1. introduction
In this article, for the high-pressure casting process of local thick parts prone to porosity and shrinkage, for porosity, shrinkage of the reasons for the analysis, the principle and structure of local extrusion, as well as local extrusion of the process of the main parameters: extrusion depth, extrusion pressure, extrusion delay time, extrusion duration, and so on; to solve the local thick parts of the porosity, shrinkage problem.
High-pressure casting is a casting method in which liquid or semi-solid metals or alloys, or liquid metals or alloys containing enhanced material phases, are filled into the cavity of the die casting mold at high pressure at a high rate, and the metals or alloys are allowed to solidify under pressure to form castings. High-pressure casting has the advantages of high efficiency, less cutting, etc. It has been widely used in automotive, communication base stations, instrumentation, transportation equipment, aviation, and other industries. Its high quality, efficiency, and ability to produce complex structures make it very suitable for mass production.
With the rapid development of the automobile industry, auto parts also have higher requirements: high performance, lightweight, and low cost are the development trends. In recent years, the rapid growth of automobile lightweight demand has driven the automotive aluminum alloy precision die casting industry’s rapid development; engine block, transmission housing, clutch housing, and other engine and chassis-related parts are commonly used for aluminum alloy die casting.
The demand for thin-walled die castings of hoods and covers with complex shapes and non-uniform wall thicknesses is increasing day by day, and they are required to pass the leakage test under high pressure, which brings challenges to the die-casting process and mold design in die-casting factories. Porosity, shrinkage holes, cold segregation, etc., are common defects of die casting; local extrusion is set up in the casting thick wall hot joints at the booster channel to implement the local pressurization, to solve the leakage, shrinkage holes at the thick wall is easy to produce, to improve the quality of casting, reduce the rate of scrap.
This article analyzes the formation mechanism of porosity and shrinkage in die-casting forming. It gives the principle and structure of local extrusion in the process of die-casting forming and analyzes the influence of extrusion depth, extrusion pressure, extrusion delay time, and extrusion duration on the quality of die-castings.
2. Die casting porosity, shrinkage causes
Due to the product structure, process parameters, mold structure, die-casting process, and other factors, die-casting parts often have some defects, such as porosity, cold segregation, shrinkage holes, insufficient pouring and oxidation slag, etc. These defects affect the product dimensions, mechanical properties, fatigue strength, high-pressure permeability, etc.; porosity and shrinkage are the main factors affecting the permeability of die casting under high pressure.
Pneumatic holes are in the metal hydraulic injection process involved in the barrel, cavity air and punch lubricant, mold releasing agent produced by the gas and can not be discharged, so air holes are generally round to be filled to the cavity inside the formation of air holes. Wall thickness is not uniform. Die casting quickly produces shrinkage holes in the thick part of the hot section. Shrinkage holes made by the reason are generally caused by volume contraction during solidification.
Take ADC12 alloy as an example. Its density in the high-temperature liquid state is 2.6g/cm3, and after solidification, it is 2.7g/cm3. There is a density difference of about 4%. Therefore, in the absence of external complementary shrinkage, about 4% of the shrinkage will result. The shrinkage cross-section tends to show an irregular pattern.
In most cases, if the die-casting manufacturer in the casting of air holes, shrinkage holes can be modified by modifying the location of the gate, adding a slag package, changing the process parameters (such as aluminum liquid temperature, low and high-speed point switching position, die-casting pressure, etc.) to improve the product qualification rate effectively. However, for some local structures, it is complex, wall thickness is not uniform, wall thickness is hanging far away from the gate, and the farthest end position is not in the parting surface. It can not set the slag bag and exhaust tank parts only by adjusting the die-casting process parameters and can not make defects to be improved. Local extrusion is a crucial method to reduce the critical part of the air holes and shrinkage holes.
3. Principle and structure of local extrusion
Localized extrusion is a method of obtaining high-quality die castings by installing cylinders directly inside the die and applying pressure directly to the part that produces shrinkage holes to inhibit them. Figure 1 shows a schematic diagram of the local extrusion structure.
Schematic diagram of local extrusion structure in die casting
In the liquid metal filling completed, after a certain period, that is, in the casting solidification process, the liquid metal in the cavity in the semi-solid state in the final solidification of the thick wall through the extrusion rod to apply pressure to force up for the shrinkage of the shrinkage hole, blowhole defects to reduce or eliminate the place, to improve the die casting pressurized leakage detection through the rate.
The local extrusion structure is mainly composed of an extrusion cylinder, extrusion pin, extrusion bushing, and related connecting devices. According to the design requirements, its structural devices for extrusion can be installed on the die frame and die core; see Figure 2.
Partial extrusion mechanical structure
The local extrusion molding method is mainly divided into the following two kinds, see Figure 3. One is in the casting molding surface pressurization; casting is the pressurized parts higher than the actual height of a certain distance above. Hence, so as not to cast a surface layer of cold material into the casting inside, the high part is directly through the subsequent machining removal; at the same time, the side wall of a single side left more than 2mm to avoid the hard layer, see figure 3a; the other is the casting of the thick-walled parts of the installation of the Local extrusion equipment, direct die casting bottom hole, see Figure 3b.
Figure 3 Local extrusion forming method
4. The main process parameters of the local extrusion process
In the local extrusion process, extrusion depth, extrusion pressure, extrusion start time, and extrusion end time are the main parameters affecting the effect of the pressurization process.
4.1 Extrusion depth
Extrusion depth, i.e., the amount of booster press-in, determines the amount of aluminum pressed into the liquid. Its size and die-casting thick parts of the organization have a direct relationship. Extrusion depth is too tiny, makeup into the dense parts of the metal liquid is not enough; makeup shrinkage is insufficient to achieve the purpose of local reduction or elimination of porosity, shrinkage and loosening, and other casting defects; Extrusion depth is too large, not only need to be a more significant pressure of the extrusion cylinder equipment, but extrusion pins due to the temperature are also easy to damage, and extrusion pins are easy to bend. Therefore, it is necessary to determine a reasonable depth of extrusion.
Figure 4 Schematic diagram of local extrusion depth
Figure 4 is a schematic diagram of the local extrusion depth. The volume of the square is V, and the volume shrinkage of the material is S. The ideal volume shrinkage is V-S, which is the amount of liquid metal replenishment. If the diameter of the extrusion rod is used as d, the ideal extrusion depth l can be calculated as:
The primary purpose of analyzing the squeeze depth is to determine the reasonableness of the squeeze delay time and to provide a basis for setting the squeeze duration.
4.2 Extrusion pressure
The size of the extrusion pressure depends on the cylinder diameter D. If the cylinder diameter D is too small, the local extrusion pressure is insufficient, which will result in inadequate make-up shrinkage; if the cylinder diameter D is too large, it will make the cylinder volume large, which will result in waste.
Analyze the extrusion pin force (see Figure 5), p for the casting pressure, d for the extrusion rod diameter, p′ for the extrusion cylinder pressure, and D for the cylinder bore. According to Pascal’s principle has:
According to the actual production experience, the extrusion pressure is more than three times the casting pressure can be determined
Figure 5 Extrusion pin force
4.3 Squeeze delay time (squeeze-in timing)
The extrusion delay time is the interval between the end of the liquid metal filling and the start of the localized extrusion process. The best extrusion results are obtained when the liquid metal is semi-solid, and the extrusion pressure is applied. The extrusion delay time is too short, the liquid metal is still in the flow state, and the thick parts did not increase the pressure of solidification and crystallization, resulting in the extrusion of the pin into the core, no extrusion effect; extrusion delay time is too long, solidification of the liquid metal, the formation of hard shells on the mold wall, resulting in the extrusion of the pin is blocked, can not make up for the shrinkage can not be realized to increase the effect of the pressure.
The optimum timing of extrusion pin insertion is related to the solidification state of the extruded part and the gate make-up shrinkage channel. Usually, the extrusion delay time needs to be determined by a test to determine the effective parameter range during the mold trial or with the help of CAE analysis; the extrusion delay time can be determined according to the temperature field of the extruded part when it is in the range of solid-liquid coexistence.
4.4 Squeeze duration
Extrusion duration refers to the time that the extrusion pin starts to extrude until the time when it retreats. Extrusion pin back too early, due to contact with the extrusion pin is not fully solidified, extrusion duration is short, will cause local extrusion molding hole bottom rupture; extrusion duration is long, will not only affect the efficiency but also the extrusion pin will be easy to pull off due to too much-tightening force. The extrusion pin is back to ensure its surrounding casting surface temperature is below the solid phase line. Factors affecting the size of the gas shrinkage hole are mainly extrusion pressure and extrusion duration.
5. Conclusion
This article analyzes the causes of porosity and shrinkage, explains the structure and principle of local extrusion, and introduces the calculation of extrusion process parameters; at the same time, the analysis and verification of specific products prove that to determine the reasonable depth of extrusion, extrusion pressure, extrusion delay time and extrusion duration, can significantly reduce the local thick-walled porosity and shrinkage, thus effectively improving the quality of the castings and the qualification rate.
FAQ
Question 1：How does the local extrusion process affect the castings’ visual quality?
Answer: The primary goal of the local extrusion method is to maximize the internal quality of castings, but it also indirectly improves the appearance quality. Local extrusion can improve castings’ overall appearance quality and surface finish by reducing internal faults, which also lowers the likelihood of surface defects (such as exposed pores, surface depressions, etc.).
Question 2：What mold requirements are there for the local extrusion process?
Answer: The local extrusion method has stringent mold requirements, which include:
High-strength materials: To survive the extrusion force, the mold material needs to be strong enough and resistant to wear.
To guarantee that the extrusion force is efficiently transmitted, the mold design must be exact, particularly about the geometry and placement of the extrusion portion.
High-temperature resistance: The mold material needs to be highly resistant to the high temperatures in the casting environment.
Cooling system optimization: To prevent thermal fatigue and cracks, an effective cooling system must be built to disperse heat swiftly.
Question 3：How does the local extrusion process affect the productivity of the production process?
Answer: The local extrusion method may marginally lower manufacturing efficiency because controlling the extrusion force during the casting process necessitates an extra extrusion step and time. Nonetheless, the procedure can raise total economic benefits and manufacturing efficiency by drastically lowering faults and trash rates and increasing the caliber of completed goods.
Question 4：Which casting materials work well with the regional extrusion method?
Answer: The local extrusion technique can cast various materials, such as copper, zinc, magnesium, and aluminum alloys. Specifically, it finds extensive application in the manufacturing of castings made of magnesium and aluminum alloys, which have strict casting quality and performance requirements and are utilized extensively in the automotive and aerospace industries.
Question 5：How does the local extrusion method differ from the conventional high-pressure casting technique regarding advantages?
Answer: In contrast to the conventional high-pressure casting method, the local extrusion technique offers the following benefits:
Defect reduction: efficiently lessen casting imperfections such as pores and shrinkage to enhance casting quality.
To improve performance, enhance the strength and durability of the castings in important areas, as well as their local density and mechanical characteristics.
Design flexibility enables the flexible application of various pressure zones throughout the casting design process, better fulfilling the performance requirements of intricate structural components.
Material saving: lower the amount of scrap produced due to casting flaws, saving money on materials and production.
