Four elements of extruded heat sink: quality requirements of ingots, requirements of molds, reduction of extrusion force, extrusion process. Aluminum alloy is widely used in radiator materials because of its light weight, beautiful appearance, good thermal conductivity and easy processing into complex shapes. There are three main types of aluminum alloy radiator profiles: flat and wide, comb or fishbone; The radiating fins on the outside of the circle or oval are radial; Dendritic. As shown in Figure 1. Their common feature is: the distance between the radiating fins is short, a groove is formed between two adjacent radiating fins, and the ratio of depth to width is large; The wall thickness difference is large, generally the heat sink is thin, and the bottom plate at the root is thick. Therefore, it brings great difficulty to the mold design, manufacture and production of heat dissipation profiles.
Some of the radiator profiles are relatively small in size and symmetric in shape, which is easier to produce. Most of the radiator profiles are flat and wide, with larger dimensions and some asymmetrical. The groove shape between the heat sinks has a large aspect ratio, and its production is difficult. In order to successfully produce radiator profiles, the cooperation of ingots, molds, and extrusion processes is required. The alloy used for extruding heat sink profiles must have good extrudability and thermal conductivity. Commonly used alloys such as 1A30, 1035 and 6063. Currently, 6063 alloy is widely used because it has good mechanical properties in addition to good extrudability and thermal conductivity.
The production of aluminum alloy radiator profiles should start with the quality of the ingot, the material and design of the mold, the reduction of extrusion force and the extrusion process.
1. Quality requirements of aluminum ingots
The alloy composition of the ingot must be strictly controlled to ensure the purity of the alloy. For 6063 alloy, the content of Fe, Mg and Si should be controlled. The content of Fe should be less than 0.2%, the content of Mg and Si are generally controlled within the lower limit of the national standard, the content of Mg is 0.45% to 0.55%, and the content of Si is 0.25% to 0.35%. The aluminum ingot must be fully homogenized to make the structure and performance of the ingot uniform.
The surface of the aluminum ingot should be smooth, and no segregation tumor or sandy mud is allowed. The end face of the ingot should be flat, and can not be cut into steps or the cutting slope is too large (the cutting slope should be within 3 mm). Because the step shape or the cutting slope is too large, when the heat dissipation profile is extruded with a flat die, if the diversion is not designed, the ingot will directly touch the die. Due to the uneven end surface of the ingot, some places contact the mold first, causing stress concentration, which is easy to squeeze the tooth shape of the mold, or cause the discharge sequence to be different, and it is easy to cause mold blockage or poor extrusion molding.
2. Requirements for extrusion die
Because the die of the radiator profile is a lot of slender fins, it has to withstand a great extrusion force, and each tooth must have high strength and toughness. If the performance is very different from each other, it is easy to cause the local fins with poor strength or toughness to break. Therefore, the quality of mold steel must be reliable, and it is best to use H13 steel produced by a reliable manufacturer, or use high-quality imported steel. The heat treatment of the mold is very important. To use vacuum heating and quenching, it is best to use high-pressure pure nitrogen quenching, which can ensure the uniform performance of all parts of the mold after quenching. After quenching, three times of tempering are required to ensure that the hardness of the mold is under the premise of HRC48-52, and it has sufficient toughness. This is an important condition to prevent the mold from breaking teeth.
For the radiator profile to be successfully extruded, the key is that the mold design must be reasonable and the manufacturing must be accurate. Generally try to avoid direct extrusion of the ingot onto the die belt. For the flat and wide comb-shaped radiator profile, design a diversion die with a small middle and large sides to make the metal flow to both sides, reduce the extrusion force on the die belt, and make the pressure distribution uniform. Since the wall thickness difference of the radiator profile section is large, the difference should be kept correspondingly when designing the mold working belt. That is to say, the working belt should be particularly enlarged in places with large wall thickness, which can be as large as 20mm～30mm, and the position of the fin tip should break through the conventions to minimize the working belt. In short, we must ensure the uniformity of metal flow everywhere. For flat and wide radiators, in order to ensure a certain rigidity of the mold, the thickness of the mold should be appropriately increased. The thickness increase is about 30% to 60%. The production of the mold should also be very fine, and the empty knife should be symmetrical up and down, left and right, and in the middle. The machining error between the fin and the fin should be less than 0.05mm. The large machining error is prone to produce partial fin teeth, that is, the thickness of the heat sink is not uniform, and even the phenomenon of fin broken.
For sections with more mature designs, inlay alloy steel molds are also a better method, because alloy steel molds have better rigidity and wear resistance, and are not easy to deform, which is beneficial to the formation of radiator profiles.
In order to prevent the mold from breaking the teeth, the extrusion force should be minimized. The extrusion force is related to factors such as the length of the ingot, the resistance to deformation of the alloy, the state of the ingot, and the degree of deformation. Therefore, the cast rod of extruded heat-dissipating aluminum profile should not be too long, which is about 0.6 to 0.85 times the length of the normal cast rod. Especially when testing the mold and extruding the first casting rod, in order to ensure the smooth production of qualified products, it is best to use a shorter casting rod, that is, a casting rod that is 0.4 to 0.6 times the normal length of the casting rod. mold.
For the cross-section of heat-dissipating profiles with complex shapes, in addition to shortening the length of the cast rods, short pure aluminum castings can also be considered for the first trial extrusion. After successful trial extrusion, use normal ingots for extrusion production.
Homogenization annealing of the ingot can not only make the structure and properties uniform, but also improve the extrusion performance and reduce the extrusion force, so the ingot must be homogenized annealing. As for the influence of the degree of deformation, since the cross-sectional area of the radiator profile is generally relatively large, and the extrusion coefficient is generally within 40, its influence is small.
4. Extrusion process
The key to the production of radiator profiles is the first trial of the extrusion die. If possible, you can do a simulation test on the computer first to see if the working belt of the mold design is reasonable, and then try the mold on the extruder. The first mold trial is very important. The operator should make the main plunger advance slowly at a low pressure lower than 8MPa when it is pushed up. It is best for someone to look at the exit of the mold with a flashlight, and wait for each heat sink of the extrusion mold to evenly squeeze the die hole, then gradually pressurize and accelerate the extrusion. When continuing to squeeze after successful mold trial, attention should be paid to controlling the squeezing speed to achieve smooth operation. Pay attention to the heating temperature of the mold when producing radiator profiles, and make the mold temperature close to the ingot temperature. If the temperature difference is too large, the temperature of the metal will drop due to the slow extrusion speed during upward pressure, which will easily cause mold blockage or uneven flow rate.
