The Role of Alloys and Melting Processes in the Cause and Elimination of Zinc Die Casting Defects
Excerpt from the Spotlight Metal October 2013 article by John Titley.
Zinc alloy pressure die castings are selected because the process-material combination will manufacture precise net shape parts, which accept a wide variety of finishes. Very high-quality standards are achievable both consistently and cost-effectively over large production quantities.
This, coupled with the range of physical and mechanical properties available and the highly productive manufacturing process, has made the Zinc alloy die-cast part the first-choice material for a huge range of mass-produced components. This includes an extensive array of applications from electronics to locks, security and hardware for doors and windows, connectors and fittings for hydraulics and pneumatics, to decorative parts for the automotive industry. All of these applications utilize more than one of the properties available from Zinc alloy pressure die-castings.
However, this diversity carries a penalty the designer must consider when undertaking the new part manufacture. The need for high-quality decorative finishes invariably means the finishing criteria becomes more critical. This affects the cost and the prospect of higher reject rates must be taken into account.
It, therefore, follows that the elimination of surface defects is a key requirement when manufacturing parts requiring high-quality surface finishes. Even minor surface blemishes may cause rejection and can occur throughout the manufacturing process. Consequently, the die caster should consider any of the following as a possible cause of casting defects - in no particular order of importance:
- Alloy chemistry: the presence of excessive iron, lead, tin, cadmium, manganese and intermetallics/oxides are all possible sources of surface defects.
- Metal temperature/metal handling: machine pot temperature and temperature variations will change casting quality. Introduction of ingot or scrap to the machine furnace at the wrong time or in the wrong condition will affect casting quality.
- Casting design: is a major controlling factor in the instigation of casting defects. Section thickness changes, lack of fillet radii, surface textures and profiles may all promote surface defect problems if the casting design does not follow recognized design guidelines.
- Die design: attention to detail is critical, including the calculation of the runner and gate sizes, attention to machine performance, position of the gate relative to casting geometry, die temperature heating/cooling, use of overflow wells and venting.
- Ejection: use of adequate ejection, size of ejector pins and their position, section thickness for ejection, removal from the die by robots, reduction of mechanical damage/impingements during ejection cycle or on conveyors etc.
- Die lubrication: control of quantity dispensed, type of lubrication, propensity for die surface deposits/coking/waxing.
- Second operations: clipping/ break off, finishing and polishing, mass media finishing such as vibro polishing etc, the effects and requirements of these need to be considered at the casting/die design stage.
It is important for the die caster to understand what defects can occur and to be able to identify the cause and the possible remedies available to resolve these issues. To implement some of these remedies, the die caster will need to have a thorough understanding of the machine performance and be conversant with the casting processes and cavity fill theories involved.
Die casting alloys
The European die casting industry uses alloy ingots manufactured to the EN 1774 -1998 standard. From its introduction in 1998, this ingot specification supersedes and renders all national specifications for countries within the European community obsolete. In most instances, die casters will be using either ZnAl400 (ZL3) or ZnAl410 (ZL5), which have similar casting characteristics and, for the most part, can be used in the same process and die. Both alloys have 4% aluminum additions with 0.05% magnesium, but ZL 5 has an additional 1% nominal copper addition offering slightly better mechanical properties than ZL 3. However, there are other alloys listed in this EU norm, which can be pressure die cast – some of which offer superior mechanical properties.
If we look at the metallurgical changes that can instigate surface defects in castings and what elements in the foundry may cause the metallurgy to change, it will become clear that this area is the least likely to generate serious casting problems or surface defects.
The surface finish of any zinc alloy pressure die casting is determined by several factors – die temperature, metal temperature at the end of the cavity fill, cavity fill time, cavity fill pattern and gate speed. Changes in the aluminum and magnesium content will influence some of these factors. An aluminum content below the minimum specified will reduce the fluidity of the alloy significantly, changing cavity conditions and increasing the fill time, both factors that have a major influence on the surface finish of cast parts. It is unlikely that the die caster will increase the levels of aluminum in the alloy by accident. In that rare event, the mechanical properties of the alloys deteriorate markedly when aluminum content is 4.5% – 5.5%. Properties improve again at around 8% aluminum allowing the die caster to use the more creep resistance ZnAl8Cu1 (ZL 8) in hot chamber pressure die cast machines.
It is important to note that magnesium is added to the alloy to improve hardness and because it controls the corrosion effects of certain impurities. But excessive magnesium has only detrimental effects on surface finish, reducing fluidity and impeding cavity fill conditions. A low magnesium zinc alloy ZL 7 was introduced to overcome the problems associated with magnesium. It is not included in the current EN standard but does offer the designer an alloy capable of being cast in thinner sections (down to 0.5mm) while retaining the high surface definition associated with the more traditional zinc alloys. (The effects of impurities in zinc alloys are covered in more detail in the "Chemistry of Zinc Foundry Alloys" technical paper found on the Brock website under Technical Resources.)
Iron is the most frequent contaminant arising within the die casting process. Most hot chamber die casting machines use cast iron or ferrous based melting/holding furnaces. The Zinc alloy is in direct contact with the furnace and will attack the iron over time, causing iron contamination in the alloy. The presence of aluminum in the alloy prevents excessive iron pick-up, while close control of the melt temperature prevents excessive erosion of the ferrous crucible. However, if overheating occurs, the attack rate will increase dramatically at pot temperatures above 450° C.
Excessive iron contamination depletes the aluminum content of the alloy, exacerbating the iron pick-up and causing the formation of inter-metallic particles in the alloy. These are primarily AlFe3 or AlFe5 particles. In severe cases, the inter-metallic will show after polishing as star-shaped impressions on the polished surface. The intermetallic particle is diamond-hard, and this deflects the polishing mop forming a star of polishing ripples around the hard particle. The presence of this problem is often referred to as hard spots or inclusions, and these are virtually impossible to remove by finishing or polishing.
It should also be noted that these phenomena will show through after plating or painting, often giving a sharp grit-like feature on the surface of the casting. Exercise care when remelting castings with this defect. After melting, it is advisable to remove the heat source and allow the melt to stand and acquiesce before removal of the dross and oxide on the surface. Continued application of heat moves the metal within the crucible causing the particles to circulate in the heat generated current and enlarges the inter-metallic particles. The fir-tree-like configuration allowing them to lock together with iron and aluminum in the melt further depletes the aluminum and increases the attack rate on the iron crucible.
Very small levels of some elements have a severe effect on casting quality and may affect the long term performance of the alloys. Lead, tin and cadmium are typical of these, being controlled down to a maximum of 50 ppm or less in zinc alloys. The grain structure of a zinc alloy allows these elements to sit at the grain boundaries. If one or more of these elements is present above the maximum specified in the standard, then certain conditions (warm with high humidity) will promote inter-granular corrosion within the casting. This is engendered by the formation of a minute galvanic cell stimulating the zinc to sacrifice itself at the grain boundaries. The effects in severe cases of this can be catastrophic, evidenced by the total collapse of the casting structure – effectively converting the casting to oxide dust.
In less severe cases, corrosion stress cracks are formed, promoting premature part failure in service. The presence of these impurities is often indicated by hot shortness cracks during casting. These cracks can also be caused by other process-based problems, so it is always advisable to investigate all potential causes before taking action. In all instances, control of metal chemistry by use of a reputable zinc alloy supplier and implementing regular machine pot analysis procedures with good housekeeping will limit the risk. The die caster should also ensure that foundry personnel are adequately trained and understand the risks associated with the presence of certain impurities.