Helder Nunes, Omid Emadinia, Manuel F. Vieira and Ana Reis
Submitted: December 5th, 2022 Reviewed: January 7th, 2023 Published: February 3rd, 2023
FROM THE EDITED VOLUME
Recent Advancements in Aluminum Alloys [Working Title]
Dr. Shashanka Rajendrachari
CHAPTER METRICS OVERVIEW
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Low- pressure casting and high-pressure casting processes are the most common liquid-based technologies used to produce aluminum components. Processing conditions such as cooling rate and pressure level greatly influence the microstructure, mechanical properties, and heat treatment response of the Al alloys produced through these casting techniques. The performance of heat treatment depends on the alloy’s chemical composition and the casting condition such as the vacuum required for high-pressure casting, thus, highlighting the low-pressure casting application that does not require a vacuum. The level of pressure applied to fill the mold cavity can affect the formation of gas porosities and oxide films in the cast. Moreover, mechanical properties are influenced by the microstructure, i.e., secondary dendritic arm spacing, grain size, and the morphology of the secondary phases in the α-matrix. Thus, the current study evaluates the most current research developments performed to reduce these defects and to improve the mechanical performance of the casts produced by low- and high-pressure casting.
- aluminum alloys
- low-pressure casting
- high-pressure die casting
- mechanical properties
Low-pressure casting (LPC) involves feeding the molten material, typically a light metal alloy such as aluminum or magnesium, into the mold cavity by applying a gas pressure onto the melt surface. This causes the melt to rise through a riser tube, placed in a crucible, and fill the mold cavity located above the furnace. This mold can be a permanent one (LPDC, low-pressure die casting) or made of sand (LPSC, low-pressure sand casting), affecting the solidification rate [1, 2]. This process can be applied to produce a vast range of components with complex geometries, such as wheels and engine crankcases . Although this process requires a higher capital cost than gravity casting, it becomes more competitive by producing better-quality melts and castings with fewer defects, especially in small or medium series, which has greater production yield and allows the application of heat treatments, unlike other processes, such as high-pressure die casting [4, 5].
High-pressure casting is an established casting process for low melting temperature alloys representing about 60% of all castings used in the automotive industries. Due to its high pressure, only permanent molds can be used, and thus it is called high-pressure die casting (HPDC). This process is characterized by a short process cycle and high productivity alongside the ability to produce parts with complex geometry, thin sections, and good surface quality. The major disadvantage of HPDC is the high cost of the equipment and dies. However, this can be compensated with production series above 5000–10,000 castings/year [6, 7, 8].
This book chapter mainly aims at comparing the process and metallurgical aspects as well as the mechanical properties of Al alloys produced by LPC and HPDC. Finally, some of the most recent developments in the casting process are discussed.
2. Metallurgy aspects of casting Al alloys
The properties of Al-Si alloy castings are significantly influenced by several microstructure features, including secondary dendrite arm spacing (SDAS), bifilms, and porosities .
Aluminum alloys used in casting are often Al-Si or Al-Si-Mg, series 4xx.x and 3xx.x, respectively. The presence of silicon is critical in these alloys, since it increases the melt fluidity and decreases the coefficient of thermal expansion, facilitating casting and improving mechanical properties. The quantity of silicon added to the aluminum depends on the casting process, regarding HPDC, because of the high solidification rate, silicon contents required are between 8 and 12%, whereas, in LPSC, silicon contents between 5 and 7% are typically used. Thus, the most used and researched alloy for LPC is the A356 alloy, also known as ISO AlSi7Mg0.3 whose chemical composition can be found in Table 1 [2, 11].
Chemical composition of the AlSi7Mg0.3 (in wt.%) alloy requirements of the EN 1706 standard .
Figure 1 presents an overview of the mechanical properties obtained by various researchers of the AlSi7Mg0.3 alloy produced by LPC [4, 5, 12, 13, 14, 15, 16, 17, 18]. Most of these researchers applied the heat treatment T6 (solution heat treatment followed by water quenching and then artificial age hardening) with the aim of enhancing the mechanical properties. In this graph, Quality Index (QI) values are also represented. This index is calculated through eq. (1):
where ultimate tensile strength (UTS) is in MPa, A% is the elongation, and d is a constant dependent of the alloy, and for the Al-Si-Mg system d it is usually 150 .
Regarding HPDC, the most widely used alloy is AlSi9Cu3(Fe) which is normally a secondary alloy, and the chemical composition of this alloy includes some Fe as shown in Table 2. The effects of the presence of Fe are discussed in the next sections.
Chemical composition of the AlSi9Cu3 (in wt.%) alloy requirements of the EN 1706 standard .
Table 3 represents some of the mechanical properties of this alloy defined in the standard NP-EN 1706 (2000)  and some results from different studies [20, 21, 22]. The biggest difference between the properties of LPC and HPDC is the elongation values, expecting lower values of elongation due to the effect of porosities. For the AlSi9Cu3(Fe) alloy, the QI can be calculated through eq. 2 :
|UTS (MPa)||YS (MPa)||A (%)||Hardness||QI|
|Standard NP EN 1706 (2000) ||min. 240||min. 140||> 1||min. 80 HB||min. 153|
|Špada et al. ||263 ± 3||—||1.9 ± 0.1||95 ± 2 HV||—|
|Cecchel et al. ||262 ± 3||158 ± 4||1.6 ± 0.1||∼98 HV||213|
|Timelli et al. ||323||252||3.8||93 HV||387|
Mechanical properties of the HPDC AlSi9Cu3(Fe) alloy.
Figure 2 illustrates the microstructure of the AlSi7Mg0.3 alloy produced by LPDC. The as-cast microstructure (Figure 2 a and b) consist of dendrites of α-Al with the eutectic phases formed in the interdendritic spaces. In this case, the eutectic is composed of fine Si particles distributed in Al resulting from the eutectic reaction. Finally, it is also possible to identify coarse dark Mg2Si particles (identified in the figure with red circles and arrows). As mentioned before, these types of alloys are normally submitted to a T6 heat treatment. The initial stages of solubilization (540°C for 6 h), quenching in water (at 40°C), and finally artificial aging (155°C for 200 min) resulted in a similar microstructure as before. However, the Si particles become coarser and more rounded, as shown in Figure 2 f. With the increase in aging time and temperature, the UTS and yield strength (YS) tend to increase and elongation decreases due to the precipitation of very fine Mg2Si phases, which are not possible to observe by optical microscopy .
Concerning the HPDC parts, the microstructure is more refined than other casting methods due to the rapid filling and fast solidification. When observing the cross section of a cylindrical casting produced by HPDC, three zones with distinct microstructures can be identified: skin layer, segregation band, and central zone (in the opposite direction of heat dissipation). When the molten Al is inside the shot sleeve, the α-Al phase starts to nucleate and grow from the walls that are commonly referred to as externally solidified crystals (ESCs). During the filling process, these crystals are forced to the center zone; thus, the solidified microstructure in this zone is composed of several coarse dendritic ESCs with sizes larger than 10 μm. Inside the die cavity, the α-Al phase continues to form however in small sizes, normally smaller than 5 μm. The high cooling rate of the melt due to the interaction with the cooler die cavity surface creates larger undercooling, promoting rapid nucleation of the α-Al phase and originating the skin layer. With ESCs continuously growing and partially interlocked during filling, liquid segregation between the central and skin zones is promoted and forms an inhomogeneous microstructure zone known as a segregation band. In this zone, the α-Al phase content is relatively low, and other phases, such as eutectic Si and intermetallic compounds, are present in larger quantities [7, 23, 24, 25].
2.2.1 Fe-rich phases
Fe is the most prejudicial contamination of Al alloys. The incorporation of these impurities occurs mainly during the recycling process and is impossible to remove by conventional methods, such as pyrometallurgy. Fe tends to react with Al to form hard and brittle intermetallic phases with a wide range of chemical formulas, sizes, and shapes. The β-Al5FeSi is the most detrimental phase due to its plate-like shape that works as a stress concentration source and fragilizes the alloys. Recycled alloys, known as secondary aluminum alloys (SAAs), are mainly used in casting due to lower chemical restrictions needed in these processes when compared with wrought alloys. However, since the SAAs in the present recycling system is the last sink of the recycled Al alloys, a scrap surplus is expected to occur soon. Thus, it is necessary to enhance the applicability of these alloys by reducing the negative effects of the Fe-rich and obtaining SAAs with mechanical properties comparable to the primary alloys [26, 27, 28].
Even though in HPDC, the Fe can aid the ejection of the casting part from the die and prolong the die life by avoiding soldering between the two materials, and the brittle Fe-rich phases also negatively affect the mechanical properties of the alloys. In this process, the Fe-rich phases, specifically α-Al(Fe, Mn)Si can form as early as in the shot sleeve stage by nucleating in oxides from the melting furnace as Jiao et al.  reported in a study with AlSi10MnMg alloy. Two types of morphology were distinguished in this case, and the particles formed in the shot sleeve presented a shape like a hexahedron with a size of around 14 μm alongside the particles formed inside the cavity due to the higher cooling rate had smaller sizes, around 6 μm, with a spherical shape. In another study  using the same alloy, three different morphologies of the Fe-rich phase shapes were identified: polyhedral – a well-defined cube; fine compact – a claw-like shape; and Chinese script-type shape – similar to a compact skeletal structure. However, to the author’s knowledge, only a few articles that study the effects of these phases on alloys produced by LPC or HPDC have been published. Thus, a more in-depth understanding of the process parameter effects on the formation of the Fe-rich phases is needed to enhance the applicability of the SAAs.
The hydrogen pickup by the melt and the formation of defects such as bifilms are the two most severe issues in Al-casting. In the LPC process, the alloy is usually heated and melted under an inert gas flux, such as argon, excessive melt oxidation, and hydrogen pickup are minimized and may provide a cleaner melt. However, the material and all equipment must be dried to remove any moisture, and the slag must be removed before casting. Since it is possible to vary the casting velocity by altering the pressure supplied to the melt, this technique is distinguished by smooth filling and good feeding capabilities. An uncontrolled filling of the mold cavity provokes a melt with high turbulence and promotes the entrapment of air. This turbulence also facilitates the molten metal to fold onto itself, which is unable to join due to the oxide layer and creates long and thin defects known as bifilms. These surface-entrained defects have been shown as the primary factor for porosity formation in LPC . As a result, it is critical to ensure many processes features in the mold design and casting methods, such as preventing “waterfall” effects, which occur when molten metal falls into a depression and providing a melt velocity in the mold cavity of fewer than 0.5 m/s. These are some examples of guidelines established by Campbell to limit the melt turbulence and the number of defects [3, 7].
In the HPDC process, it is commonly verified that scrap rates of 5 to 10% due to the occurrence up to 30 specific types of defects can occur. Some of the most common defects that have a direct effect on mechanical properties are gas porosity and oxide films, similar to LPC [6, 8, 29]. Other defects can occur when further processes are applied to HPDC parts, specifically heat treatment. The high pressure associated with this process creates a high quantity of entrapped gasses in the Al. These gasses are originated from the decomposition of the die lubricants and from the entrapped air during the injection. During heat treatment, especially due to the high temperatures of the solution treatment stage, the gasses expand forming the defect known as blisters turning the piece unsuitable to use. And thus, commonly the alloys produced by HPDC are considered as not heat treatable .
3. LPC and HPDC methodologies and processing parameters
In this section, process cycles, working parameters, and recent advancements in LPC and HPDC are presented, as well as some effects of these aspects on the microstructure and mechanical properties of the alloys.
The equipment used in LPC and cold chamber HPDC is shown in Figure 3. A furnace, a mold – which may be composed of metal or sand – and a feeder tube – which lets the metal rise from the crucible to the mold cavity – are the most common parts of the equipment required for LPC. Whereas, a shot sleeve with a hydraulic operated plunger, an intricate, and costly metal die, as well as complex systems for mold fixing, part ejection, and die cooling are the main equipment components for cold chamber HPDC .
3.2 Process cycle
A comparison between the two process cycles of LPC and HPDC is represented in Figures 4 and 5, respectively. Some similarities can be observed which are mainly the stages of filling and solidification under pressure. LPC consists initially of melting the alloy inside the furnace used for casting or by the loading of already molten Al by a ladle. When the molten is prepared for casting, the feeder tube and mold are placed on top of the furnace. By applying gas pressure, commonly with Ar, the melt rises through the feeder tube and fills the mold cavity. The pressure is maintained during the solidification of the material inside the mold. When the pressure is released, the remaining molten material falls back into the crucible. The mold is then opened when dies are used, or the sand mold is destroyed by vibrations. Finally, parts can be moved to post-processing, such as heat treatment and sand-blasting .
The HPDC cycle mainly includes five steps, as shown in Figure 5: spraying and closing of the die; dosing of molten Al into the shot sleeve; injection of the melt by the application of pressures between 7 and 140 MPa through a plunger; solidification under pressure; and the opening of the die and the ejection of the part. Then the part follows to post-process, such as trimming. Mostly, the die is clamped to securely close together the two halves of the die that are already attached to the casting machine with enough force to guarantee that the die does not open during injection of the molten metal or solidification. The surface of these dies must be clean and lubricated to facilitate the ejection of the parts. The molten Al that was previously transferred into a chamber and part of this is then injected into the die cavity .
3.3 Design of die for LPC
One of the most recent studies about LPC is regarding the geometry design of the transition zone from the feeder tube to the mold cavity. While HPDC’s most recent studies focus on applying a vacuum in the casting process.
Different numerical studies have been carried out to investigate the effects of geometrical parameters of the die design and the feeder tube. Yaki  evaluated the influence of cylindrical and cone-shaped riser tubes on liquid rising pressure and stability. The later geometry promotes a lower liquid pressure during rising and a more stable filling. This can be observed in Figure 6, which represents the velocity vector diagram of liquid at 10s of rising. In the case of the cylindrical tube, it is possible to observe a vortex inside the tube that increases the turbulence of the melt. This does not occur with a conic riser tube allowing the melt to rise with more stability.
Bedel et al.  evaluated the impact of die geometry on filling dynamics through simulation and experimentation. Another study  observed that the horizontal section’s geometric parameters of the furnace, the rising tube, and the mold cavity were responsible for oscillation during filling. According to specific geometric parameters studied, the section changes ratio and the section transition height impacted filling dynamics, concluding that the melt flow will be more unstable by applying greater pressure ramp and section change between the furnace and feeder tube. Thus, these researchers  aimed at designing an algorithm to be applied in LPC. This algorithm may be used to construct the filling system to find the proper filling pressure ramp for any complicated component. Some processes on this algorithm consist of determining the possible orientation of the parts, computation of the maximal vertical section change for each orientation, and selection of the orientation with the lowest corresponding value. It can be useful for the determination of the transition height (trans) and the actual section change (R) in a filling system to be used with a specific feeder tube. With the 3D map developed by Bedel et al. or any equivalent Lagrangian model, the maximal pressure ramp can be determined as a function of the R and trans-values. And thus, the maximum filling pressure ramp and the minimum filling system height can be determined for any component.
3.4 Vacuum-assisted HPDC
Vacuum-assisted high-pressure die casting (VHPDC) has been studied with the main purpose of reduction of entrapped air and quantities of oxide films in the cast. This is done by applying a low atmospheric pressure in the shot sleeve and cavity during injection and filling . With applying vacuum, some process parameters are altered, such as the filling time, which tends to be faster. In a simulation study, Kan et al. , employing 500 Pa of pressure and a melt speed of 1 m/s, verified that the mold cavity under vacuum was filled in 0.95 seconds whereas the filling process took 1.2 seconds in the non-vacuum study. These results for vacuum and non-vacuum real experiments were 0.6 and 0.8 s, respectively, thus, saving about 21% of the time cycle.
In Figure 7, it is possible to observe the microstructures of AlSi9Cu3(Fe) alloy produced by VHPDC. These images did not show any grain size deference from similar microstructures of alloys produced without vacuum, as shown in Figure 8 from the same study . The phases α-Al, eutectic Si, and Fe-rich phases also did not vary significantly with the usage of vacuum. Thus, the main differences were porosity levels and thus increased casting integrity. Reducing trapped air allows the application of heat treatment to the cast without causing the blister defects mentioned above . The pores tended to be fewer and smaller, with a decrease in volumetric porosity, from 0.34 to 0.09%, and were distributed more evenly with VHPDC. This reduction in porosity enhanced the fatigue life (about 16%) of the alloy with a 4% increase in fatigue strength. The static tensile properties were improved slightly, especially UTS from 314 to 326 MPa and elongation from 2.11 to 2.81% . In another study, Hu et al.  proved that increasing the vacuum levels can indeed improve the YS of the alloys with the reduction in pores volume.
In a study of the effect of T6 heat treatment on the alloy AlSi11MgMn, Liu et al.  observed a decrease in UTS, while the elongation increased for alloys produced by VHPDC. In this alloy, the microcracks formed near the large α-Fe intermetallic and not due to the eutectic Si particles. The heat treatment could change Si morphology from fibrous particles to more globular shapes. Thus, the type of fracture observed corresponded to a more ductile behavior than the fracture of the non-heat-treated alloy.
3.5 Melt treatments
In LPC, some processes should be performed on the molten alloy to guarantee good melt quality such as grain refinement and eutectic silicon modification to provide the desired mechanical properties. Grain refining of α-Al grains seeks to improve the alloys’ mechanical properties, such as ultimate tensile strength and fatigue strength. It is commonly accomplished by adding B and Ti via master alloys, constituted by Al-Ti-B compounds, by the creation of Al3Ti and/or TiB particles as nucleation agents during solidification. Although the performance of the latter type is mostly reported, a recent study revealed the better performance of Al2.2Ti1B-Mg grain refiner. This master alloy leads to the growth of an Al2.2Ti1B-Mg layer on the TiB2 particles. Decreasing the mismatches between TiB2 and Al promotes the nucleation of α-Al and results in a higher efficiency refining process than the other master alloys .
Besides, the primary goal of silicon modification is to reduce the size and form of eutectic particles to increase elongation values. The eutectic silicon modification is also done by master-alloy additions containing specific elements, such as Sr. or Na, that force the nucleation of the eutectic silicon to occur after the formation of eutectic aluminum. The presence of these elements guarantees that silicon grows between these α-Al grains and acquires a fibrous morphology, and shorter length . However, the effect of these additions on defect formation is not yet thoroughly studied, with several studies showing contradictory observations. Sr additions influence the number of bifilms and the size of the pores, whereas B stimulates the formation of defects in the castings’ cores. Furthermore, the Sr combines with the Al2O3 to generate the spinel Sr.Al2O3, facilitating the oxides to break into smaller ones [4, 42, 43]. Therefore, the additions using master alloys are a significant step in the casting process to enhance the mechanical properties by modifying the microstructures of the alloys. Moreover, alloys modified and refined have been shown to present lower porosities and higher density values than alloys without these treatments . To avoid excessive porosity originating from the dissolved hydrogen, normally degassing processes are carried out before casting. Several technologies can be applied to degas the melt such as rotor degassing with argon and ultrasonic melt treatment [45, 46].
3.6 Effects of process parameters
In LPC, several parameters have a notorious effect on the properties and quality of the alloys. Some of these parameters include the mold material, the filling conditions, and the holding pressure (HP).
The type of materials used in the casting molds affects the cost and quality of the castings. Sand molds are typically less often used than permanent ones, and the use of dies allows higher productivity. However, compared to sand molds, this kind of mold requires a higher capital investment. The mechanical properties might also be impacted by the type of mold. The refined of α-Al phase, which exhibits smaller dendritic arm spacing, and eutectic Si particles in the metallic dies to induce higher values of UTS and elongation. This refinement is attributed to shorter solidification times .
Puga et al.  evaluated the effects of mold-filling parameters effects on the mechanical properties of an LPSC AlSi7Mg0.3 alloy. In this study, two different pressure-time curves were evaluated for two temperatures (650 and 700°C). The main difference in these curves is the number of ramps. While one curve only has two ramps, the other curve presents an intermediate third ramp which controls and reduces the filling velocity to smaller than 0.5 m/s. This last curve allowed a smoother filling with lower pressurized speed (Pa/s) and created a casting with fewer defects, such as porosities and oxides. And thus, the casting produced with a 3-ramp curve at 650°C presented the highest values of UTS (253 ± 9 MPa), yield strength (YS = 215 ± 5 MPa), and elongation (2.4 ± 0.2%). These authors revealed that the application of ultrasonic degassing treatment promoted the refinement of the alloys. This treatment at the lowest temperature (650°C) provoked a more intensive grain refinement and a more globular microstructure and enhanced the mechanical properties.
The LPC allows solidification to occur under a certain pressure known as holding pressure (HP). Several researchers, for example, Timelli et al.  and Wu et al. , reported a relation between HP with several characteristics such as SDAS and porosity. The local cooling rate during solidification has an impact on SDAS, which is a commonly used aspect to determine the grain size in casting alloys. Smaller SDAS values indicate a more refined microstructure, which improves some mechanical properties . Timelli et al. concluded that by increasing HP from 35 to 50 kPa, the SDAS decreased from 67 to 58 μm and the porosity levels reduced from 0.3 to 0.1%. On the other hand, Wu et al. studied an even higher HP of 85 at 300 kPa. With the highest pressure, the researchers obtained SDAS values of 39 ± 6 μm (for a cooling rate of 1°C/s) and 21 ± 2 μm (for a cooling rate of 10°C/s). The density of the alloys also reached the highest values for these conditions. With the smallest SDAS and lowest porosity levels, the alloy solidified under the highest pressure and cooled faster, presenting the highest UTS value (293 ± 11 MPa) and elongation (14 ± 1%) of all the alloys studied.
It has been widely reported that the process parameters of HPDC affect the mechanical properties and microstructures of the alloys. Cho et al.  observed a strong proportional relationship between dendrite arm spacing and cooling rate. With the increase of the cooling rate from 15 to 100°C/sec, the dendrite arm spacing of AlSi9Cu3 and AlSi11Cu3 alloys reduced to more than half, from 12 to 5 μm and from 8 to 5 μm, respectively.
Santos et al.  observed no clear correlation between pressure (35 or 70 MPa) and injection temperature (579, 643, or 709°C) with the porosity of the AISi9Cu3(Fe) alloy produced by HPDC. Samples with 70 MPa have the lowest and highest values of porosity, 3 ± 1 (579°C) and 5 ± 1 (709°C). Moreover, these parameters had some effects on the microstructure of the alloys, specifically the α-Al phase. Higher temperatures promoted a refinement of this phase, while at lower temperatures the dendrite structure tends to be fragmented. The injection temperature seems to have no significant effect on mechanical properties, but the highest values of UTS were absorbed with the highest temperature (244 ± 12 and 265 ± 8 MPa, with 35 and 70 MPa injection pressure, respectively). The elongation values were constant, between 4 and 5%, in all alloys. However, no correlation between UTS and YS could be determined with these parameters. In another study of the same alloy, Obieka et al.  concluded that a higher pressure of about 140 MPa provoked an increase in all the mechanical properties of the alloy: UTS, YS, elongation, hardness, and impact strength. These results were mainly attributed to the refinement of the microstructure and the various phases.
This book chapter allowed for some direct comparisons between low-pressure casting and high-pressure die casting, as follows:
- Even though HPDC needs a higher initial investment, usually it is still more profitable than LPC due to its lower cycle time and higher productivity. The cycle time is one of the major differences between the processes.
- The microstructure of the most used alloys (for LPSC AlSi7Mg and HPDC AlSi9Cu3(Fe)) obtained from the process presents some similarities, such as α-Al dendrites with eutectic particles in-between. However, the HPDC provokes a refinement of the structure due to the fast filling and cooling causing small values of SDAS.
- Castings from HPDC tend to show higher quantities of defects than LPC ones, especially porosities due to the entrapment of air and turbulence during filling.
- Some of the most recent developments in both processes were analyzed. In recent studies, algorithms have been defined to establish rules for die design dies for LPC. In HPDC, applying a vacuum has been studied to improve some mechanical properties and to allow the application of heat treatments.
The authors gratefully acknowledge FCT – Portuguese Foundation for Science and Technology (2022.11466.BD), the funding of Project AM2R – Agenda Mobilizadora para a inovação empresarial do setor das Duas Rodas (C644866475-00000012) and, Hi-rEV – Recuperação do Setor de Componentes Automóveis (C644864375-00000002), cofinanced by Plano de Recuperação e Resiliência (PRR), República Portuguesa through NextGeneration EU.
Conflict of interest
The authors declare no conflict of interest.
List of abbreviations
Elongation at breakESCs
Externally solidified crystalsHP
High-pressure die castingLPC
Low-pressure die castingLPSC
Low-pressure sand castingQI
Section change in the filling system for low-pressure castingSAA
Secondary aluminum alloysSDAS
Secondary dendritic arm spacingtrans
Transition height in the filling system for low-pressure castingUTS
Ultimate tensile strengthVHPDC
Vacuum-assisted high-pressure die castingYS
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