Die casting simplefied

CASTING CHARACTERISTICS OF ALUMINUM DIE CASTING ALLOYS.

Casting characteristics of an alloy are those properties of the alloy that characterize the alloy’s behavior in the casting process. The casting characteristics that should be considered in a die casting process are the tendencies of the alloy towards die soldering, hot tearing, and forming sludge, the alloys fluidity (flowability), machinability, porosity formation, macrosegregation, and feedability. 

We have many variety of alloys (Aluminum alloy) designated in standard EN1706 for high pressure die casting applications. However if we see critically those alloys are developed mostly on the basis of their applications & emphasis is given on their physical properties like tensile strength, yield strength, elongation, hardenability etc:. Those properties are very much essential to meet the product requirements. But as a die caster we also need to see the casting characters of those alloys. The reason behind such need is to see how best we can contribute to produce a better quality casting with optimum cost & best productivity. If you see what is mentioned in first paragraph (bold letters) regarding the casting characteristic, then you will obviously agree that as a die caster those are of utmost importance to cast an alloy in an efficient way. 

A very common process fault that we envisage in daily shop floor practice is "Die Soldering" & "Sludge formation". We will try to understand why such defects occurs & what kind of counteraction is required to minimise those problems. In doing so we will concentrate mainly on the alloys & their casting characteristics to understand the causes. We can see that such characteristics are very critical & contribute a lot to those defects. Eventually by controlling such critical issues we can actually control our rejections & enhance profit. As I mentioned in my early publications that three major contributor for a good casting are "Metal, Die & Process". In today's discussion we will focus on two problems "Die Soldering & Sludge formation" related to Metal (Aluminum alloy).

Soldering often occurs between aluminum alloys and ferrous die material and is a major problem in die casting. Sludge usually forms in aluminum die casting alloys and settle to the bottom of holding furnaces, thus reducing furnace capacity, and when entered into the casting cavity, causes hot spots in die cast components. Die castings may have machining problems caused by hard spots that are formed by entered sludge particles, and also because of the relatively hard surface layer caused by the fast cooling at the surface.

DIE SOLDERING

Die soldering is referred to the phenomenon that molten aluminum sticks to the surface of the die material and remains there after the ejection of the cast part. In die casting a challenge is to minimize the cycle time of the casting process to increase productivity and lower operational costs. Die soldering is an impediment to this challenge in that it leads to malfunctioning of die inserts that require replacement or repair, thus causing significant decrease in productivity. So, it is a major concern in the die casting industry. Die Soldering is the result of an interface reaction between molten aluminum and the die material during the impact of the high-velocity molten aluminum onto the die surface and the intimate contact between alloy and die at high temperature. The high velocity destroys the protective film (coating and lubricant) on the die surface. Here, the molten aluminum comes in contact with the virgin die surface. Subsequently, iron in the die dissolves into the molten aluminum and a layer of intermetallic phases is formed. This we call soldering in die, which is difficult to prevent.It has been found that soldering occurs more frequently around the gate where conditions of high temperature and high melt velocity exist. It is also observed that soldering takes place in thin section of die geometry & around the small core pins specially when located near the gate area. It has been observed that the mechanism of soldering is purely based on the diffusion and chemical reactions of the elements in the die (solid) and the liquid metal. So basically we derive that soldering is a diffusion driven reaction – iron diffusing out of the tool steel into the molten aluminum and forming the intermediate layers.

Various die casting parameters have been identified as playing critical roles during die soldering. The dominant ones are: 

· Temperature of the metal and die. 

· Chemistries of casting alloy and intermetallic layers. 

· Die lubrication and coating. 

· The die design and operating parameters. 

Temperature of the Metal and Die - The temperature of the cast metal and the die surface plays a critical role in causing die soldering. High metal and die surface temperatures lower the surface hardness and make the die surface less wear resistant. This makes the die more susceptible to erosion. High temperatures favor the growth of intermetallic phases by increasing the diffusion rate of the atoms of iron and aluminum. High die temperature may also break the lubricant film, and decrease its ability of preventing soldering. Hence high melt and die surface temperatures facilitate soldering and they must not be too high. There should not be hot spots in die surface, or inside the core. Metal and die temperatures also should not be too cold to cause poor filling. The temperature of the melt is a critical factor in creating "Soldering" on the die surface. It is found that holding temperature of the melt at ~ 663°C could minimize the occurrence of soldering. The die preheating temperature must be between 180 to 220 C. Higher temperatures will result in the inadequate application of the lubricant. Lower temperatures might result in the formation of cold shut in casting. The die surface must be polished to a medium level finish with no irregular pattern. A mirror polished of die surface enhances die soldering hence not recommended.

Chemistries of casting alloy and intermetallic layers - On reviewing it can be inferred that the formation of the intermetallic layers is purely based on the diffusion and chemical reactions of the elements in the die (solid) and the liquid metal. Experienced aluminum die casters have observed that different grades of aluminum alloys differ from each other in their tendency towards soldering. It is found that zones of intermetallic compounds consisting of Fe2Al5, Fe3Al and FeAl3 phases (Aluminium Ferite). However, the presence of other alloying elements in aluminum alloys such as Si, Cu, Mg, etc., resulted in the formation of a number of complex intermetallic compounds in the intermediate layer, which actually plays a very insignificant roles in Soldering. It is established through research that higher tendency of Soldering is witnessed in the hypoeutectic Al-Si alloy, & the Al-Si-Cu, and the eutectic Al-Si, which has the least soldering tendency. This happens because of the presence of increasing amounts of silicon in the aluminum decreases the growth rate of intermetallic layers. Iron content in the casting alloy plays a critical role in causing soldering. 

Iron content in the casting alloy plays a critical role in causing soldering. The maximum solubility of iron in aluminum is 1.8 % at 650°C and 3 %, at 700°C. The soldering phenomenon diminishes as the iron content approaches this value. Also, the iron content influences the growth of intermetallics, which has a direct influence on the soldering. It is found through research that the soldering tendency of an alloy with 0.8 % iron is high, and that of an alloy with 1.1 % iron is very low. This is because as the iron content in the cast metal reaches its saturation level the chemical potential gradient is greatly reduced. The amount of free silicon is high at the intermetallic layers because silicon breaks free from the casting alloy at the die interface and exists as large silicon crystals in the soldered microstructure. Aluminum alloys greater than 12 % silicon have a high propensity to form hard Si particles. So we can conclude as under for the intermetallic layers

1. Higher Silicon will facilitate to reduce the intermetallic layers thus will reduce the die soldering 
2. Aluminum alloys containing high iron content should have a limiting range between 0.9 and 1.15 percent by weight. Any excess Iron will facilitate die soldering.

Die Lubrication and Coating - The main propose of applying a lubricant or coating is to create a partition between the metal and the die surface. They reduce the soldering tendency by preventing contact between melt and die. Lubricant also assists melt flow and casting release. To effectively separate the melt from die surface the lubricants should form a film on the die surface. This film should be firmly attached on the die surface and be strong enough to resist the melt washout and the attack of melt heat. The film should also be uniform and continuously cover the entire die surface, especially the area, where the soldering is prone to occur. The lubricants tend to breakdown at elevated temperatures. To successfully apply lubricants onto the die surface, the die surface temperature should not be too high; otherwise, the heat breaks down the emulsion, and evaporates the water in the spray, and thereby leaves the lubricant solids on the die surface. When the lubricant breaks down in any region, it may be washed off in high melt velocity areas such as in thin wall sections or at the gate surround and the aluminum liquid attaches itself to the steel die surface. When there is a hot spot in the die, the surface energy of the lubricant material tends to increase and hence, causes the lubricant to get detached from the die and to flow to regions, which are at lower temperatures. This exposes the die metal to the liquid alloy and results in soldering

The die design and operating parameters - Thick sections of the die are potential sites for die soldering. Coring out to reduce the excess metal will help reduce soldering. Formation of a thin film of soldering roughens that area of the die surface, and this roughness promotes further soldering. Once soldering occurs, there is a rapid build-up of the aluminum alloy layer over the soldered layer after subsequent shots. Undercuts from die manufacturing or die casting operations facilitate soldering. Insufficient draft is another major factor, which affects the soldering phenomenon. As a die designer one must keep in mind when selecting the gate area & gate thickness. This is a tricky area where designer must ensure free flow of metal as well as to ensure no excess heat in die cavity or critical die geometry is protected from direct hitting of metal flow.

SLUDGE FORMATION - This sludge is made up of oxides, such as alumina (Al2O3) and magnesia (MgO), and primary crystals that contain Al, Si, Fe, Mn, Mg and/or Cr. These oxides and crystals have high melting points and high specific gravities; therefore, they tend to accumulate on the floors of furnaces and thus reduce the effective capacity of the furnace. Moreover, when the sludge crystals are entrained into castings, they decrease the alloy’s fluidity and form “hardspot” inclusions, which make machining difficult and degrade the mechanical properties of the cast component. The formation of sludge may also increase the die-soldering tendency of the alloy because sludge crystals are composed mainly of Fe and Mn rich compounds and their formation causes a depletion of Fe and Mn in the melt. The oxides form during alloy melting and treatment. Studying and finding the measures to reduce these oxides implies mainly to deal with melting and melt treating operations. Sludge formation has been shown to be dependent on the alloy’s chemistry, melting and holding temperatures, and time. The  studied of sludging phenomenon has defined a sludge factor (SF) for Al-Si-Cu alloys. This factor is calculated from the Fe, Mn, and Cr contents of the alloys as follows,

Sludge factor (SF) = (1 X wt%Fe) + (2 X wt%Mn) + (3 X wt%Cr) 

It is found that the Fe, Mn and Cr contents of the alloy as well as the cooling rate significantly affected the morphology, quantity, and size of the sludge particles. It is also found that sludge did not form until a certain temperature was reached for a given Fe content and the sludge forming temperature depended on the Fe content of the alloy.

The study shows different criteria were formed and it may cause confusion in comparing the sludge formation tendencies for different alloy. Even for a given alloy the predictions are different from different studies. One study showed that for a given alloy there was a critical temperature at which sludge formed and grew at the fastest speed. In addition, the time duration at the critical temperature is very important for sludge formation.

So what the die casting engineer will do to control the sludge & subsequently the hard spot in casting? Few counter measures are found sustainable as under.

1. Do not take the holding furnace temperature above 680 to 690 C
2. Holding the melt for long in furnace will lead to formation of sludge & Hard spot.
3. Check the sludge factor of every lot of Heat that you are receiving from alloy supplier.
4. Melting & Holding on a same furnace having the same hearth will lead to hardspot.
5. Change your crucible as per the schedule & also get it tested from time to time.
6. Keep your pouring cup clean & properly quoted. 
7. Do not remelt overflows & thin flashes in melting furnace.
8. Keep the runner & biscuit free from dirt/oil & other contamination.
9. Implement filtering of metal while tapping from melting furnace
10. Check lining of melting furnace on regular basis to avoid sludge.