1. Introduction
The mold is a key tool for aluminum profile extrusion. During the profile extrusion process, the mold needs to withstand high temperature, high pressure, and high friction. During long-term use, it will cause mold wear, plastic deformation, and fatigue damage. In severe cases, it may cause mold breaks.
2. Failure forms and causes of molds
2.1 Wear failure
Wear is the main form that leads to the failure of extrusion die, which will cause the size of aluminum profiles to be out of order and the surface quality to decline. During extrusion, aluminum profiles meet the open part of the mold cavity through the extrusion material under high temperature and high pressure without lubrication processing. One side directly contacts with the plane of the caliper strip, and the other side slides, resulting in great friction. The surface of the cavity and the surface of the caliper belt are subjected to wear and failure. At the same time, during the friction process of the mold, some billet metal is adhered to the working surface of the mold, which makes the geometry of the mold change and cannot be used, and is also regarded as a wear failure, which is expressed in the form of passivation of the cutting edge, rounded edges, plane sinking, surface grooves, peeling, etc.
The specific form of die wear is related to many factors such as the speed of the friction process, such as the chemical composition and mechanical properties of the die material and the processed billet, the surface roughness of the die and billet, and the pressure, temperature, and speed during the extrusion process. The wear of aluminum extrusion mold is mainly thermal wear, thermal wear is caused by friction, the metal surface softening due to rising temperature and the surface of the mold cavity interlocking. After the surface of the mold cavity is softened at high temperature, its wear resistance is greatly reduced. In the process of thermal wear, temperature is the main factor affecting thermal wear. The higher the temperature, the more serious the thermal wear.
2.2 Plastic deformation
The plastic deformation of the aluminum profile extrusion die is the yielding process of the die metal material.
Since the extrusion die is in a state of high temperature, high pressure, and high friction with the extruded metal for a long time when it is working, the surface temperature of the die increases and causes softening.
Under very high load conditions, a large amount of plastic deformation will occur, causing the work belt to collapse or create an ellipse, and the shape of the product produced will change. Even if the mold does not produce cracks, it will fail because the dimensional accuracy of the aluminum profile cannot be guaranteed.
In addition, the surface of the extrusion die is subject to temperature differences caused by repeated heating and cooling, which produces alternating thermal stresses of tension and compression on the surface. At the same time, the microstructure also undergoes transformations to varying degrees. Under this combined effect, mold wear and surface plastic deformation will occur.
2.3 Fatigue damage
Thermal fatigue damage is also one of the most common forms of mold failure. When the heated aluminum rod comes into contact with the surface of the extrusion die, the surface temperature of the aluminum rod rises much faster than the internal temperature, and compressive stress is generated on the surface due to expansion.
At the same time, the yield strength of the mold surface decreases due to the increase in temperature. When the increase in pressure exceeds the yield strength of the surface metal at the corresponding temperature, plastic compression strain appears on the surface. When the profile leaves the mold, the surface temperature decreases. But when the temperature inside the profile is still high, tensile strain will form.
Similarly, when the increase in tensile stress exceeds the yield strength of the profile surface, plastic tensile strain will occur. When the local strain of the mold exceeds the elastic limit and enters the plastic strain region, the gradual accumulation of small plastic strains may form fatigue cracks.
Therefore, in order to prevent or reduce fatigue damage of the mold, appropriate materials should be selected and an appropriate heat treatment system should be adopted. At the same time, attention should be paid to improving the use environment of the mold.
2.4 Mold breakage
In actual production, cracks are distributed in certain parts of the mold. After a certain service period, small cracks are generated and gradually expand in depth. After the cracks expand to a certain size, the load-bearing capacity of the mold will be severely weakened and cause fracture. Or microcracks have already occurred during the original heat treatment and processing of the mold, making it easy for the mold to expand and cause early cracks during use.
In terms of design, the main reasons for failure are the mold strength design and the selection of the fillet radius at the transition. In terms of manufacturing, the main reasons are material pre-inspection and attention to surface roughness and damage during processing, as well as the impact of heat treatment and surface treatment quality.
During use, attention should be paid to the control of mold preheating, extrusion ratio and ingot temperature, as well as control of extrusion speed and metal deformation flow.
3. Improvement of mold life
In the production of aluminum profiles, mold costs account for a large proportion of the profile extrusion production costs.
The quality of the mold also directly affects the quality of the product. Since the working conditions of the extrusion mold in profile extrusion production are very harsh, it is necessary to strictly control the mold from the design and material selection to the final production of the mold and subsequent use and maintenance.
Especially during the production process, the mold must have high thermal stability, thermal fatigue, thermal wear resistance and sufficient toughness to extend the service life of the mold and reduce production costs.
3.1 Selection of mold materials
The extrusion process of aluminum profiles is a high-temperature, high-load processing process, and the aluminum extrusion die is subjected to very harsh usage conditions.
The extrusion die is subjected to high temperatures, and the local surface temperature can reach 600 degrees Celsius. The surface of the extrusion die is repeatedly heated and cooled, causing thermal fatigue.
When extruding aluminum alloys, the mold must withstand high compression, bending and shear stresses, which will cause adhesive wear and abrasive wear.
Depending on the working conditions of the extrusion die, the required properties of the material can be determined.
First of all, the material needs to have good process performance. The material needs to be easy to smelt, forge, process and heat treat. In addition, the material needs to have high strength and high hardness. Extrusion dies generally work under high temperature and high pressure. When extruding aluminum alloys, the tensile strength of the die material at room temperature is required to be greater than 1500MPa.
It needs to have high heat resistance, that is, the ability to resist mechanical load at high temperatures during extrusion. It needs to have high impact toughness and fracture toughness values at normal temperature and high temperature, to prevent the mold from brittle fracture under stress conditions or impact loads.
It needs to have high wear resistance, that is, the surface has the ability to resist wear under long-term high temperature, high pressure and poor lubrication, especially when extruding aluminum alloys, it has the ability to resist metal adhesion and wear.
Good hardenability is required to ensure high and uniform mechanical properties across the entire cross section of the tool.
High thermal conductivity is required to quickly dissipate heat from the working surface of the tool mold to prevent local overburning or excessive loss of mechanical strength of the extruded workpiece and the mold itself.
It needs to have strong resistance to repeated cyclic stress, that is, it requires high lasting strength to prevent premature fatigue damage. It also needs to have certain corrosion resistance and good nitridability properties.
3.2 Reasonable design of mold
Reasonable design of the mold is an important part of extending its service life. A correctly designed mold structure should ensure that there is no possibility of impact rupture and stress concentration under normal use conditions. Therefore, when designing the mold, try to make the stress on each part even, and pay attention to avoid sharp corners, concave corners, wall thickness difference, flat wide thin wall section, etc., to avoid excessive stress concentration. Then，cause heat treatment deformation, cracking and brittle fracture or early hot cracking during use, while the standardized design is also conducive to the exchange of storage and maintenance of the mold.
3.3 Improve the quality of heat treatment and surface treatment
The service life of the extrusion die largely depends on the quality of heat treatment. Therefore, advanced heat treatment methods and heat treatment processes as well as toughening and surface strengthening treatments are particularly important to improve the service life of the mold.
At the same time, heat treatment and surface strengthening processes are strictly controlled to prevent heat treatment defects. Adjusting quenching and tempering process parameters, increasing the number of pretreatment, stabilization treatment and tempering, paying attention to temperature control, heating and cooling intensity, using new quenching media and studying new processes and new equipment such as strengthening and toughening treatment and various surface strengthening treatment, are conducive to improving the service life of the mold.
3.4 Improve the quality of mold manufacturing
During the processing of molds, common processing methods include mechanical processing, wire cutting, electrical discharge processing, etc. Mechanical processing is an indispensable and important process in the mold processing process. It not only changes the appearance size of the mold, but also directly affects the quality of the profile and the service life of the mold.
Wire cutting of die holes is a widely used process method in mold processing. It improves processing efficiency and processing accuracy, but it also brings some special problems. For example, if a mold processed by wire cutting is used directly for production without tempering, slag, peeling, etc. will easily occur, which will reduce the service life of the mold. Therefore, sufficient tempering of the mold after wire cutting can improve the surface tensile stress state, reduce residual stress, and increase the service life of the mold.
Stress concentration is the main cause of mold fracture. Within the scope allowed by the drawing design, the larger the diameter of the wire cutting wire, the better. This not only helps improve processing efficiency, but also greatly improves the distribution of stress to prevent the occurrence of stress concentration.
Electrical discharge machining is a kind of electrical corrosion machining performed by the superposition of material vaporization, melting and machining fluid evaporation produced during discharge. The problem is that due to the heat of heating and cooling acting on the machining fluid and the electrochemical action of the machining fluid, a modified layer is formed in the machining part to produce strain and stress. In the case of oil, the carbon atoms decomposed due to the combustion of the oil diffuse and carburize to the workpiece. When the thermal stress increases, the deteriorated layer becomes brittle and hard and is prone to cracks. At the same time, residual stress is formed and attached to the workpiece. This will result in reduced fatigue strength, accelerated fracture, stress corrosion and other phenomena. Therefore, during the processing process, we should try to avoid the above problems and improve the processing quality.
3.5 Improve working conditions and extrusion process conditions
The working conditions of the extrusion die are very poor, and the working environment is also very bad. Therefore, improving the extrusion process method and process parameters, and improving the working conditions and working environment are beneficial to improving the life of the die. Therefore, before extrusion, it is necessary to carefully formulate the extrusion plan, select the best equipment system and material specifications, formulate the best extrusion process parameters (such as extrusion temperature, speed, extrusion coefficient and extrusion pressure, etc.) and improve the working environment during extrusion (such as water cooling or nitrogen cooling, sufficient lubrication, etc.), thus reducing the working burden of the mold (such as reducing extrusion pressure, reducing chill heat and alternating load, etc.), establish and improve the process operating procedures and safe use procedures.
4 Conclusion
With the development of aluminum industry trends, in recent years everyone is seeking better development models to improve efficiency, save costs, and increase benefits. The extrusion die is undoubtedly an important control node for the production of aluminum profiles.
There are many factors affecting the life of aluminum extrusion die. In addition to the internal factors such as the structural design and strength of the die, die materials, cold and thermal processing and electrical processing technology, heat treatment and surface treatment technology, there are extruding process and use conditions, die maintenance and repair, extrusion product material characteristics and shape, specifications and scientific management of the die.
At the same time, the influencing factors are not a single, but a complex multi-factor comprehensive problem, to improve its life of course is also a systemic problem, in the actual production and use of the process, need to optimize the design, mold processing, use maintenance and other main aspects of control, and then improve the service life of the mold, reduce production costs, improve production efficiency.
Edited by May Jiang from MAT Aluminum
Post time: Aug-14-2024
