The invention relates to the technical field of nonferrous metal material manufacturing, in particular to a method for improving the service life of a hollow aluminum profile extrusion die. Background Art

[0002] The aluminum profile industry is the largest sub-industry in the non-ferrous metal field and is closely related to many areas of the national economy. Aluminum profiles are usually directly extruded on an extruder using an extrusion die. The extrusion die directly determines the size and precision of the extruded profile. The service life of the extrusion die significantly affects the production efficiency and production cost of aluminum profiles. Therefore, the extrusion die is the core technology of aluminum profile manufacturing. Usually, aluminum profile extrusion dies are made of H13 steel and are generally used under medium temperature conditions of about 500°C. They are repeatedly subjected to loads and thermal cycles. Therefore, it is very easy to form low-cycle fatigue cracks in the stress-concentrated parts, resulting in early fracture and failure of the die. Especially for the diverter extrusion die for hollow aluminum profiles, the stress concentration coefficient at the root of the diverter bridge is large, and micro cracks are often formed after extruding 1-2 batches or more than 30 ingots, and then continue to expand to form macro cracks, resulting in early failure and scrapping of the die, increasing production costs.

[0003] The development and use of extrusion dies in the industry can generally be divided into the following processes: the first step is to design the die according to the geometric structure of the profile; the second step is to process the die, including machining, electrical processing and heat treatment processes; the third step is to test the die. If the test is unsuccessful, the die needs to be removed, the metal aluminum in the die needs to be cleaned with hot alkali etching, and then the die needs to be repaired; if the test is successful, the extrusion profile production can be started directly; the fourth step is a nitriding process. After the successful die is used for several extrusion productions over a certain period of time, the die hole of the extrusion die will reduce the surface quality of the aluminum profile due to wear, and the die must be stopped and removed, that is, the die has completed the first batch of extrusion; then it is etched and polished before nitriding treatment to improve the surface hardness of the die working belt. For the diversion extrusion die, after the first batch of extrusion, the root of the die bridge usually produces microcracks due to strong stress concentration, but the cracks expand very slowly; the fifth step is to nitriding the die again. After the second batch of extrusion production, the surface of the extrusion die working belt will be worn again, making the surface quality of the extruded profile worse. Therefore, it is necessary to stop the machine again to remove the die, and then nitriding treatment is performed again after etching and grinding. In this way, the extrusion die can generally be extruded and nitrided for 5-6 batches. However, if the die bridge cracks due to fatigue crack expansion at the root of the die bridge, the die will be directly scrapped, and its service life can only complete 2-4 batches, not 5-6 batches. This situation belongs to the early failure of the die, and the expected service life has not been reached.

[0004] In the industry, to solve this problem, some adopt the design method of increasing the width of the root of the die bridge to improve the load-bearing capacity of the die bridge, but this will reduce the welding quality of the extruded profiles; others adopt the design method of increasing the size of the splitter holes to reduce the extrusion pressure, but this will reduce the stiffness of the die. By forging the die blank repeatedly for many times, the metallurgical quality of the die material can be effectively improved, the crack propagation rate can be reduced, and thus the service life of the die can be improved, but this will increase the machining cost of the die and prolong the die development cycle. Therefore, the aluminum profile industry is eagerly looking forward to a simple, feasible and low-cost method for improving the service life of extrusion dies. Summary of the Invention

[0005] (1) Technical Problems to be Solved

[0006] Aiming at the common problem of the low service life of extrusion dies in the aluminum profile industry, the present invention provides a simple, feasible and low-cost method for improving the service life of extrusion dies. To achieve the above object, the present invention adopts the following technical solutions:

[0007] (2) Technical Solutions

[0008] A method for improving the service life of hollow aluminum profile extrusion dies. After the extrusion die is extruded in the first or second batch, it is removed from the extruder, the metallic aluminum in the extrusion die is removed by hot caustic etching, and after manual inspection, when there are no obvious macroscopic cracks, the working surface of the extrusion die is polished, and after nitriding treatment, it is reinstalled on the extruder. Before the second or third batch of extrusion, an empty die heavy load is applied to the extrusion die through the extrusion rod, wherein the loading temperature is 500°C ± 20°C, the load magnitude is 1.2 - 1.6 times the extrusion breakthrough pressure, and the loading time is 0.5 - 1.0 seconds.

[0009] Further, before applying the empty die heavy load operation, the extrusion die needs to be preheated to 450 - 470°C first, and then reinstalled on the extruder.

[0010] Further, when applying the empty die heavy load operation, a steel gasket is placed in the central part of the extrusion die, and the temperature of the steel gasket is the same as the preheating temperature of the extrusion die.

[0011] Further, the timing of applying the empty die heavy load operation needs to be determined according to the usage of the extrusion die: if the number of extrusion times in the first batch is less than 30 times, the empty die heavy load is arranged to be completed before the third batch of extrusion; if the number of extrusion times in the first batch is between 30 - 50 times, the empty die heavy load is arranged to be completed before the second batch of extrusion or before the third batch of extrusion; if the number of extrusion times in the first batch is more than 50 times, the empty die heavy load is arranged to be completed before the second batch of extrusion.

[0012] (3) Beneficial Effects

[0013] The present invention is a technical method designed based on the principle of fracture mechanics. During the actual use of an extrusion die, it repeatedly bears extrusion force and thermal cycle, and is extremely likely to initiate fatigue cracks at the stress concentration site. When the crack extends to a certain extent, it will inevitably lead to the fracture failure of the die. The basic law of fatigue failure is that micro-cracks are preferentially initiated at the stress concentration site. After a long-term slow expansion, the micro-cracks form macro-cracks, and the continuous expansion of the macro-cracks leads to fracture. By intervening in the crack expansion process through special measures, the expansion rate can be effectively changed. After a micro-crack is formed at the root of the die bridge in the present invention, through a single empty-die loading, and the applied load is higher than the cyclic load stress amplitude under the actual working conditions, an overload effect is generated, so that a large plastic deformation zone is generated at the crack tip under the action of this load; the plastic deformation will effectively relax the stress concentration at the crack tip, and a compressive stress state will be formed at the boundary of the plastic zone at the crack tip after unloading. This plastic relaxation and compressive stress state will have a significant inhibitory effect on crack expansion during the subsequent loading process, thereby delaying crack expansion and increasing the service life of the die.

[0014] Compared with the prior art, the present invention has obvious advantages and beneficial effects. Specifically, this special method of empty-die heavy-load treatment is simple to operate, without adding any cost, but can effectively reduce the cracking probability of the die bridge of the extrusion die, increase the service life of the die, and save production costs. Statistics show that for the split-flow extrusion die treated by the method of the present invention, compared with the same type of die not treated by the method of the present invention, the fracture rate of the die bridge significantly decreases, and the average service life significantly increases. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] Figure 1 is a schematic structural diagram of the present invention.

[0016] Description of the reference numerals in the drawings:

[0017] 1. Extrusion rod 2. Extrusion die 3. Steel gasket

[0018] 4. Extrusion cylinder 5. Die cushion DETAILED DESCRIPTION OF THE EMBODIMENTS

[0019] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the preferred embodiments and comparative examples of the present invention will be specifically described below.

[0020] The traditional method is that after the extrusion die 2 is extruded in the first or second batch, the extrusion die 2 includes an upper die and a lower die. The upper die and the lower die are removed from the die cushion 5 of the extruder, and the metallic aluminum inside the upper die and the lower die is removed by hot alkali etching and washing. After manual inspection and determination that there are no obvious macroscopic cracks, the working surface of the extrusion die is polished. After nitriding treatment, it is reinstalled on the die cushion 5 of the extruder. In this application, before the application of the empty die heavy load operation, the extrusion die 5 is first preheated to 450 - 470 °C. Before the second or third batch of extrusion, the extrusion rod 1 passes through the extrusion cylinder 4 to apply an empty die heavy load to the extrusion die 2 once. Among them, the loading temperature is 500 °C ± 20 °C, the load magnitude is 1.2 - 1.6 times the extrusion breakthrough pressure, and the loading time is 0.5 - 1.0 seconds. When the empty die heavy load operation is applied, a steel gasket 3 is placed in the central part of the extrusion die 2. The radius of the steel gasket 3 is about half of the inner radius of the extrusion cylinder 4. The steel gasket 3 has the same preheating temperature as the extrusion die 2. The extrusion rod 1 pressurizes the extrusion die 2 through the steel gasket 3, which is convenient to quickly load the thrust applied by the extrusion rod 1 at the central part of the split-flow extrusion die 2 and immediately release it.

[0021] As shown in Table 1, taking the extrusion dies of three kinds of hollow aluminum profiles (A, B, C) as examples, the implementation timing, loading magnitude, etc. of the empty die heavy load scheme of the present invention are tested and implemented in production, and then the service life of the extrusion dies is statistically counted.

[0022] For the extrusion die of the hollow profile A, since only 20 extrusions are completed in the first batch before nitriding and no microcracks are formed inside the die bridge, no obvious effect is obtained by implementing the empty die heavy load treatment on it before the second batch of extrusion after the first nitriding. Subsequently, 5 sets of dies still failed due to die bridge fracture; after the second batch of extrusion, microcracks are formed inside the die bridge. After the second nitriding and before the third batch of extrusion, the empty die heavy load is implemented on the die, and the effect is obvious. The dies that failed due to die bridge fracture are reduced to 33%. Compared with the dies without the empty die heavy load treatment, the average service life is increased by 453 extrusions after being treated by the method of the present invention.

[0023] For the extrusion die of the hollow profile B, with a larger size and greater extrusion force, only 55 extrusions are completed in the first batch and microcracks are already formed inside the die bridge. When the empty die heavy load is implemented on the die before the second batch of extrusion after the first nitriding, since the load magnitude is the same as the maximum breakthrough pressure of the actual extrusion and no overload effect is formed, no obvious effect is obtained. Subsequently, 80% of the dies still failed due to die bridge fracture; but if the load is too large, reaching 2 times the maximum extrusion breakthrough pressure, 40% of the dies will crack directly; under the overload condition of 1.5 times the extrusion breakthrough pressure, the internal cracks of the die are significantly passivated, and the die bridge cracking is reduced to 25%, and the average service life is increased by 413 extrusions.

[0024] For the extrusion die of the hollow profile C, 30 extrusions were completed in the first batch, and microcracks had formed inside the die bridge. Applying an appropriate-sized empty die overload to the die before the second or third batch of extrusions achieved obvious results. The cracking failure ratio of the die bridge was reduced from 60% to below 25% and 33% respectively, and the average service life of the die was increased by more than 340 extrusions.

[0025] Table 1 shows the process conditions and results of three examples and the comparative example

[0026]

[0027]

[0028] Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions described in the foregoing embodiments, or perform equivalent replacements for some of the technical features. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principle of the present invention shall be included within the protection scope of the present invention.

Claims

1. A method for improving the service life of a hollow aluminum profile extrusion die. After the extrusion die is removed from the extruder after being extruded in the 1st or 2nd batch, it is cleaned of metallic aluminum in the die by hot alkali etching and inspected manually. After no obvious macroscopic cracks are found, the working surface of the extrusion die is polished, nitride-treated, and then reinstalled on the extruder. It is characterized in that: Before the 2nd or 3rd batch of extrusion, an empty-die heavy load is applied to the extrusion die through the extrusion rod. Among them, the loading temperature is 500°C ± 20°C, the load magnitude is 1.2 - 1.6 times the extrusion breakthrough pressure, and the loading time is 0.5 - 1.0 seconds. The timing of applying the empty-die heavy load operation needs to be determined according to the usage of the extrusion die: If the number of extrusion times in the 1st batch is less than 30 times, the empty-die heavy load is arranged to be completed before the 3rd batch of extrusion; If the number of extrusion times in the 1st batch is between 30 - 50 times, the empty-die heavy load is arranged to be completed before the 2nd batch of extrusion or before the 3rd batch of extrusion; If the number of extrusion times in the 1st batch is more than 50 times, the empty-die heavy load is arranged to be completed before the 2nd batch of extrusion; When the empty-die heavy load operation is applied, a steel gasket is placed in the central part of the extrusion die.

2. The method for improving the service life of a hollow aluminum profile extrusion die according to claim 1. It is characterized in that: Before applying the empty-die heavy load operation, the extrusion die needs to be preheated to 450 - 470°C first, and then reinstalled on the extruder.

3. The method for improving the service life of a hollow aluminum profile extrusion die according to claim 2. It is characterized in that: The steel gasket has the same preheating temperature as the extrusion die. ​

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