Squeeze Casting Process: Trends and Opportunities

Abstract

This chapter introduces the importance of casting process, particularly in ferrous foundries. It opens with a high level functional classification of casting processes, with focus on squeeze casting, and its application in the design of metal matrix composites. To lay a suitable foundation on the subject, detailed discussions on the process parameters, process sequence, cost effectiveness, factors governing the selection of the process, associated casting defects, merits and demerits of the process are included. Special emphasis is given to discussions on the casting defects remedial measures and casting quality, types of squeeze casting processes, differences between them, area of application and components that can be manufactured using squeeze casting. The chapter closes with a brief discussion on the future trends and opportunities for improving the squeeze casting process.

1. Introduction

Metal matrix composites (MMCs) are frequently produced through squeeze casting. Because of their superior stiffness and strength than homogenous materials, MMCs are frequently used to replace engineered materials. MMCs have been used in aeronautical, automotive, and defense engineering structural applications. MMCs are typically made in industries using squeeze casting, stir casting, infiltration, and spray deposition techniques [1]. It is particularly well-known in the automotive industry for the production of diesel engine pistons. This chapter describes the process, parameters, applications and casting flaws that occurs during squeeze casting of metal matrix composites [2]. The mechanism of the defects creation is examined, as well as its implications for squeeze casting’s future.

2. Casting processes

Molten materials, usually metals and their alloys, are used in casting operations. After that, the molten material is poured into a mold cavity, which takes the shape of the finished part. The molten material then cools until it solidifies into the required shape, with heat being extracted and conducted mostly through the mold. Despite the fact that the above represents a reasonably straightforward operation, casting is inherently a difficult process due to the metallurgy of working with molten metal [3].

2.1 Classification of casting processes

Casting techniques can be divided into two categories based on the type of mold.

2.2 Materials

• expendable mold processes;

• Permanent mold processes.

The molds are destroyed in expendable mold operations in order to remove the casting. Sand, plaster, and ceramics mixed with a bonding agent are common mold materials. Permanent mold procedures, on the other hand, require the mold to be created in such a way that the casting may be easily removed. Permanent molds are typically formed of metals that keep strength at high temperatures.

Various casting procedures can be used, as shown in Figure 1. The majority of them can handle complex geometries in a variety of weights and sizes. Overall, these casting procedures are utilized, because:

• they may make complex shapes with internal holes or hollow sections;

• several casting methods can generate extremely big components;

• they can be used to process materials that would be difficult to treat otherwise;

• casting may be the most cost-effective way of manufacturing depending on the lot size.

Figure 1.

General classification of casting processes [4].

Other factors are considered while determining the suitability of distinct casting techniques for a given part. Under the category of general qualities, these are discussed. Finally, casting processes are used to classify a variety of plastic processing techniques [5].

2.2.1 Importance of casting processes in ferrous foundries

1. Casting can result in products with intricate shapes and internal voids.

2. It can be utilized to make pieces that are a few hundred grams to several kilograms in weight (thousands of kilograms).

3. Any desired complicated shape can be produced.

4. Casting can be used to process both ferrous and non-ferrous materials.

5. It is cost-effective and waste-free, with excess metal from each casting being re-melted and re-used.

6. Cast metals and alloys exhibit isotropic properties. It has the same physical and mechanical qualities in both directions and along both sides.

7. Casting process is very adaptable to the needs of mass manufacturing, enabling for the rapid production of a large number of castings. The automotive industry, for example, mass-produces cast engine blocks, gearbox cases and other engine components.

8. The required casting tools are less expensive and easy to use.

9. Certain metals and alloys can only be processed through casting.

3. Squeeze casting

Because of its ability to mass produce, simpler process parameter control, improvements in wettability of the reinforcements by the liquid metal, better metallurgical quality of matrix alloys due to solidification under pressure, and the ability to reinforce only selected regions of components, squeeze casting is the preferred metal matrix composite manufacturing process for a wide range of commercial applications [6].

Squeeze casting is a hybrid of casting and forging in which molten metal is injected into a warmed die and the upper die is closed after solidification to form the mold cavity. Squeeze casting causes the metal to completely fill the cavity due to the pressure produced by the higher die, resulting in a good surface quality and low shrinkage.

Both ferrous and non-ferrous alloys can be squeeze cast, however aluminum and magnesium alloys are the most frequent due to their lower melting temperatures. Parts for automobiles are a popular use [7].

The liquid metal is forced against the die walls, preventing air gaps from forming at the casting–die interface. Because pressure is applied, defects such as porosity and shrinkage are minimized, allowing for the production of finer grain castings with higher strength [8].

Preheating the die containing the preform to 300–400°C is the first step in the casting process. The punch is then driven into the die cavity at a constant ram speed of around 10 m/s after the molten metal has been injected into the die. In most circumstances, a pressure of 20–30 MPa is ideal. During solidification and a subsequent cooling time of 5–10 min, the pressure is maintained [9]. After then, the ram is removed and the composite is ejected. Squeeze casting enables for the elimination, or at least reduction, of not just gas porosity, but also flaws caused by solidification shrinkage. The origins of squeeze casting can be traced back to squeeze forming, which is a three-phased process:

1. pouring a known amount of molten metal into a pre-heated die cavity on the press’s lower plate;

2. closing the die and pressurizing the liquid metal;

3. maintaining the pressure until complete solidification and extracting the casting

A schematic of a squeeze casting procedure is shown in Figure 2.

Figure 2.

Schematic representative of a typical squeeze casting machine [10].

3.1 Types of squeeze casting process

The two basic forms of squeeze casting process may be distinguished, depending on the natural pressure applied as shown in Figure 3.

1. The direct squeeze casting mode.

2. The indirect squeeze casting mode.

• Direct squeeze Casting

Figure 3.

Direct and indirect squeeze casting [11].

Direct squeeze casting (DSC) is also known as liquid metal forging. The DSC method involves Pouring liquid metal into a warmed, lubricated die and forging it while it solidifies [12]. The pressure is applied shortly after the metal begins to freeze and maintained until the entire casting has solidified. Casting ejection and handling are identical to closed die forging ejection and handling.

• Indirect Squeeze Casting

Direct squeeze casting (DSC) is often performed on a vertical machine (akin to a forging press), whereas indirect squeeze casting (ISC) is performed on both vertical and horizontal machines. During indirect squeeze casting, molten metal is fed to the shot sleeve and then injected into the die cavity through relatively large gates and at a low velocity (usually less than 0.5 m/s). The plunger applies high pressure “indirectly” through the huge gating system to solidify the melt in the die cavity. Figure 4 compares the metal flow in a typical die casting method to an indirect squeeze casting method [12]. The slower injection speed of the ISC method supports planar filling of the metal face within the die cavity, removing trapped gases from the castings (Table 1).

Figure 4.

Schematic illustration of metal flow in (a) conventional die casting; and (b) indirect squeeze casting process [12].

Direct squeeze casting method	Indirect squeeze casting method
The pressure for preform penetration is provided directly to the melt in the direct squeeze casting method.	The melt is forced into the preform by a gate system in indirect squeeze casting.
There is no gate mechanism, direct squeeze casting tooling is relatively easy.	The tooling is more complicated, and a gating mechanism is present.
The existence of oxide residue in the composite is another difference.	The oxide residue in the composite is stopped by the gating system.
In most cases, this is done on a vertical machine (similar to a forging press).	Indirect squeeze casting (ISC), which uses both vertical and horizontal machines, is more analogous to traditional high-pressure die casting.

Table 1.

Differences between direct squeeze casting method and indirect squeeze casting method [13].

3.2 Reasons for squeeze casting

There has been a continuing need and necessity to make automobiles lighter and more fuel efficient while also improving passenger comfort. Automobile makers have been looking for solutions to keep or reduce vehicle mass. Dies have prompted die casting producers to develop new parts that were formerly iron castings or stamped steel assemblies, as well as stronger die castings that can be welded and painted. Because it gives characteristics to the metal that are difficult to accomplish with GPM casting and standard die casting, squeeze casting is typically referred to as a ‘high integrity’ method. Reduced porosity in the metal matrix, improved mechanical capabilities, and increased wear resistance are among the improved qualities. Squeeze castings can also be heated-treated, which is not possible with traditional die castings. In comparison to traditional die castings, thicker runner systems and, in particular, massive in-gates are utilized. The casting can be solidified under sufficient pressure to avoid practically all shrinkage by properly positioning the in-gates and maintaining high pressure on the molten alloys, as well as the use of pressure pins (if needed) during solidification. The high pressure used during solidification retains the molten metal in direct contact with the die surface, resulting in castings that are faithful to the die dimensions. Because the filling rates, also known as in-gate velocities, are low, entrapped gas in the casting is usually prevented with correct venting. As a result, the part is pore-free or substantially pore-free. Squeeze casting is a procedure that combines the benefits of both casting and forging into a single operation. The process’s main selling points are the possible cost savings compared to forging and the metallurgical advantages compared to alternative manufacturing techniques. It has been demonstrated that the squeeze casting technique can generate sound and fine equiaxed grain structures in most commonly used cast alloys even some that are generally only employed in wrought form. This sound cast structure is bound to give the material isotropic properties. With excellent dimensional accuracy and repeatability, the squeeze casting technique may produce complex shapes. This allows designers to construct near-net shapes, reducing the need for further machining. Automobiles are subjected to extensive research and development in order to increase their efficiency and functionality. These enhancements frequently result in increased vehicle weight and decreased engine performance, resulting in poor fuel efficiency. Meanwhile, in order to address global environmental challenges, the desire for automobiles that are lighter and consume less gasoline is increasing. One viable option for addressing these criteria is to replace steel with aluminum. Engine blocks and gearbox cases are two examples of structural parts made from aluminum die casting. Die casting goods are currently being used in important safety elements such as suspension and space frames, which demand a high degree of strength, elongation, and yield strength.

3.3 Application of squeeze casting in the design of metal matrix composites (MMC)

Depending on the type of hardening particles used and the intended application condition, metal matrix composite materials are made via casting or powder metallurgy. Aluminum, magnesium, copper, titanium alloys, and super alloys are the most frequent metal matrixes [14]. Graphite, carbon, oxides, carbides, boron, molybdenum, and tungsten are common hardening particle or fiber materials. Casting route is used to make the majority of reinforced aluminum. Depending on the composition of the aluminum alloys, several casting procedures are used. The most common casting methods are gravity, vacuum, rotary centrifugal, squeeze, and extrusion casting. In some cases, an automatic bottom pouring stir casting furnace (Figure 5) for melting aluminum and/or magnesium metals and alloys is provided, along with the various casting methods. The bottom pouring stir casting furnace has a bottom furnace that pours molten metal composites directly into the casting equipment by activating a single mechanism while the stirring process continues [16].

Figure 5.

Bottom type stir casting set up with squeeze casting attachment [15].

As a result, new casting procedures have been developed to minimize these flaws. Squeeze casting, out of all the current casting processes, offers the greatest potential for producing fewer defective cast components. Figure 6 shows the micrograph of a) squeeze casting and b) conventional casting (Table 2).

Figure 6.

A micrograph of (a) squeeze casting and (b) conventional casting [17].

Process parameter	Influence if increased
Melt temperature	Improved fluidity, dissolved gas content, and vulnerability to hot tears
Die temperature	Solidification is slowed, and tolerances are altered
Punch speed	Faster die filling means a greater possibility of jetting
Pressure	Better consolidation, fewer flaws, and a higher risk of seizing and galling
Pressure hold time	Interfacial bonding between different materials is improved
Lubrication thickness	Better fluidity, lubricant may be embedded into the part surface (poor surface quality), and lubricant buildup can jam moving die components, particularly ejectors

Table 2.

Squeeze casting factors that have an impact on part quality [9].

3.4 Squeeze casting process sequence

1. A quantity of molten metal is poured into a warmed die cavity on a hydraulic press’s bed.

2. The press is turned on to pressurize the liquid metal and shut the die cavity. This is the process of molten metal solidifying under pressure.

3. The metal is held under pressure until it has solidified completely. Not only does this boost the rate of heat flow, but it also has the potential to remove macro/micro shrinkage porosity. Furthermore, because gas porosity nucleation is pressure-dependent, porosity formation due to dissolving gases in molten metal is limited.

4. The punch is finally removed, and the component is ejected.

The squeeze casting sequence of operation is schematically illustrated in Figure 7.

Figure 7.

Schematic illustrating squeeze casting process sequence of operations. (a) Melt charge, preheat, and lubricate tooling. (b) Transfer of melts into die cavity. (c) Close tooling, solidify melt under pressure. (d) Eject casting, clean dies, charge melt stock [18].

3.5 Process parameters

The primary process parameters during squeeze casting are as follows:

1. The temperature of molten metal varies depending on the alloy and part geometry. The starting point is usually 35–55°C above the melting point.

2. Die temperatures typically range from 300 to 400 degrees Celsius.

3. Squeeze pressures of 20–30 MPa are commonly utilized, with 25 MPa being the most common, depending on the part geometry and mechanical qualities required.

4. Pressure duration: can range from 30 to 120 seconds, depending on part geometry.

5. Lubrication level: When sprayed on the warm dies before to casting, a suitable grade of graphite spray lubricant has proven sufficient for aluminum, magnesium, and copper alloys. To prevent welding between the casting and the metal die surfaces, ceramic-type coatings are necessary.

6. Punch to-die-clearance button metal volume

7. Quality and quantity of the melted product

Process parameters: a number of parameters that can affect the casting quality for both direct and indirect Squeeze casting are explained below;

• The first is the alloy and its quality; in reality, the alloy’s melting temperature and thermal conductivity influence die life and dictate casting parameters like die temperature. As a result, squeeze casting is preferred for low melting temperature alloys like Al and Mg. Metal cleaning is also crucial to avoid dross and oxide impurities in the casting.

• As previously stated, melt quantity is critical in direct SC, and precision control systems are required to ensure casting dimensional control. The die cavity can be built to accommodate the presence of an enlarged appendix in a noncritical area for the distribution of any extra metal. Lynch offered a compensating hydraulic piston and cylinder to control the exact amount of metal in the die as an alternative. Overflows are also a viable option.

• The heat transfer rate and alloy cooling are affected by the operating temperature of the die cavity and punch. In reality, too low a temperature in the die can produce premature solidification and cold laps in the casting, while too high a temperature in the die can create surface flaws and metallization (casting and die welding). For Al and Mg alloys, the die temperature is normally between 200 and 300°C, with the lower temperature being ideal for thicker section parts. A lubricating agent, usually made of graphite, should be employed.

• The time between the actual pouring of liquid metal and the instant the punch begins forcing the alloy into the die cavity is known as the time delay. A time delay is proposed to allow cooling of the metal pool before squeezing in order to prevent shrinkage porosity. This time varies based on the melt/pouring temperature and the casting’s complexity. Two additional important parameters are the magnitude and duration of the applied pressure. Pressure has a direct impact on the microstructure and mechanical qualities of squeeze cast components because it determines the solidification temperature.

Squeeze casting is simple and low-cost, makes good use of raw materials, and has a lot of potential for automation at high speeds. The technique produces cast products with the best mechanical qualities possible. A fluid metal is solidified under pressure during solidification in the squeeze casting process, resulting in a high cooling rate and temperature gradient. Squeeze casting has a lot of advantages, including low porosity density, heat treatability, consistency, and good mechanical qualities [11].

3.6 Merits and demerits of squeeze casting

Merits

1. Its mechanical qualities are superior.

2. It has a finer structure and less porosity.

3. It has a smoother, better-finishing surface.

4. It is extremely profitable.

5. Possibility of use in composite goods.

6. Post-casting machining is minimal or non-existent.

Demerits

1. The metallic mold’s life expectancy is reduced.

2. High precision control is required.

3. Because of the intricate tooling, costs are extremely expensive.

4. Tooling has no versatility.

3.7 Associated casting defects

1. Oxide inclusions: Failure to maintain clean melt-handling and melt-transfer systems is the cause of oxide inclusions. Filters should be included in the melt-transfer system, or molten metal turbulence should be controlled when filling the die cavity, to reduce the chances of introducing metallic inclusions. It’s also a good idea to keep foreign materials out of open dies.

   The image below represents oxide inclusion (Figure 8).

2. Porosity and voids: When not enough pressure is applied during squeeze casting procedures, this can happen. When the other factors are tuned, porosity and/or voids are usually reduced by increasing the casting pressure. The image below represents porosity on cast materials (Figure 9).

3. Extrusion Segregation: Squeeze cast components have significantly less relative micro segregation than other cast components. Defects like these can be avoided by properly designing dies, using a multiple gate system, increasing die temperature, or reducing the delay time before die closure (Figure 10).

4. Centerline segregation: At lower solute temperatures, it’s a fault that is commonly found in high-alloy wrought aluminum alloys. As the lower-melting solute is contained within the center sections of the extruded projections or more massive areas of the casting as solidification occurs on the die walls, the liquid phase becomes more concentrated. Such flaws can be eliminated by increasing die temperature, reducing die closure time, or choosing a different alloy (Figure 11) [18].

5. Blistering: During turbulent die filling, trapped air or gas from the melt generates blisters on the cast surface when the pressure is released or during subsequent solution heat treatments. Degassing the melt and preheating the handling transfer equipment, utilizing a slower die closing speed, increasing the die and punch venting, and lowering the pouring temperature are all ways to avoid such problems (Figure 12).

6. Cold laps: Molten metal covering previously solidified layers causes inadequate bonding between the two. It’s vital to raise the pouring temperature or the die temperature to avoid chilly laps. It has also been discovered that reducing the die closure time is useful (Figure 13).

7. Hot tearing: occurs in alloys with a wide freezing temperature range. Contraction of the solid around the stiff mold surface can cause rupture in partially solidified portions when solid and liquid coexist over a wide range of temperatures. Reduced pouring temperature, reduced die temperature, increased pressurization duration, and increased draft angles on the casting are some of the strategies utilized to avoid hot ripping in squeeze cast goods (Figure 14).

8. Sticking. Rapid cycling of the process without proper die/punch cooling and lubrication causes a thin layer of casting skin to adhere to the die surface. Reduce the temperature of the die or the temperature of the pouring liquid to avoid sticking (Figure 15).

9. Extrusion debonding When the metal sits in the open die for a long time before being extruded to fill the die cavity, this happens. After the melt has been extruded around the partially hardened crust in the die, the oxide remains there, resulting in the absence of a metal-to-metal link at oxide stringer positions. Increase the tooling or pouring temperatures to prevent extrusion debonding. The production of oxide on the semi-liquid metal in the die can be reduced by reducing the die closure time (Figure 16) [28].

Figure 8.

Oxide inclusion on materials [19].

Figure 9.

Porosity on cast materials [20].

Figure 10.

Image of segregation on cast materials [21].

Figure 11.

Optical image of centerline segregation [22].

Figure 12.

Image of blistering on metal [23].

Figure 13.

Image of cold lapping of a metal represented below [24].

Figure 14.

Hot tearing shown on a cast metal [25].

Figure 15.

Image of stcking [26].

Figure 16.

Image of extrusion on a metal [27].

4. Future trends and oppportunities for improving squeeze casting process

The introduction of squeeze casting as a production technique has provided a solution to these criteria, with the present emphasis on lowering materials use through near-net shape processing and the necessity for both higher strength and high ductility parts. Squeeze casting has been used in manufacturing in the United States, the United Kingdom, and Japan in recent years. Squeeze casting is gaining popularity in the industry. Squeeze casting was first advertised as a solution for components that had difficulty with traditional castings. This could have been due to design constraints, new applications resulting in higher loads, increasing pressure tightness requirements, or a desire to improve customer reliability. Following this initial surge in the industry, replacements for aluminum forgings and conversions from ferrous castings, such as ductile iron, were the following growth areas. While smaller tonnage machines are frequently utilized in Japan, where customers are more diverse, squeeze castings are mostly used in the automotive industry in the United States. These uses are driven by the need to reduce vehicle weight. This necessitates the conversion of iron to aluminum, as well as the requirement for cost savings, which necessitates the conversion of existing processes such as forging and permanent mold to new ones. The majority of the automotive items are steering and suspension components, such as steering knuckles and control arms, as well as air conditioning components. Squeeze casting is the recommended method when fatigue and ductility are critical component qualities. There is a significant chance for squeeze castings to flourish as industry expertise with squeeze casting is combined with technical understanding. In the past, the method has been used on defective parts or poorly designed designs, with disastrous outcomes. However, when a deeper understanding of the process’ capabilities is gained, and technically competent models and algorithms for squeeze casting optimization are established, new applications will consistently meet or surpass expectations. Squeeze casting parts and components can be used to replace cast iron and pressed steel parts in automobiles. Furthermore, when the pressure increases, structural improvements occur, allowing the tensile characteristics to be comparable to wrought aluminum [29].

4.1 Some components produced by squeeze casting

The squeeze casting process has a number of applications which include;

Dome, blades, disks, automotive wheels, pistons, gears, hydraulic brake valve, Brake master cylinder, steering knuckles, control arms.

5. Conclusion

The process sequence, parameters and properties of squeeze casting operation with the attendant technical advantages over other conventional casting production techniques have been discussed in the foregoing sections. Squeeze casting of metal matrix composites is amenable to mass production, especially in the automotive industry. The process and production sequences can be subjected to automatic process control with the gains of consistent high casting quality and production rates. There exist a plethora of opportunities for future improvement and optimization of squeeze casting process parameters as researches and development efforts in the automobile industry and metal matrix composites is intensified.