Overview of investment casting
When wax is used to make a pattern, investment casting is also called “lost wax casting”. Investment casting usually refers to the casting scheme in which the shape is made from fusible materials, the surface of the shape is coated with several layers of refractory materials to make a mold shell, and then the mold is melted out of the mold shell, so as to obtain the mold without parting surface, which can be filled with sand and poured after high temperature roasting. Investment casting is often called “lost wax casting” because of the extensive use of waxy materials to manufacture the pattern.
The types of alloy produced by investment casting are carbon steel, alloy steel, heat-resistant alloy, stainless steel, precision alloy, permanent magnet alloy, bearing alloy, copper alloy, aluminum alloy, titanium alloy and nodular cast iron, etc.
Generally, the shape of investment castings is relatively complex. The minimum diameter of the casting hole can reach 0.5mm, and the minimum wall thickness of the casting is 0.3mm. In production, some parts originally composed of several parts can be designed into a whole part and directly cast by investment casting through changing the structure of the parts, in order to save processing hours and metal material consumption, so that the structure of the parts is more reasonable.
Casting process of aluminum alloy
Aluminum alloy casting refers to aluminum alloy profiles for which parts can be immediately obtained by casting forming process of metal materials. The alloying element composition of aluminum alloy castings generally exceeds the composition of the corresponding deformed aluminum alloy profiles. Aluminum alloy casting is a traditional metal material, because of its relatively small density, high strength characteristics, widely used in aviation companies, aerospace, vehicles, mechanical equipment and other fields.
The features of aluminum alloys
Aluminum alloy is made of aluminum ingot as raw material, adding some elements, such as copper, silicon, magnesium, zinc, manganese and other auxiliary materials through a series of complex processes, is the most widely used in the industry of a class of non-ferrous structural materials. It is widely used in aviation, aerospace, automobile, mechanical manufacturing and chemical industry. At present, aluminum alloy is the most versatile and durable alloy.
Characteristics of aluminum alloy:
1. Low density
2. High intensity
3. Good conduction and thermal conductivity
4. Corrosion resistance
5. Easy to process
Types of Aluminum Used in Investment Casting
1. Aluminum silicon alloy, also known as “silicon aluminum amine” or “silicon aluminum amine”. It has good casting performance and some very wear-resistant properties, and the coefficient of thermal expansion is relatively small. Among the cast aluminum alloy, the variety is the most, the largest amount of alloy, the silicon content is 4% ~ 13%. Sometimes add 0.2% ~ 0.6% magnesium silicon aluminum alloy, widely used in structural parts. Such alloys are widely used in parts such as pistons.
2. Aluminum-copper alloy, alloy with copper content of 4.5% ~ 5.3% has the best strengthening effect. Appropriate addition of manganese and titanium can significantly improve the strength and casting performance at room temperature and high temperature. Mainly used for making sand castings bearing large dynamic and static load and shape is not complicated.
3. Aluminum magnesium alloy, the cast aluminum alloy with the lowest density (2.55g/cm3) and the highest strength (about 355MPa), containing 12% magnesium, has the best strengthening effect. The alloy has good corrosion resistance in atmosphere and sea water, and good mechanical properties and machinability at room temperature.
4, aluminum zinc alloy, in order to improve the performance often add silicon, magnesium, often known as “zinc silicon aluminum Ming”. Under casting conditions, the alloy has quenching effect, that is, “self-quenching”. Can be used without heat treatment, to metamorphic heat treatment, castings have high strength.
What are the alloys in aluminum casting?
Aluminum alloy materials in aluminum alloy die casting are mainly divided into three kinds of materials: aluminum silicon alloy, aluminum silicon copper alloy and aluminum magnesium alloy.
1. Al-si alloy: mainly includes YL102 (ADC1, A413.0, etc.), YL104 (ADC3, A360);
2. Aluminum silicon copper alloy: mainly includes YL112 (A380, ADC10, etc.), YL113 (3830), YL117 (B390, ADC14), ADC12, etc.;
3. Aluminum magnesium alloy: mainly includes 302 (5180, ADC5,) ADC6, etc.
The features of aluminum alloys investment casting
Aluminum alloy casting is a very versatile process that offers a range of additional benefits including:
1. High strength to weight ratio.
2. Excellent chemical stability.
3. Excellent corrosion resistance.
4. High temperature resistance.
5. Excellent thermal and electrical conductivity.
6. Smooth, cast finish.
The difference between stainless steel castings and aluminum alloy castings
Here are some key differences between aluminum castings and stainless steel castings that affect the choice of alloys for industrial parts:
1. Machinability. Aluminum is more malleable and easier to work with than stainless steel.
2. Weld. Stainless steel is easier to weld than aluminum.
3. Strength and weight. While aluminum is not as strong as stainless steel, it weighs about a third as much, making it a good choice in situations where strength-to-weight ratios are important.
4. Electrical conductivity. Aluminum is a very good conductor, while stainless steel is very bad.
5. Heat resistance. Aluminum softens at 400 degrees Fahrenheit, while stainless steel can withstand higher temperatures.
6. Cost. Aluminum usually costs less than stainless steel.
The pros of choosing aluminum investment castings
Investment casting aluminum is a kind of less cutting or no cutting casting technology, is an excellent technology in the casting industry, its application is very wide. It is not only suitable for casting of various types and various alloys, but also the dimensional accuracy and surface quality of the cast aluminum parts produced are higher than other casting methods. Even the complex, high temperature resistant, not easy to process cast aluminum parts that are difficult to be cast by other casting methods can be cast by investment precision casting.
The biggest advantage of investment casting aluminum is that because investment castings have very high dimensional accuracy and surface finish, mechanical processing can be reduced. Only a little processing allowance can be left in the parts with higher requirements. Even some cast aluminum parts only leave grinding and polishing allowance, and can be used without mechanical processing. It can be seen that the method of investment casting aluminum can save a lot of machine tools and processing hours, and greatly save metal raw materials.
Another advantage of the investment casting aluminum method is that it can cast a variety of complex castings, especially superalloy castings. For example, the blade of a jet engine, its streamlined outline and cooling cavity can hardly be formed by machining technology. Using the process of investment casting aluminum can not only achieve mass production, ensure the consistency of castings, but also avoid the stress concentration of the residual blade after machining.
The cons of choosing aluminum investment casting
1. Oxidizing slag inclusion
Defect characteristics: Oxidized slag is mainly distributed on the upper surface of the casting, in the corner of the casting mold ventilation. The fracture is mostly grayish white or yellow, which is found by X-ray perspective or in mechanical processing, and can also be found in alkali washing, pickling or anodizing.
2. Pores and bubbles
Defect characteristics: The pores in the wall of the three castings are generally round or oval, with smooth surface, generally shiny oxide skin, sometimes oil yellow.
3. Shrink loose
Defect characteristics: Shrinkage of aluminum castings generally occurs at the root thickness of the fly riser near the inner runner, at the wall thickness transition and at the thin wall with a large flat surface. The fracture is gray in the casting state, pale yellow after heat treatment, pale yellow or gray black. The X-ray film shows a cloudy shape. The serious filamentous shrinkage can be found through X-ray, fluorescence fracture and other inspection methods.
4. Cracks
Causes: The structure design of the casting is unreasonable, there are sharp corners, the thickness of the wall is too wide, the sand mold is not good, the local overheating of the casting mold.
Aluminum alloy castings are widely used in aviation industry
Aluminum alloy has a series of advantages such as low density, high specific strength, good corrosion resistance and formability, low cost, and has a wide range of application prospects and irreplaceable status in aviation, aerospace, shipbuilding, nuclear industry and weapons industry. In aviation, aluminum alloy is the main material of aircraft body structure.
Although the density of aluminum alloy is low, but its strength is relatively high, close to or more than high-quality steel, good plasticity, can be processed into various profiles, has excellent electrical conductivity, thermal conductivity and corrosion resistance, widely used in industry, the use of second only to steel
Aluminum alloy can be divided into deformation aluminum alloy and casting aluminum alloy according to processing methods. Cast aluminum alloy, used in the as-cast state; Deformed aluminum alloy, can withstand pressure processing, mechanical properties are higher than as-cast. Can be processed into various forms, specifications of aluminum alloy. Mainly used for manufacturing aviation equipment, daily necessities, building doors and Windows.
Aluminum alloy castings are widely used in civil industry
Aluminum alloy die casting has some advantages that other castings cannot match, such as beauty, light quality, corrosion resistance and other advantages, so that it is widely favored by users. Aluminum alloy die casting, especially since the automobile lightweight, casting aluminum alloy castings have been widely used in the automobile industry.
The density of cast aluminum alloy is lower than cast iron and cast steel, but the specific strength is higher. Therefore, under the same load condition using aluminum alloy castings, can reduce the weight of the structure, so in the aviation industry and power machinery and transport machinery manufacturing, aluminum alloy die casting parts are widely used. Aluminum alloy has good surface luster, good corrosion resistance in the atmosphere and fresh water, so it has a wide range of uses in the manufacture of civilian vessels. Pure aluminum in nitric acid, acetic acid and other oxidizing acid medium has good corrosion resistance, so aluminum alloy die casting has a certain use in the chemical industry. Pure aluminum and aluminum alloy have good thermal conductivity, heat exchange devices used in chemical production, and power machinery requirements with good thermal conductivity parts, such as the cylinder cover and piston of internal combustion engine, are also suitable for manufacturing with aluminum alloy.
What problems can arise during the process of casting aluminum alloy?
Common problems:
1. Pinholes
2. Bubbles
3. Sand holes
To solve the above problems mainly solve the problem of “water”. To solve the water, the essence is to solve the hydrogen in aluminum alloy melting engineering. Characteristics of aluminum alloy: easy oxidation, easy to absorb the air and the moisture of aluminum ingot itself. So to control the phenomenon of pinhole, bubble and sand hole, to prevent and control from the source, including the requirements of temperature, the use of slag agent and degassing
