Aluminium die-casting die is an important processing technology and equipment. Its good or bad performance, high or low life, directly affects the quality of enterprise products and economic benefits. And mould material and heat treatment technology are the main factors affecting the life of aluminium and magnesium alloy die casting mould. In this article, we analyze the following 3 aspects to improve the mould’s thermal stability, hardenability, wear resistance, thermal fatigue, thermal conductivity, and other properties to improve the mould life.
①The main failure forms of aluminium die casting mould
②Briefly introduce the typical mould steel and common heat treatment methods
③Point out the reasonable selection of mould materials and heat treatment process
Table of Contents
Aluminium die-casting mould failure form
Aluminium alloy die-casting mold is used in the die-casting machine, the working cavity surface temperature can reach 600 ℃, and molten aluminum alloy liquid is easy to adhere to the steel material, must work on the mold cavity frequently coated with an anti-adhesive coating, resulting in the cavity surface temperature fluctuations, its failure is mainly in the form of sticky mold, erosion, thermal fatigue and wear. When the mold cavity structure is complex, and there is stress concentration, the mold will also fracture and fail under the combined effect of thermal and mechanical load.
Pic: Die casting mould fixed core, H13 steel
Aluminum alloy parts and mold surface Ming to movement, due to surface unevenness, some contact points local stress exceeds the yield strength of the material bonding occurs, the bonded junction occurs shear fracture and yank away, so that the mold surface material transfer to the workpiece or off.
When the mold surface is in contact with aluminum alloy liquid for relative movement, the bubble between the liquid and the mold ruptures to produce immediate impact and high temperature, causing the mold surface to form tiny pockmarks and craters. Aluminum alloy liquid and tiny solid particles fall at high speed, repeatedly impacting the mold surface so that the local material is lost in the metal surface into pockmarks and pits. Repeated action: The mould county’s surface will sprout fatigue cracks and cause a local fracture.
The die surface is repeatedly subjected to heating and cooling stress caused by fatigue and cracking formation. The main reason for cracking aluminium die-casting mould is the difference between the mould’s pouring and preheating temperatures. The greater the temperature difference, the faster the cooling speed, and the easier the thermal fatigue cracking, followed by the speed of the thermal cycle. The mould heat treatment process and surface treatment also have a close relationship.
As the friction between the mould surface and the processed high-temperature aluminium alloy parts can not get lubrication, oxidation by the high-temperature workpiece, the mould cavity surface is tempered and softened, low hardness and intensify the wear, serious wear so that can’t the mould processed out of qualified products and scrap failure.
Aluminium die-casting mould in the work of large cracks or partial separation and loss of normal serviceability of the phenomenon known as fracture failure. Die fracture usually manifests as local broken pieces or the entire mould broken into several parts.
Aluminum alloy die casting mould steel selection
3Cr2W8V (H21) steel
It contains more tungsten, chromium and vanadium elements, so it has higher hardenability, tempering stability and thermal strength, and is suitable for die casting dies with high load capacity, high thermal strength and high tempering stability.
4Cr5MoSiV1 (H13) steel
It has high toughness and hot and cold fatigue resistance, not easy to produce thermal fatigue cracks, even if there are thermal fatigue cracks are thin and short, not easy to expand, no need to preheat before use, and can use tap water spray cooling, with high thermal strength.
4Cr5Mo2MnSiV1 (Y10) steel
Molybdenum with a mass fraction of about 2%, supplemented by vanadium and mud to improve thermal stability, and a moderate amount of silicon and manganese to improve the strength of the matrix, with good thermal fatigue properties and resistance to molten metal corrosion.
4Cr5MoSiV (H11) steel
It is a tungsten hot work steel with good toughness at medium temperature, good thermal strength, thermal fatigue performance and certain wear resistance, air quenched at lower austenitizing temperature, small heat treatment deformation, small tendency to produce oxidation skin when air quenched, and can resist the corrosion effect of molten aluminium.
3Cr3Mo3VNb (HM3) steel
A new type of high toughness hot forging dies steel. Its carbon content is low when adding trace elements Nb, improves tempering resistance and thermal strength, has a significant secondary hardening effect, and can effectively overcome the early failure of the die caused by thermal wear. Thermal fatigue, thermal cracking, etc…
4Cr3Mo3SiV (H10) steel
In the working temperature of 500 ~ 600 ℃ with higher hardness, thermal strength and wear resistance, and has very good hardenability and high toughness, tempering resistance and thermal stability than H13 steel, impact toughness and fracture toughness than 3Cr2W8V steel, when the tempering temperature exceeds 260 ℃, the hardness of the steel is higher than H13 steel.
The use of high toughness mold material to improve the performance of aluminum alloy die-casting die and extend the mould’s service life is very important. For example, a mold made of 3Cr2W8V steel is processed directly from the material. Overall dimensions φ180x85mm, hardness requirements 42 ~ 46HRC, production use only 249,900 pieces of die-casting. Later, it was manufactured with 4Cr3Mo3SiV, and the service life was increased to 1000.000 pieces.
Auminium alloy die-casting, die heat treatment process selection
The heat treatment of the aluminium die casting die changes the organization of the die steel so that the die can get the required organization and performance, which can prolong the life of the die. It should determine heat treatment process specification according to the die’s material, shape, size and complexity.
Die casting mould pre-heat treatment can be used continuous annealing, isothermal annealing and tempering heat treatment three processes. The purpose is to obtain a uniform organization and diffuse distribution of carbide before the final heat treatment to improve the steel toughness. The continuous annealing process is relatively simple and can obtain a better grain pearlite organization. The isothermal annealing can obtain an ideal grain pearlite organization for the die casting mould with complex shape and high requirements.
Die-casting mold steel is the poorest thermal conductivity of high-alloy steel. The quenching and heating often take pre-heating measures, the number of pre-heating and temperature, depending on the composition of the mold steel and the requirements of the mold deformation. The quenching temperature is low. The shape is simple, and the deformation requirements of the mold are not high in the case of no cracking, quenching and heating to a pre-heating (800 ℃ ~ 850 ℃). For higher temperature quenching and shape complex, high deformation requirements of the mold must be the second pre-heating (600 ~ 650 ℃, 800 ~ 850 ℃). The purpose is to reduce the stress generated during heating while making the overall mold organization uniform.
It can execute the quenching heating temperature of die casting mold according to each steel grade’s quenching heating specification. For example, the quenching temperature of 3Cr2W8V steel is 1050~1150℃, and the quenching temperature of H13 steel is 1020~1100℃. To ensure that the carbide is fully dissolved, to obtain a uniform composition of austenite, and to obtain good high-temperature performance, die-casting mold quenching heating and holding time should be properly extended, generally in the salt bath furnace heating and holding factor of 0.8 ~ 1.0min/mm.
Quenching and cooling
Oil quenching fast cooling speed can obtain good performance, but the tendency of deformation cracking. Generally, for simple shape, deformation requirements of the die-casting die using oil cooling; for complex shape, deformation requirements of high die-casting die, to prevent mold deformation and to crack, it is appropriate to use graded quenching. Quenching and cooling should be as slow as possible to reduce quenching deformation, heating and quenching in the vacuum resistance furnace. Cooling can take gas quenching. Cooling can be taken to graded quenching in the salt bath in Shanghai heating quenching and other methods. Die quenching and cooling are generally cooled to 150 ~ 200 ℃ even heat should be immediately after tempering, do not allow cooling to room temperature.
The hardness requirement of die casting mold is achieved by tempering, and the hardness of die casting mould cavity affects the mould’s cold and hot fatigue life. Different materials, quenching temperatures, and tempering temperatures are also different. Such as 3Cr2W8V steel and aluminium alloy die casting, die hardness requirements are generally 42 ~ 48HRC, and the tempering temperature is generally between 560 ~ 620 ℃ to choose, but if the high-temperature quenching, the tempering temperature is up to 670 ℃. 1150 ℃ quenching 650 ℃ tempering hardness of 45HRC; and 1050 ℃ quenching 650 ℃ tempering hardness of 35HRC.
Surface hardening treatment
The surface hardness of the die casting die is not very high after quenching and tempering. To make the surface of the die casting die to get very high. Hardness and wear resistance, while the heart maintains sufficient strength and toughness, improve the aluminium alloy die casting die anti-stick performance, die casting die surface nitriding or nitrogen-carbon infiltration treatment. The use of toughening treatment and surface strengthening process is an important way to improve the performance and life of the die. Such as H13 die casting die nitriding heat treatment medium for ammonia + ethanol, process 580 ℃ x 4.5h. After 1030 ℃ quenching 600 ℃ tempering and then 580 gas nitriding heat treatment, die surface hardness of 900HV or more, the matrix hardness 46 ~ 48HRC, die wear resistance, fatigue resistance, corrosion resistance, etc. has been greatly improved.
Aluminum alloy die-casting mold production according to the mold working conditions, analysis and research failure cause correct choice of mold materials, reasonable development of heat treatment process, to ensure the mold surface hardness, wear resistance and heart strength, toughness, to prevent metal liquid erosion, sticky mold, effectively reduce the rate of scrap, significantly improve the service life of the mold. Production practice proves that the aluminum alloy die-casting mould pre-heating to an effective and economical temperature can reduce the temperature difference between the mold and the workpiece, reduce the mold cracks, extend the service life of the mold, and improve productivity. Of course, the correct use, good management and careful maintenance of the aluminum alloy die-casting mold in the process of use are also effective measures to reduce the mould’s early fracture failure and improve the mould’s service life.
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