Various manufacturing processes for metal parts – Part 1 – Castings

Material forming method is an important aspect of metal part design, a crucial factor in the manufacturing process, and a matter of great concern for manufacturers. Today, we will take a look at various metal-forming processes.

The production method of pouring liquid metal into a mold cavity that conforms to the shape and size of the part, waiting for it to cool and solidify to obtain a blank or part, is usually called metal liquid forming or casting.

Liquid metal → Filling(Casting) → Shrinking & Solidifying→ Castings

• It can produce parts with any complex shape, especially those with complex inner cavities.
• It has strong adaptability and is not limited by the type of alloy or the size of the casting.
• There is a wide range of available materials, and scrap can be remelted with low equipment investment.
• The scrap rate is high, the surface quality is low, and the working conditions are poor.

The classification of casting processes includes sand casting, metal casting, pressure casting, precision casting, centrifugal casting, continuous casting, special casting, etc.

Sand casting is a casting method that produces castings in sand molds. Steel, iron, and most non-ferrous alloys can be obtained using the sand-casting method.

• It is suitable for producing blanks with complex shapes, especially those with complex internal cavities.
• It has wide adaptability and low cost.
• For some materials with poor plasticity, such as cast iron, sand casting is the only forming process for manufacturing their parts or blanks.

The main application of sand casting is for parts such as engine cylinder blocks, cylinder heads, crankshafts, etc., in automobiles.

Investment casting, also known as lost-wax casting, is a metal casting process in which a wax pattern is coated with multiple layers of refractory material to create a ceramic shell. The wax is then melted out of the shell, leaving behind a mold with no parting lines. The mold is then filled with molten metal after being heated to high temperatures, resulting in a casting. This process is commonly referred to as “lost-wax casting”.

• High dimensional accuracy and geometric precision
• High surface roughness
• Capable of casting complex-shaped castings, and the cast alloy is not restricted

The process is complex and costly.

It is suitable for producing small parts with complex shapes, high precision requirements, or that are difficult to process by other methods, such as turbine engine blades.

Die casting is a casting process that uses high pressure to inject molten metal into a precision metal mold cavity. The molten metal solidifies and forms the casting under the pressure of the mold.

• The molten metal is subjected to high pressure and flows quickly during die casting.
• The product has good quality, stable size, and good interchangeability.
• High production efficiency, and the die casting mold can be used many times.
• It is suitable for mass production and has good economic benefits.

• Castings are prone to tiny porosity and shrinkage.
• Die-cast parts have low plasticity and are not suitable for working under impact loads and vibration.
• The mold life is low while casting high-melting-point alloys, which affects the expansion of die-casting production.

Die-cast parts were first used in the automotive and instrument industries, and later gradually expanded to various industries such as agricultural machinery, machine tool industry, electronic industry, national defense industry, computers, medical equipment, clocks, cameras, and daily hardware.

Low pressure casting refers to the method of filling the mold with liquid metal under low pressure (0.02~0.06MPa) and crystallizing under pressure to form the casting.

• The pressure and speed during pouring can be adjusted, making it suitable for various casting types (such as metal molds, sand molds, etc.), casting various alloys, and casting parts of various sizes.
• Low-pressure casting adopts the bottom-filling method. The metal liquid filling is stable without splashing, which can avoid the entry of gas and the scouring of the mold wall and core, and improve the qualification rate of the castings.
• Castings crystallize under pressure, with dense and clear contours, smooth surfaces, and higher mechanical properties. This is particularly advantageous for the casting of large, thin-walled parts.
• Elimination of welding and brazing joints increases the metal utilization rate to 90-98%.
• Low labor intensity, good working conditions, simple equipment, easy-to-achieve mechanization, and automation.

Products of low-pressure casting are mainly traditional metal products, such as cylinder heads, wheel hubs, cylinder blocks, etc.

Centrifugal casting is a casting method in which metal liquid is poured into a rotating mold, and under the action of centrifugal force, the mold is filled and solidified into a shape.

Gravity casting is a molding method in which liquid metal is filled into a metal mold under the action of gravity and cooled and solidified in the mold to obtain a casting.

• The thermal conductivity and heat capacity of metal molds are high, and the cooling speed is fast, resulting in dense casting structures and mechanical properties about 15% higher than those of sand castings.
• Gravity casting with metal molds can achieve high dimensional accuracy and low surface roughness of castings, with good quality stability.
• Since there is little or no use of sand cores, the gravity-casting process can improve the environment, reduce dust and harmful gases, and reduce labor intensity.

• The metal mold used in gravity casting itself has no permeability, so certain measures must be taken to remove the air in the mold cavity and the gas generated by the sand core.
• The metal mold has no yielding property, so it is easy to produce cracks when the casting solidifies.
• The manufacturing cycle of metal molds is long, and the cost is high. Therefore, it can only show good economic benefits when mass-produced in large quantities.

Gravity casting with metal molds is suitable not only for mass production of complex-shaped non-ferrous alloy castings such as aluminum and magnesium alloys, but also for the production of castings and ingots of steel and other metal alloys.

Vacuum casting is a kind of advanced die-casting process that eliminates or significantly reduces the porosity and dissolved gases in die-casting by removing the gas in the cavity of the moulds during the casting process, so as to improve the mechanical properties of the die-castings and the surface quality.

• The use of vacuum die casting technology can eliminate or reduce porosity in the die casting, improve the mechanical properties and surface quality of the die casting, and improve plating performance.
• By reducing the counter pressure in the mold cavity, alloys with poor casting properties can be cast using lower pressure, and it is possible to cast larger castings with a small machine using vacuum die casting technology.
• Improved filling conditions can allow for the casting of thinner castings through die casting.

• The seal structure of the mold is complex, and its manufacturing and installation are difficult, resulting in higher costs.
• If not controlled properly, the effect of vacuum die casting may not be significant.

Squeeze casting is a method of solidifying and forming liquid or semi-solid metals under high pressure to directly obtain finished products or blanks. It has advantages such as high utilization of liquid metal, simplified processes, and stable quality. It is an energy-saving and potentially promising metal-forming technology.

Squeezing (Extrusion) casting is divided into direct squeezing casting and indirect squeezing casting, and the process flow of the two is as follows:

The process flow of direct squeeze casting: Mold coating – pouring alloy – mold closing – pressurization – holding pressure – pressure relief – mold opening – castings demolding – resetting.

The process flow of indirect squeeze casting: Mold coating – mold closing – feeding – filling – pressurization – holding pressure – pressure relief – mold opening – castings demolding – resetting.

• The use of squeeze casting can eliminate internal defects such as air holes, shrinkage holes, and shrinkage porosity.
• The surface roughness of the castings is low, and the dimensional accuracy is high.
• It can prevent the appearance of casting cracks.
• It is easy to realize mechanization and automation.

The squeeze casting process can be used to produce various types of alloys, such as aluminum alloys, zinc alloys, copper alloys, nodular cast iron, etc.

Lost foam casting is a new type of casting method that combines wax or foam models similar in size and shape to the castings into a cluster, brushes with refractory coatings and dries them, and buries them in dry quartz sand for vibration molding. The model is gasified under negative pressure, the liquid metal occupies the model position, and after solidification and cooling, the casting is formed.

The process flow of lost foam casting includes: pre-foaming -> foam molding -> foam model coating -> drying -> molding -> pouring -> sand falling -> cleaning

• Lost foam casting has no sand cores, reducing processing time.
• There is no parting line, and the design is flexible with a high degree of freedom.
• Clean production with no pollution
• It lowers investment and production costs.

Lost foam casting is suitable for producing castings of various sizes and complex structures without limitation on the type of alloy or production batch. Examples include gray iron engine blocks and high manganese steel bend pipes.

Squeeze casting is a method of solidifying and forming liquid or semi-solid metals under high pressure to directly obtain finished products or blanks. It has advantages such as high utilization of liquid metal, simplified processes, and stable quality. It is an energy-saving and potentially promising metal-forming technology.

• In continuous casting, the metal is quickly cooled, resulting in a dense and uniform crystalline structure and better mechanical properties.
• It saves metal and improves the yield of castings.
• The process is simplified, eliminating molding and other processes, thus reducing labor intensity and the required production area.
• Continual casting production is easy to realize through mechanization and automation, improving production efficiency.

Continual casting can be used to pour long castings of various alloys with unchanged cross-sectional shapes, such as ingots, slabs, billets, pipes, etc., including steel, iron, copper alloy, aluminum alloy, magnesium alloy, and so on.