Measures to improve the service life of die casting die

Measures to improve the service life of die casting die

The main reasons for the failure of die casting die are: ① the alternating stress of thermal expansion and cold contraction, repeated circulation for a long time, and thermal fatigue turtle cracks on the die surface; ② Cracking and damage of the whole mold caused by thermal stress and mechanical stress; (3) Under the action of injection force and thermal stress, the mold will crack at the weakest part, which will rupture the cavity; ④ Mold erosion caused by chemical corrosion, mechanical wear, scouring erosion and melting loss erosion; ⑤ Plastic deformation of the mold under the action of mold locking, core insertion pressure and filling pressure. The reasons for the failure of these molds are complicated and varied. Based on practical application, this paper discusses some measures to improve the service life of die casting die.

1 Selection of die casting die materials

In order to improve the thermal shock toughness, the chemical purity requirements of commonly used steel H 13 are as follows: the S content (mass fraction, the same below) of superior steel should be less than 0.005%; Super H 13 steel requires S content less than 0.003%; The content of phosphorus is less than 0.0 15%. There is no * * crystal carbide inclusion at the grain boundary of steel, and the strength of massive * * crystal carbide and impurities is very small, which can not resist thermal fatigue and reduce the plasticity of steel, and is the origin point of cracking. The electroslag remelting furnace should be used to refine steel, which has high purity, compact structure, excellent thermal fatigue resistance, good thermal cracking resistance, excellent toughness and plasticity, excellent polishing performance and good anisotropy. The uniformity of steel requires that the microstructure of materials should be uniform, and the mechanical properties of billets should be the same in any direction, and there should be no performance differences in longitudinal, transverse and depth directions.

Correct selection of die materials and high-strength alloy materials can improve the service life of the die. Sweden 8407, Germany 2344, American H13 (4R5MOVLSI) and Japanese SKD6 1 materials are preferred. DAC55 and ZHD435 of Hitachi, Japan have good toughness and high temperature strength under high hardness, and the die life is also very long.

2 heat treatment of die casting mold

Different heat treatment processes will make the quality and performance of die casting die different. The heat treatment process and metallographic structure of H 13 die steel should refer to the North American Die Casting Society (NADCA 207? 2003). It is suggested that die steel manufacturers should be responsible for the heat treatment of dies to avoid quality differences caused by different materials and heat treatment manufacturers.

H 13 steel should be quenched in a high-pressure liquid nitrogen-cooled high-vacuum furnace, which can effectively prevent decarburization, oxidation, deformation and cracking on the die surface. The quenching temperature is raised to 1020 ~ 1050℃, and the temperature and holding time are properly controlled according to the size of the module material and the required strength and toughness of each component, so that the alloy carbide can be fully dissolved into austenite, and the mold cracking caused by insufficient dissolution of carbide at grain boundaries during heat treatment can be reduced. However, attention should be paid to the critical points ac 1, Ac3 and holding time of steel to prevent austenite from coarsening. After quenching, temper for three times at different temperatures, and pay special attention to the tempering effect. If nitriding treatment is needed, one tempering treatment can be reduced.

Annealing can reduce or eliminate cutting stress, metamorphic layer stress caused by EDM and thermal fatigue stress caused by die casting. The die should be annealed regularly to relieve stress: the first stress relief annealing should be arranged before quenching (annealing temperature is 700 ~ 750℃), and the second stress relief annealing should be arranged before mass production after passing the die test, and then annealed once during die casting of 65438+100000 and 30000 dies, and nitriding once can replace the first annealing treatment. The stress relieving annealing temperature of H 13 steel is 20 ~ 40℃ lower than the last tempering temperature during quenching, and the holding time is1.0 ~1.5 h. ..

Reasonable selection of the hardness (HRC) of the mold. When AISI H 13 ESR material is used in die casting dies, if the hardness is low, it is easy to cause die sticking and early cracking, and if the hardness is too high, it may crack. Therefore, it is generally recommended that the hardness (HRC) of zinc alloy die casting die is 47 ~ 52; Small and medium-sized aluminum-magnesium alloy die casting dies are 46 ~ 48; For large aluminum and magnesium alloy castings and molds with thick or complex shapes, the hardness (HRC) should be reduced to 44~46. Hitachi DAC55, ZHD435 and DIEVAR steels have good toughness and high temperature strength under high hardness, and the hardness (HRC) can be increased by 2 ~ 4 compared with H 13.

Surface strengthening treatments such as nitriding and carbonitriding should be selected for the parts and all cores on the surface of die-casting mold cavity to reduce die sticking or erosion. At present, KANUC in Japan is widely used for processing. If nitriding is needed, the total depth of nitriding layer on the die surface should be less than 0.2 ~ 0.3 mm, and it should be controlled at 0.04~0.08mm from thick to thin according to the wall thickness of the casting. There should be no compound white bright layer to prevent the die from cracking after too many white bright layers are broken. For parts that are easy to stick to the mold, surface treatment such as nitriding can be carried out every 1 ~ 20000 times of die casting. When the die is die-cast for 800 ~ 654.38+ten thousand times, and it is easy to stick to the die due to the decrease of hardness, nitriding treatment can also be carried out. Before and after each annealing and nitriding, the surface of the mold should be polished. In order to prevent the die cavity from oxidative corrosion before mass production, the die should be pre-oxidized at 530 ~ 560℃ 1.5 ~ 2.0 h after passing the test.

3 design of die-casting die

The wall thickness of die casting should be as uniform as possible (generally, the thickness of small parts is 2.5? 1mm, and the thickness of the middle piece is 3.0? 1 mm, and the thickness of large parts is 4.0? 1mm), corner transition should have rounded corners or slopes to reduce stress concentration, and rib structure can be used to eliminate hot spots formed by castings. Excessive thickness of die casting has coarse grains, which will form pores, shrinkage porosity, oxidation and internal cracks, accompanied by stress sources, making its strength and durability lower than that of products formed by reinforcing ribs.

The easy-to-crack parts and easy-to-damage parts of the mold should adopt the insert structure as far as possible, which is convenient for maintenance and replacement after damage. However, the distance between the insert hole on the molded part including the core hole and another hole near the edge of the mold should not be too small, and the inner corner of the insert hole should be chamfered to avoid becoming a weak part of the early cracking of the mold.

In order to improve the rigidity of die design, it is necessary to analyze the stress of each part of die cavity. The forces acting on the cavity include the pressure, bulging force and impact force when the alloy liquid is filled, the tension and friction force when demoulding, the thermal stress caused by temperature change, the pressure, tension and pre-tightening force when the mold is opened, closed and inserted into the core. When designing, all parts in the mold should have enough thickness and width, and the mold should have enough rigidity to withstand various stresses. It is also important to balance these forces properly to prevent mold deformation and cracking. Pay attention to the thin section of the mold and the concave corner root of the module when manufacturing, which are sensitive parts of the mold fracture. To ensure the matching accuracy, if the pre-tightening force of the module is too large, the clamping force will be concentrated at one point, which is the main factor for the large-area fracture of the mold.

So as to better prevent the overall deformation of the mold. Correctly design the stress center position of the cavity to make it as close as possible to the stress center of the die casting machine. The two pads behind the moving die should be supported on the cavity insert of the die as far as possible, not only on the sleeve plate outside the cavity insert; The supporting area of the supporting column or the supporting block in the middle behind the moving die should be large enough, otherwise the end face of the supporting block (even the die mounting plate of the die casting machine) will be easily deformed and lose its supporting function.

Parts with concave corners on the die are prone to stress concentration. Try to have a large transition fillet at the corner of the product to avoid narrow and deep concave corners and grooves. The fillet radius of the die cavity of Al-Mg alloy die casting die should be greater than 65438±0.0mm, and the surface roughness should be small to avoid premature cracking at the fillet. Near the inner gate, increasing the radius of fillet as much as possible can better delay the appearance of turtle crack in the early stage of die. Choose the combination structure of insert and movable slider reasonably to avoid sharp corners on the module; The joint area of the sealing surface of the embedded contact is large, and when the slider retreats, the aluminum liquid will not escape from the sealing surface into the guide groove of the slider; In order to prevent the movement from being stuck, the side of the slider is equipped with an inclined plane.

Correctly design the gating system, design the position of the inner gate and the filling direction, and try to avoid the impact of high-speed filling aluminum liquid on the mold wall or core. When designing the cross section of the inner gate, if the injection filling speed is too fast, a lot of kinetic energy will be converted into heat energy and transferred to the mold, which will increase the temperature of the mold and cause mold sticking, cracking and erosion defects. The maximum filling speed of die-casting aluminum liquid shall not exceed 56m/s, and the filling speed shall be? 46 m/s is better. When designing the thickness of the inner gate, it is best to choose a thicker and larger inner gate under the condition of ensuring the surface quality of the product, so as to increase the flow without increasing the impact on the mold.

The fitting tolerance and surface roughness of each component must be correctly selected. Because the die expands unevenly when heated, the fit tolerance will change, and the movement of parts will fail, which will lead to the damage of the die surface and the increase of the clamping gap between the moving and fixed modules, resulting in flash and flash. In order to prevent flash, the cavity insert plane of the moving and fixed dies should be slightly higher than that of the sleeve plates of the moving and fixed dies, generally within the range of 0 ~ 0.080 mm, especially the gap between the sleeve plates of the moving and fixed dies should be within the range of 0.030 ~ 0. 100 mm after the die is closed. The depth of the shallowest part of the exhaust passage on the sleeve plate is 0. 12 ~ 0. 15 mm, which must include the gap between the movable die sleeve plate and the fixed die sleeve plate after die closing. Only in this way can we prevent flash, flying material and die sticking. Try to make the parting surface of each part of the sleeve plate consistent with the parting surface of the module and flush with the sleeve plate of the module, so as to reduce the steps of the parting surface, facilitate the exhaust and prevent the flash from sticking to the mold.

Try not to set the words, marks and thimbles of products on the cavity plane near the inner gate. These will cause the mold to crack prematurely, and also make the lettering marks unclear too early.

Try to use -Q2 diagram, so that the die can be well matched with the die casting machine, improve the product qualification rate and production efficiency, and prolong the service life of the die.

Design of cooling and heating system for die casting mold

In order to control the mold temperature and prevent the mold from deformation and cracking, it is necessary to design a cooling and heating temperature control system for the mold. Usually, a tube with a diameter of (6 ~ 12) mm is opened in the mold module, and a cooling hole with a diameter of (3 ~ 12) mm is opened in the core and module, which is cooled by water and heated by hot oil. Electric heating tube can also be used in die casting plants without mold temperature machine (controlling heating temperature? 400℃) and thermometer? Mold, automatic heating preheating mold.

On the back of the cavity module, process a hole (6 ~ 8) mm, and the spacing between the holes should be (25? 5)mm, more than 50 mm away from the cooling water or heating oil passage, insert a thermocouple and connect it to the thermometer of the die casting machine.

Near the runner, branch runner and inner gate of the mold, the thick-walled cavity and core of the casting should be cooled with water. Hot oil or electric heating tube heating dies should be designed for thin-walled cavities, sliding block core pulling away from the inner gate, and some parts in the die cavity that absorb less heat and dissipate heat quickly. The temperature of hot oil is generally 200 ~ 350℃. Pay attention to the cooling water channel of the mold to have enough distance from the mold surface or corner to avoid premature cracking of these parts or cracking of the mold surface.

Each inlet pipe joint of the mold should have a switch to control the cooling water flow so as to adjust the temperature of each part of the mold. Rust and scale in the cooling water pipeline will affect the cooling effect of the mold and should be removed in time. It is recommended to use copper and stainless steel for pipes and joints connected with molds to prevent pipes from being blocked after rusting.

5. The influence of die casting die manufacturing on die life.

The dimensional accuracy and matching accuracy of mold manufacturing should be high, and the mating surface of sealing contact must be sealed and matched, and the area of sealing contact should be large to prevent molten aluminum from drilling in. Try to avoid welding repair treatment caused by human factors, because the repaired parts of the mold are easy to crack.

After electric pulse discharge machining, the surface of cavity will produce metamorphic layer. The chemical composition, metallographic structure and mechanical properties (strength, hardness and toughness) of this layer have changed. The metamorphic layer is hard and brittle, with stress and a large number of microcracks, which will cause early cracking of the mold. When finishing with electric pulse or WEDM, low current and high frequency should be used as much as possible to reduce the overburning depth of the die surface. Good use of special oil for EDM can play the role of cleaning, cooling, lubrication, insulation, ionization prevention and reduction of deteriorated layer. When discharging, soaking in oil can reduce the metamorphic layer more than washing oil. No matter how deep the metamorphic layer is, there is great stress on the die surface. If the white layer and residual stress are not eliminated, the die surface will crack, corrode and crack earlier in use.

When finishing the mold cavity, the tool feed should be small and no tool marks should be left. If necessary, polishing allowance should be left. All surfaces of the mold cavity, even those without traces of machining tools, should be polished once to eliminate the hardened layer and white layer produced by tool machining or electric discharge machining. However, it should be noted that the mold should not be overheated locally during grinding, so as not to burn the surface of the mold and reduce the hardness of the mold. The methods to eliminate hardened layer, white layer and stress include: ① polishing of oilstone, grinding and polishing, and chemical corrosion removal; (2) Glass shot peening can not only remove the surface melting and solidification layer and eliminate the residual tensile stress, but also form compressive stress, which is a good method to delay cracking at present; (3) Low temperature tempering can also greatly reduce the surface stress of the die without reducing the hardness. When polishing the surface of mold cavity, the roughness depends on the product: ① the surface of thin-walled and bright product should be polished properly, and the surface roughness Ra is 0.2 ~ 0.4? m； ② On the surface of thick-walled products with general surface requirements, the cavity surface can be polished, and the surface roughness Ra is 0.4 ~ 0.8? m； ③ Generally, mirror surface is not required for polishing, so that the release agent can be uniformly attached to the surface of the mold, but the tool marks must be polished to avoid premature cracking of the mold; (4) Pay attention to cross grinding, and the marks on the surface of the mold cannot have obvious grinding direction.

The influence of die casting process and production operation on the life of die casting die.

Increasing the iron content in die-casting aluminum alloy can effectively reduce the degree of mold sticking. General requirements of aluminum alloy iron content? 1.5%, it is best to control the iron content of molten aluminum in the range of 0.65% ~ 0.90% in actual production. In the process of die casting, the temperature fluctuation of aluminum liquid should be within? In the range of 10℃, it is suggested that the pouring temperature of ADCl2 aluminum alloy should be less than 660℃ in spring and autumn, and it can be changed up and down in winter and summer 10℃, which can eliminate seasonal defects. It is easy to crack and corrode near the inner gate of the mold, and it is not easy to crack and corrode away from the inner gate. This is mainly because the high-temperature molten aluminum near the inner gate transfers more heat to the mold, resulting in higher mold temperature. Therefore, the pouring temperature of molten aluminum should be reduced as much as possible without affecting the product quality.

Under the condition of meeting the forming requirements, low injection speed and high injection speed should be adopted as far as possible. Too fast filling speed will cause mold sticking, erosion and cracking; When the low-speed injection velocity is high, the molten metal is wrapped by more gas, and when the high-speed injection velocity is high, the gas will expand in the low-pressure area of the cavity, resulting in detonation. The gas will drive the molten aluminum to impact and erode the surface of the cavity at high speed, resulting in cavitation defects on the surface of the cavity (such cavitation will also occur at the gate of the overflow channel), and cracks will also appear on the eroded surface.

Under the condition of satisfying good molding, the pressure should be as small as possible. Shell-shaped and round products can be observed. After thousands of dies are die-cast, the turtle crack on the outer surface of the same part of the product is much larger than that on the inner surface, which shows that under the same conditions, the direction of the extrusion and expansion force of aluminum liquid on the dies is different, resulting in great differences in the size of the defects that cause the dies to crack. Especially at the concave corner of the mold cavity, tensile stress and thermal stress will concentrate here, and cracks and cracks will appear prematurely at the concave corner; However, when the convex corner and core surface of the die are squeezed and subjected to thermal shock, the die will stick, but the stress concentration is very small and the die is not easy to crack. It can be seen that the pressure and stress direction of molten aluminum have great influence on die cracking. Sometimes, in order to match the life of modules that are not easy to crack, better mold materials or heat treatment methods can be adopted to improve the life of modules that are easy to crack.

In the process of die casting, the surface temperature of the die rises from 100℃ to 6 10℃, which is more likely to cause cracks than from 200℃ to 6 10℃, and the surface temperature rises from 200℃ to 680℃ and from 200℃ to 6 10℃. The mold is more prone to crack when it is kept at above 500℃ for 6 s than for 3 s, so it is necessary to make the mold withstand low temperature at high temperature, with small temperature difference change and short time. In general, the surface temperature of the die (or the internal temperature of the die with thermocouple) should not be higher than 40% ~ 45% of the molten alloy temperature, that is, the temperature of the aluminum alloy die should be less than 320℃, preferably 200 ~ 280℃. When closing the mold, the surface temperature of the mold should not be lower than 20% of the pouring temperature of the alloy, and generally 130 ~ 2 10℃ is appropriate.

The die-casting aluminum alloy die is preheated to 180 ~ 300℃ and then poured and injected. Compared with the die-casting aluminum liquid preheating die, it can delay the appearance of turtle cracks on the die surface. Because the mold is preheated by direct pouring and injection of aluminum liquid, the temperature difference on the surface of the mold is relatively large. For the first batch of 10 ~ 20 die castings die-cast after die preheating, low-speed injection should be adopted to reduce the close contact between molten aluminum and the die, reduce the speed of heat transfer to the die, and achieve the purpose of slow heating.

Spraying release agent evenly during die casting operation can reduce the die sticking and wear of aluminum liquid to the die. In order to prevent the release agent from chilling the mold, it is best to preheat the aqueous release agent to 20 ~ 30℃ in winter. Spray release agent to form mist, and the nozzle should be far away from the mold surface (20? 10)cm, angle of inclined die face 15? 5? It works best. Don't spray too much release agent, and control the spraying time between 0.5 and 2.5s; Spraying and pouring spraying are prohibited to prevent the mold surface from cooling rapidly. In order to reduce the chilling speed, the method of exchanging spraying between moving and fixed dies for many times can be adopted. In addition, after the casting is ejected, it needs to be painted and lubricated at the top of the thimble before returning to prevent the thimble from being stuck.

For many molds, glass balls, ceramic balls or micro-electric pulses are often used to polish the roughness of some parts of the mold, and even fine mesh bars with an interval of 0.5 ~ 1.5 mm are repaired on the surface of the mold. This can not only prevent cracking, prolong the service life of the mold, but also reduce the flow speed of aluminum liquid and eliminate cold insulation and patterns on the surface of the product; It can improve the heat absorption speed of the mold surface, make the surface of the product solidify quickly, and increase the temperature of the mold surface due to the rapid heat absorption of the mold surface, accelerate the volatilization of paint and water, eliminate residual moisture, and prevent the casting from appearing bubbles and blackening.

7 Use and Maintenance of Die-casting Mold

When installing the mold, at least six pressure plate bolts should be installed in each half of the moving mold and the fixed mold. If only four pressure plate bolts are installed in each mold half, as long as one bolt is loose and the other three bolts are seriously unbalanced, the bolts will be deformed or broken quickly, and even the mold will be pulled off.

In the process of die casting, it is necessary to polish and polish the mold sticking marks in the mold cavity in time, but be careful not to chisel or knock the mold with hard tools. When the surface roughness of the mold cavity becomes larger, it should be polished well. When all or part of the product is stuck in the mold cavity, it should be handled by experienced mold repairers to prevent die-casting workers from damaging the mold during the handling.

Lubricate the slider, guide post and ejector pin of the mold once per shift, and check and clean the cooling water channel of the mold per shift to make it smooth and sealed. Observe the sealing fit between the parting surface of the mold and the slider in each shift, find and repair the flash and seam of the mold as early as possible to prevent it from causing serious crushing, depression, deformation and flash defects.

When the mold is not used, it is best not to spray paint on the mold after the last die casting. If paint has been sprayed, compressed air should be used to blow off the residual moisture on the surface and deep cavity of the mold to prevent the mold from rusting. After each batch of production is completed, or every time 10 thousand molds are produced, the molds should be maintained. Every maintenance, it is necessary to apply red lead powder to check the deformation and sealing fit of the mold, eliminate gaps to prevent flash, eliminate uneven stress on the module or slider, and prevent the module from crushing and bursting. After maintenance, the mold cavity, core-pulling slider, ejector pin, guide post and parting surface should be coated with antirust oil.

When the mold is corroded, fractured, defective and cracked in a small area and the insert cannot be replaced, argon arc welding can only be used for repair. In order to effectively prevent the die casting mold from cracking easily after welding repair, the argon arc welding covered electrode designated by the die steel manufacturer should be selected first, and it should be noted that the specifications of covered electrode used before and after die quenching treatment may be different. Before argon arc welding, the mold cracks and other defects should be polished off to expose the metal matrix, and the module should be preheated to 300 ~ 450℃ with an electric furnace (if the mold is slowly baked with the flame of acetylene oxygen welding, the preheating range may not reach the required temperature range, and the temperature is uneven, which has no obvious effect on preventing cracks after butt welding. ) and clean the surface before argon arc welding to prevent blowholes during welding repair; When the mold temperature is higher than 475℃, welding repair should be stopped and welding should be carried out after the mold is cooled; Attention should be paid to welding, and it must be repaired alternately, instead of welding line by line, so as to better reduce the temperature rise and stress generated during welding. Welding repair after quenching, and then stress relief annealing at 20 ~ 50℃ below quenching tempering temperature for 2 ~ 3h (welding repair before quenching, annealing temperature is 750℃), can well eliminate the stress generated during welding.

For the carbon deposit on the coating adhered to the surface of the mold, in addition to polishing with oilstone and sandpaper, the carbon deposit can be removed evenly and effectively by pneumatic spraying glass balls or ceramic balls without affecting the dimensional accuracy of the mold.

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