Die casting

[edit] Introduction

Die casting is an effective, economical process for producing complex shapes with excellent dimensional accuracy, repeatability, and surface finish. Because of its advantages it is on of the most common processes in the metalworking industry and is therefore a promising area for research and development in energy and material efficiency.^[1]

Information on Die casting can be found at Wikipedia. What's this?

[edit] How it Works

[edit] Basic Process

Figure 1: An artistic rendering of the hot-chamber process.

Figure 2: An artistic rendering of the cold chamber process.

Die Casting is a casting method in which molten metal is injected into the die^W at a high pressure and the cast is maintained at that pressure throughout solidification. Injection is performed by a moving piston within a cylindrical chamber.

There are two basic methods of die casting, hot-chamber and cold-chamber. These two methods are shown in Figures 1 and 2. Hot-chamber machines rely upon a pool of molten metal to feed the die. At the beginning of the cycle the piston of the machine is retracted, which allows the molten metal to fill the "gooseneck" that leads to the die. The gas or oil powered piston then forces this metal out of the gooseneck into the die. The advantages of this system include fast cycle times and the convenience of melting the metal in the casting machine.^[2] In a cold-chamber machine the metal is melted in a separate furnace and ladled into the pressure chamber, resulting in longer cycle times.^[3]

The remainder of the process is common to both the hot and cold chamber method. Various die components are used to produce a wide range of finished products. Cores are sections of the die which create a void in the cast, they may be retractable, fixed in the die, or may be loose, meaning they are placed in the die before each cycle and subsequently ejected with the part and removed later. Prefabricated inserts may be inserted into the die prior to casting so that they are encased in the part during solidification and become part of the finished part. Water cooling passages may be incorporated in the die to control cooling rates at various points in the cast. Chills^W which are portions of the die with higher thermal conductivity may be also used to control cooling rates at various points in the cast. Once the cast has fully solidified the die is opened, the finished cast is ejected, and the die is prepared for reuse in the next cycle.^[4]

In the Figures above only one cavity is shown within the die for simplicity. In practice there are as many cavities in each die as possible to allow many casts to be made in one process cycle or "shot". What limits the number of casts per shot is the passages required to carry the melt to each die cavity. If there are too many cavities the network of passages will be long and complex and the melt will begin to solidify before it has even reached the die cavity.

[edit] Process Variations

• Pore free casting process^W

When a cast is required to have minimal porosity then the pore-free casting process is used. It is identical to the standard process except oxygen is injected into the die before each shot. This forces air out of the die leaving a pure oxygen atmosphere. The oxygen reacts with the melt leaving small, evenly dispersed oxides in the cast but virtually no gas porosity^W. This reduction of porosity greatly increases the strength of the final cast. These castings can still be heat treated^W and welded^W because of the lack of porosity. This process can be performed on aluminum, zinc, and lead alloys.^[2]

The pore free casting process has some disadvantages:

-Increased cycle times due to the added step

-Parts with many thin offshoots, ribs, etc. are not suitable for the pore free casting process since air will not be fully purged from all of the thin elements, where strength is most important

-Dispersed oxides will have a slight negative effect on the properties of the final cast

• Heated manifold direct injection die casting^W

Heated-manifold direct-injection die casting, also known as direct-injection die casting or runnerless die casting, is a zinc die casting process where molten zinc is forced through a heated manifold^W and then through heated mini-nozzles, which lead into the molding cavity. This process has the advantages of lower cost per part, through the reduction of scrap (by the elimination of sprues, gates and runners) and energy conservation, and better surface quality through slower cooling cycles.^[2]

• Vacuum die casting

In vacuum die casting the die has two openings, a vacuum outlet at the top and a sprue at the bottom, the sprue is submerged into the melt and a vacuum is drawn at the vacuum outlet. The resulting pressure difference draws the melt into the die.^[5]

[edit] Pressure Effects

One of the main components of die casting that sets it apart from other types of casting is the high pressures involved. The high-pressure injection leads to a quick fill of the die, which is required so the entire cavity fills before any part of the casting solidifies. In this way, discontinuities are avoided even if the shape requires difficult-to-fill thin sections. This creates the problem of air entrapment, because when the mold is filled quickly there is little time for the air to escape. This problem is minimized by including vents along the parting lines^W. Turbulence during high pressure injection also causes air entrapment so that even in a highly refined process there will still be some porosity^W in the center of the casting.^[2]

High pressure also affects the microstructure^W and mechanical properties of the final cast. Increased pressure leads to finer grain sizes ^[6], and reduced pore size ^[7]. Due to the effect of pressure to distribute phases homogenously and reduce porosity, both hardness, and tensile strength increase with pressure.^[7]

[edit] Material Limitations

Die casting is typically limited to non-ferrous alloys which have low melting points and viscosities^W relative to ferrous alloys. Die casting machines and dies are typically made from steel. Non-ferrous alloys will flow through the machinery due to their relatively low viscosities and will not bind with the surfaces of machinery because they are at a temperature much lower than the melting point of the steel machinery.^[8] There are however some alloys which will pick up iron from the machinery surfaces, it is for these alloys that cold-chamber machines are used in order to limit contact between the molten metal and the machinery.^[2] Ferrous alloys cannot be used with machinery that is also composed of ferrous alloys because it will melt, damage, and bind with the machinery.^[8] This can be solved by modifying the machinery in a variety of ways such as ceramic components, insulating coatings etc.

[edit] Energy Efficiency

[edit] The Process Itself

There are several methods being developed to improve the efficiency of the die casting process. One method is using sensors to allow real-time monitoring of process parameters and the other is increasing die life through coatings and improved materials.

[edit] Sensors

There are many parameters which are tailored in each die casting process to suit the particular part being fabricated. These parameters include melt temperature, injection pressure, and holding pressure (the pressure at which the die is held during solidification), as well as the composition of the melt. The most important parameter is of course the composition of the melt. The following are two methods being developed to monitor melt composition in order to improve the quality of the final cast and facilitate control of the casting process in order to maximize efficiency.

• Laser-induced breakdown spectroscopy^W (LIBS)

LIBS has been adapted to the casting process to measure the composition of the melt and provide feedback allowing for better process control. A good description of how this process works is found {[WP|LIBS|here}}. Disadvantages are difficulty ensuring that the material being tested is below the slag layer and that emissions from the slag layer are not affecting the measurement.^[9]

• Galvanic Cell^W Based Chemical Sensors

The composition of the melt can also be determined using galvanic cell based chemical sensors. In these sensors an anode and a cathode are separated by an electrolyte, one is placed in a reference material and the other in the melt. The cell is calibrated such that the magnitude of the voltage generated across the electrolyte is proportional to the ratio of the logarithms of the concentrations of the element to be detected in the reference material and the melt. These sensors are more simple than a LIBS system but can only analyze one element so multiple sensors are required.^[9]

[edit] Improved Die Materials

The dies used in the die casting process must be replaced periodically due to wear and failure. The amount of time each die lasts is called "die life". This is an important factor affecting the energy efficency of the die casting process because of the energy that goes into fabricating dies and replacing them.

There are two main factors contributing to die failure. The first is washout which includes erosion, corrosion, and soldering^W (when the melt fuses to the die walls). The second factor is thermal fatigue^W caused by the drastic temperature changes experienced by the die during each cycle.^[10]

• PVD coated dies

Various Physical vapor deposition^W (PVD) coatings on dies are being developed and implemented to improve the resistance of dies to mechanical and thermal wear. The exact composition of the coating is dependant on the composition of the alloy being cast. ^[10]

• Die Material and Method of Fabrication

Figure 3: An artistic rendering of soldering process in metal mold casting.

By modifying the die material and method of fabrication the occurance of die soldering can be minimized. The mechanism of soldering in metal mold casting is as follows. The initial attack of the melt loosens harder grains of the tool steel causing pitting. This is followed by the formation of intermetallic phases inside the pits and around broken grains at the surface of the die. The intermetallic phases are composed of ferrous(tool steel) and nonferrous(melt) components. These growths increase in size and fuse to form a layer on the die. The melt begins to stick to this layer creating a second layer of soldered melt. At some point the mechanism slows significantly, in the case of aluminum this occurs when the ratio of thicknesses of the intermetallic layer and the soldered layer is ~1:5. This mechanism is shown in Figure 3.

Understanding the mechanism that causes soldering it can be seen that by choosing a die material that has an even composition will remove the ability of the melt to attack the surface at different rates and cause pitting. Also a die material with reduced kinetics of formation of reaction products between die material and melt will reduce the growth of an intermetallic layer. Since thermal cracking creates the same effect as pits a material resistant to thermal fatigue is also benefitial.

Dies that are made by casting do not have uniform composition, microstructre, or properties, dies made from powder-metallurgy such as sintering have much less variation of properties and are therefore more usefel for minimizing soldering. ^[11]

[edit] Widening It's Application

The energy consumed by the melt furnaces is much higher than that consumed by the hydraulic systems, in some cases ten times more.^{[12] [13]} Because the melt furnaces operate continuously while the casting machine is operating the more casts that can be produced per unit time the higher the energy efficiency of the casting process. It is for this reason that the short cycle times associated with die casting make it the most energetically efficient casting process.

Low weight, high strength materials are in high demand, particularily in the automotive and aviation industries. Die casting is suited for producing non-ferrous low weight parts, but because of the porosity associated with the die casting process these parts do not have the required strength. So most parts requiring high strength are produced with other, less efficient casting methods. If the strength of die cast parts could be brought up to that required, many parts could be manufactured using the more efficient die casting method resulting in large energy savings.

Outlined below are two methods of accomplishing this strength increase. The first, rheo diecasting, is very promising as it eliminates porosity and the associated strength reductions. The second is the use of vibrations, this method increases the equiaxed zone of the cast but also causes finer grain size so the net effect on strength of the final cast is much less then that of the rheo die-casting method and possibly even a negative effect.

[edit] Rheo Die-Casting

The reason die cast parts have considerable porosity is air entrapment caused by turbulent flow during melt injection. Rheo die-casting increases the viscosity of the melt to a level where turbulence and the associated porosity are eliminated. This is accomplished by converting the melt from a liquid to a semi-solid state before it is passed to a conventional cold chamber die casting machine. The process is outlined below.

At the start of each cycle the required amount of melt is transferred from the furnace to a mechanical mixer. The fluid flow in the mixer is characterized by high shear rates and high turbulence, and it is under these conditions that the melt begins to solidify. Because of the shear and turbulence and dendritic formations or even slight irregularities are sheared off of solidifying particles. This results in the melt solidifying to a semi solid state (30-65% solid), with a globular solid phase surrounded by the liquid phase. At this point the melt has the proper viscosity to allow it to flow with minimal turbulence. The melt is then transferred to the chamber of a conventional cold chamber die casting machine for the remainder of the cycle.^[14][15]

Rheo-diecast samples have very low porosity and fine and uniform microstructure giving them improved tensile strength and ductility compared to sample produced by conventional high-pressure diecasting. This makes them suitable for use in high strength applications such as vehicle parts.^[16]

[edit] Vibration Induced Cavitation

• Mechanical Vibration

It has been shown that mechanical vibrations modify the microstructure of the final cast, namely through the reduction of undesirable dendritic^W/columnar^W zones and the increase of desirable equiaxed^W zones. This can be explained by two phenomenon. Firstly, the vibrations agitate the melt and disperse crystals producing more numerous, evenly distributed nuclei. Secondly, the vibrations cause cavitation^W, in short, due to pressure differentials in the melt, bubbles form and collapse. When these bubbles collapse they create huge pressures and therefore shockwaves within the melt which causes dislocation of growing crystals. This dislocation contributes to the production of more numerous, evenly distributed nuclei. The net effect is a finer grain structure and the related favourable properties in the final cast.^[17].

These vibrations are normally transmitted to the melt through coupling rods inserted directly into the melt. These rods are rapidly dissolved when immersed into molten alloys, which contaminates the melt. Also, the intensity of cavitation is not evenly distributed throughout the melt, it is concentrated close to the coupling rod.^[17] The high pressures involved in die casting further complicate the application of this method to die casting.

• Electromagnetic Vibration

An alternative method for producing vibrations in the melt is the use of electromagnetic^W vibration. Applying a stationary magnetic field Bo and a sinusoidal electric current i = Isinωt to an alloy during solidification generates an electromagnetic vibrating pressure described by:

where L is the length of a parallelepipedic tank in which the alloy is contained during solidification.^[17]

This process has addressed many of the disadvantages of mechanical vibration, direct contact with the melt is not required so holding pressure is no longer an issue, and the vibrations are more evenly distributed throughout the melt.

Unfortunately this process has very large energy requirements and is not practical. But if the vibrations were constantly adjusted to maintain them at the resonance frequency of the melt sufficient vibrations could be produced with much lower energy requirements. This is currently being researched.^[17]

[edit] Conclusion

The die casting process is for many applications the most efficient casting method. Some improvements to its efficiency can be made by using sensors to closely monitor process parameters and using PVD coatings and improved die materials to increase die life. Even larger energy savings can be made by implementing the rheo die-casting process in order to allow parts requiring high strength to be made with the die casting method rather than other, less efficient casting methods.

[edit] References

1. ↑ FAQ about Die Casting, http://www.diecasting.org/faq/, (14 Nov 2008)
2. ↑ ^{2.0 2.1 2.2 2.3 2.4} Degarmo, E. Paul, J Black, R Kohser, Materials and Processes in Manufacturing (9th ed.), p. 328-331, Wiley, ISBN 0-471-65653-4.
3. ↑ Die Casting, http://www.efunda.com/processes/metal_processing/die_casting.cfm (10 November 2008).
4. ↑ Designining Die-Cast Parts For Manufacturability, http://ezinearticles.com/?Designing-Die-Cast-Parts-For-Manufacturability&id=1492778 (10 November 2008).
5. ↑ Vacuum Die Casting, http://www.aurorametals.com/vd.htm (19 Novemeber 2008)
6. ↑ V Gertsman, J Li, S Xu, J Thomson, and M Sahoo, “Microstructure and second-phase particles in low- and high-pressure die-cast magnesium alloy AM50”, Metallurgical and Materials Transactions A, 36(8), 1989-1997, 2005 http://www.springerlink.com.proxy.queensu.ca/content/u2n16008040421x2/ (12 November 2008).
7. ↑ ^{7.0 7.1} M Kaplan, F Yakuphanoglu, A Yildiz. “Effects of Mold Pressure on Mechanical, Microstructures, Oxidation Behavior, and Thermodynamic Properties of an Al-Based Alloy”, Materials and Manufacturing Processes, 21(1), 97-104, 2006 http://www.informaworld.com.proxy.queensu.ca/smpp/content~content=a727103403~db=all~order=page (12 November 2008).
8. ↑ ^{8.0 8.1} Y Miura, N Kashiwagi, Z Mochizuki, "Apparatus for Die Casting Ferrous Metals", http://www.freepatentsonline.com/3672440.html (Nov 13 2008)
9. ↑ ^{9.0 9.1} JW Fergus, "Sensors for Monitoring the Quality of Molten Aluminum During Casting", http://www.springerlink.com.proxy.queensu.ca/content/2k03788106679p46/fulltext.pdf (14 November 2008)
10. ↑ ^{10.0 10.1} M. Rosso, D. Ugues, E. Torres, M. Perucca, P. Kapranos, "Performance enhancements of die casting tools through PVD nanocoatings", http://www.springerlink.com.proxy.queensu.ca/content/y28w6r8j0h102844/fulltext.pdf (14 November 2008)
11. ↑ S. Shankar and D. Apelian, "Mechanism and preventive measures for die soldering during Al casting in a ferrous mold", JOM Journal of the Minerals, 54(8), 47-54, 2002 http://www.springerlink.com/content/n726h8006x3qw72m/fulltext.pdf (24 November 2008)
12. ↑ "New, highly efficient melting furnaces supplied to die-casting company", http://hitempproducts.thomasnet.com/Asset/APM-Tubes-Tubothal-Aluminium-Case-History.pdf (Nov 23 2008)
13. ↑ "Controlled magnesium melt process, system and components there for", http://www.patentstorm.us/patents/5643528/description.html (Nov 23 2008)
14. ↑ KP Young, "Semi-Solid MEtal Casting: Reducing the Cost of Copper Alloy Parts", p.1, http://web.archive.org/web/20061007163908/http://www.mass.gov/envir/ota/publications/pdf/semi_solid_metal_fact_sheet.pdf (14 November 2008)
15. ↑ Z. Fan. “Rheo-diecasting of Al-Alloys”, http://www.brunel.ac.uk/controls/common/getImage.aspx?imageId=64 (12 November 2008).
16. ↑ Z. Fan, G. Liu, Y. Wang, "Microstructure and mechanical properties of rheo-diecast AZ91D magnesium alloy", http://www.springerlink.com.proxy.queensu.ca/content/m4277811427p1348/fulltext.pdf (14 November 2008)
17. ↑ ^{17.0 17.1 17.2 17.3} C, Vives. "Grain Refinement in Aluminum Alloys by Means of Electromagnetic Vibrations Including Cavitation Phenomena", http://www.tms.org/pubs/journals/JOM/9802/Vives/ (12 November 2008).