Aluminium Casting Processes: History
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Aluminium castings have been widely used in many industries, including automotive, aerospace, telecommunication, construction, consumer products, etc., due to their lightweight, good electric and thermal conductivity, and electromagnetic interference/radio frequency interference (EMI/RFI) shielding properties. There are many aluminium casting processes, including sand casting, shell mould casting, pressure die casting, lost foam casting, permanent mould casting, investment (lost wax) casting, centrifugal casting, squeezing casting, semi-solid casting, continuous casting, etc. Aluminium castings made through different casting processes may have different surface quality, different gas contents, different porosity, different mechanical properties, etc. These different properties of aluminium castings from different casting processes, especially gas content, porosity, and ductility, will affect the weldability/joinability and joint quality.

  • aluminium castings
  • porosity
  • hot cracking
  • Joinability

1. Introduction

Aluminium castings have been used in many industry sectors, including automotive, aerospace, telecommunications, construction, consumer products, etc. For example, they have been used in a wide range of networking, telecommunications, and computing equipment as housing because of their good EMI/RFI shielding ability and heat dissipating ability; they have been used in small electronic products because of their durability, lightweight, and EMI/RFI shielding ability; and they are ideal for electric connectors because they are lightweight and have good electric conductivity. The main applications of aluminium castings are in the automotive industry. Due to global warming and government legislation, automotive vehicles are required to increase their fuel efficiency and reduce greenhouse gas emissions. Lightweighting is a good practice in addition to vehicle electrification. To reduce the gross weight of vehicles, more and more lightweight aluminium castings are introduced into their structures. Cast aluminium has been used in automotive applications for the power train, such as engine blocks [1], cylinder heads, and transmissions, since the early 1900s, and its applications in structural components have increased greatly, including alloy wheels, longitudinal members, cross members [2], pillars [2], front steering knuckles, steering wheel cores, connection nodes, shock towers, etc. Aluminium die casting has been used as connection knots to join different aluminium alloy extruded profiles, as presented in Audi A2 and A8 aluminium space frames [3].
Applications of aluminium castings in automotive vehicles are mainly in two situations: 1. Complex structures, such as engine blocks; 2. Parts integration. In order to further reduce the weight and simplify the vehicle assembly process, the castings used in cars are getting larger with many previously individual parts integrated together. Tesla is pioneering in this area. Recently, Tesla produced some mega-castings with the enormous IDRA giga press (about 19.5 m long, 7.3 m wide and 5.3 m high) at Gigafactory Texas. Tesla is planning to use two huge single castings for the front and rear underbody and to connect them with a battery pack that is acting as part of the body structure [4]. The rear underbody casting is the integration of 70 different parts, and all together this new 3-section assembly strategy will reduce the total number of parts of this structure by 370.
However, due to the features of cast aluminium, such as porosity, poor surface quality, a tendency toward hot cracking, and low ductility, joining these materials is problematic. From the material point of view, aluminium weldability by fusion welding is mainly influenced by these characteristics: the existence of a surface layer of aluminium oxide and release agent residuals from casting, which will deteriorate wettability and introduce gases and inclusions in the weld; high thermal conductivity, which will consistently remove a large amount of heat from the welding zone; a relatively high thermal expansion coefficient, which will increase residual stress and cause greater distortion; hydrogen content in the alloy, which will cause porosity in the welds; a wide solidification range, which will cause segregation of alloying elements and hot cracking [5]. Hot cracking, including solidification cracking and liquidation cracking, can happen during fusion welding of aluminium castings. Fusion welding of aluminium cast parts generally requires a low gas content, especially a low hydrogen content. The air pockets and hydrogen contents in aluminium cast parts will cause porosity in the weld bead. Although mechanical joining methods, such as self-piercing riveting (SPR) and clinching, are less sensitive to the gas content of the aluminium castings, they require large plastic deformations of the materials. Since casting materials are normally more brittle and have low elongation, SPR and clinching will cause cracking during the joining processes.

2. Aluminium Casting Processes

There are many aluminium casting processes, including sand casting, shell mould casting, pressure die casting, lost foam casting, permanent mould casting, investment (lost wax) casting, centrifugal casting, squeezing casting, semi-solid casting, continuous casting, etc. Aluminium castings made through different casting processes may have different surface quality, different gas contents, different porosity, different mechanical properties, etc. These different properties of aluminium castings from different casting processes, especially gas content, porosity, and ductility, will affect the weldability/joinability and joint quality, so it is worth giving a brief introduction of different aluminium casting processes.
Sand casting is a casting process using a sand-made mould. A pattern with the same shape as the part to be casted is made from wood, metal, or plastic. The pattern is then put inside the flask, embedded with sand and bonding agents, and pressed tightly. The cavity required is formed after removing the pattern. Through the gating system, molten aluminium is poured into the mould cavity and solidifies. As the casting is cooled down, the mould is broken, and the casting is collected. Sand-casted aluminium normally has a rough surface finish. The cooling rate of sand casting is low, and it can be slightly changed by using sands with different heat capacities.
Shell mould casting is a casting technology with a mould made of thermosetting phenolic resin and sand [6]. First, the two halves of the pattern are designed and created from metal, which is then heated and coated with lubrication. The pattern is then put into the sand chamber with the thermoset resin, and the chamber is turned upside down. The mixture of resin and sand sticks to the pattern and hardens to form a shell. Two shells with a thickness of 10–20 mm are formed when the pattern is removed. These two shell moulds are assembled to form a complete mould. Liquid aluminium is poured into the mould and solidified. After breaking the shell of the mould, the casting can be collected. Compared with sand casting, shell mould casting can produce castings with a better surface quality and a similar cooling rate but is more expensive.
In gravity casting, the liquid aluminium is poured into a vertical opening (sprue) and flows into the casting cavity by force of gravity without the use of other measures such as pressure, vacuum, etc. Due to the thermal contraction of the aluminium during solidification and cooling, the volume of the aluminium in the cavity will decrease by several percent. If liquid aluminium is not continuously fed while the casting is solidifying, porosity will occur in the casting that will degrade the quality of the cast part.
Permanent mould aluminium casting, also known as metal mould casting, is one of the aluminium casting methods that uses metal as the mould material, similar to pressure die casting. Accordingly, the liquid aluminium is pushed into the mould by its gravity, so the pouring speed is quite low. Due to the metal-made mould, the casting’s cooling speed is fast. Moulds have a long service life, so they are called permanent moulds. The aluminium castings from permanent mould casting have high mechanical properties due to the fast-cooling rate of the casting, and low shrinkage and gas porosity defects [6].
In low-pressure die casting, the mould is above the sprue, and the liquid aluminium is pressured up the sprue and into the runner system and the casting cavity. The metal flow is accomplished by pressurizing the furnace. The rate of liquid aluminium flow is controlled by the pressurization level of the furnace.
High-pressure die casting (HPDC) is a manufacturing process in which liquid aluminium is injected at high pressure and speed into a steel mould to produce parts. HPDC can have a very high cooling rate, generally between 50 to 125 K/s [7]. The liquid aluminium is poured into a cylindrical tube and injected into the runner system with a piston at high-speed (a few m/s). The result is that the cavity filling time is much shorter, in tens or hundreds of milliseconds, instead of tens of seconds as in gravity and low-pressure casting [8]. No machining is required for most high-pressure die casting parts, due to the excellent dimensional accuracy and the smooth surfaces. High-pressure die casting production is fast when compared to other casting processes. Although high-pressure die casting processes can produce thin-walled and lightweight parts, the associated turbulent conditions remain the major source of interior and surface casting defects, such as pores. As an emerging technology, vacuum HPDC can facilitate degassing and reduce porosity, which could improve mechanical properties and the performance of the welded HPDC aluminium parts [9][10][11]. HPDC is normally used for large production volumes, and for low production volumes, other casting processes, such as low-pressure die casting, permanent mould casting, and sand casting can be used.
Centrifugal casting is a casting process that uses centrifugal force through high-speed rotation to evenly distribute the molten metal onto the mould wall. A central cavity can be created without a central core. Unlike most other casting processes, centrifugal casting is mainly used to manufacture rotationally symmetric stock materials in standard sizes for further machining, rather than the final products for specific applications.
Investment casting is also called lost-wax casting. Investment casting is so named because the process invests (surrounds) the wax or plastic pattern with refractory material to make a mould, and a molten metal is casted into the mould. It can make aluminium castings with a high finishing surface, a high dimensional accuracy, and it is possible to cast complex aluminium casting parts, but this process is more expensive and has a long cycle time.
Continuous casting is a casting process in which molten aluminium alloy is continuously poured into a mould with a circulating water-cooling system. Wherever the casting is made, it is immediately cooled and moved away in a continuous mode. Normally, a continuous stamping or rolling line will follow. It is normally used to cast simple bars, plates, or pipes.
Direct-chill (DC) casting is currently the most common semi-continuous casting practice in non-ferrous metallurgy. During the process, molten aluminium is fed through a bottomless and water-cooled mould. The mould is normally made of high thermal conductive materials and water cooled. There are holes arranged along the bottom edge of the water-cooling cavity, so water can be directly jetted from the holes onto the surface of the emerging ingot to provide direct chilling and solidification. Most of heat (about 80%) is extracted by the water-jet direct chill (the secondary cooling) and only 20% is removed through the mould wall (the primary cooling). Direct Chill casting is a method for the manufacturing of cylindrical or rectangular ingots from non-ferrous metals. The ingots are usually further processed by other methods, such as rolling and forging, etc. More than half of global aluminum production uses the DC casting process.
Squeeze casting is a hybrid casting process with the combination of low-pressure casting and high-pressure casting, and it has the potential to eliminate the gas defects associated with HPDC and to make the castings heat treatable. In squeeze casting, the die is filled slowly with metal to maintain laminar flow. Once the cavity is full, a pressure is added to the melt to over 100 MPa and held to compensate for shrinkage until the casting has solidified [8]. Zyska and Boroń [12] compared the porosity of three aluminium castings, AlMg9, AlSi7Mg, and AlCu4Ti, made by gravity die casting and squeeze casting. The results showed that the porosity in the castings made by squeeze casting was almost half that of the castings made by gravity die casting. It is demonstrated that squeeze casting mainly reduces shrinkage porosity in the centre of the slab.
Semi-solid metal (SSM) casting is a casting process that involves filling a mould with the metal in a semi-solid (partial molten) state in which globules of solidified metal are homogeneously dispersed in the liquid. It is a combination of solid metal forging and liquid casting. Normally, a vigorous shearing deformation is used to generate the semi-solid metal with a fine microstructure. There are many benefits of semisolid casting, including: (1) reduced shrinkage due to the lower casting temperature; (2) low gas porosity, making the castings heat treatable; (3) super mechanical properties owing to the uniquely fine microstructures of the SSM castings; and (4) outstanding fine surface finish [8]. Semi-solid die casting offers all the benefits of die casting and, in the meantime, eliminates most, if not all, of the defects, such as porosity. Semi-solid casting has very good tool and die life. The tool life in semi-solid casting is double that of conventional diecasting, and three to five times that of squeeze casting.
Although squeeze casting and SSM casting processes can produce casts with much less porosity, they are more expensive than most of the other casting processes. The advantages, disadvantages, porosity, inclusions, surface finish, and production cost are summarized in Table 1.
The quality and performance of an aluminium casting strongly depend on the quality of the molten aluminium alloy and the technology used to produce it. Aluminium alloy casting is not an easy process because these alloys are prone to form dendritic and heterogeneous structures and to absorb hydrogen during melting. Thus, a specific melt processing operation is required to reduce and control the level of porosity in the microstructure after solidification. Optimising the casting process can improve the weldability of casting aluminium. Wiesner [13] found that sparse use of wax-free, low concentrated lubricants and release agents can improve welding quality.
Table 1. Summary of different aluminium casting processes.
Controlling the microstructure of aluminium alloys is very important in order to achieve high mechanical performance, and this can be achieved by proper cooling methods, suitable degassing techniques, composition modification, grain refinement, etc. There are many conventional and well-established casting processes that have been used for the manufacture of a wide variety of aluminium components. Nevertheless, the performance that can be achieved is limited due to defects that emerge during melt and solidification processes. The microstructure of casting aluminium can be controlled by the cooling rate, which determines the secondary dendrite arm spacing (SDAS) and the size and distribution of secondary phases. As SDAS becomes smaller, porosity and second phase constituents are dispersed more finely and evenly. Different casting processes may have different cooling abilities due to the different features of the processes and the different thermal properties of the mould materials. However, the cooling rate during an aluminium casting process is not only related to the casting process but also to the geometry of the parts, such as size and wall thickness. The refinement of the microstructure had been proven leading to substantial improvement in tensile properties (e.g., ultimate tensile strength (UTS) and elongation) [33]. In the meantime, microstructure refinement can also be realized by adding some grain refiners, such as strontium (Sr). The addition of Sr can transform the morphology of the eutectic silicon phase present in Al–Si casting alloys from coarse plate-like to fine fibrous networks and produces several benefits [34][35]. Sr can also decrease the mean aspect ratio and size of the eutectic particles [33][36]. It had been demonstrated that addition of about 280 ppm Sr to EN AC-46000 alloy generated fully refined Si-particles regardless of the cooling conditions [33]. Investigations indicate that Sr co-segregates with Al and Si within the eutectic Si phase, which is responsible for the formation of multiple twins in a Si crystal, Si crystal growth in different crystallographic directions, and the restriction of Si crystal growth and branching [34]. Sr can also refine iron- and copper-containing phases [35]. It has been shown that Sr modification may improve strength, ductility, fracture, fatigue, and impact properties [37][38].
When joining aluminium castings, it is important to know which casting process is used to make them and what the mechanical and physical properties of the castings are.

This entry is adapted from the peer-reviewed paper 10.3390/coatings13050958


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