Permanent molding casts for larger market.
During the past two decades, the demand for stronger and lighter metal components--particularly in the transportation industry--together with developments in aluminum alloys, has opened new markets and spurred increased production for aluminum permanent mold foundries, such as those built by Montupet near Montreal and by TEKSID at Dickson, TN, testify to the viability of the process.
In opening remarks at the first AFS-sponsored International Conference on Permanent Mold Casting of Aluminum held in April in Novi, MI, John L. Jorstad, Reynolds Metals Co, commented on the viability of aluminum permanent molding: "I have heard estimates of 400 million pounds-plus of aluminum permanent mold castings by the mid-1990s. It's estimated that annual sales value of aluminum permanent mold castings in the U.S. and Canada will be more than $3 billion by 1995, and will make up more than 25% of all aluminum castings made in North America."
Permanent Mold Processes
The different aluminum permanent mold processes include: gravity pour (both static and tilt); low pressure; squeeze; and centrifugal casting. A new high-pressure vertical shot process defies the distinction usually made between diecasting and permanent molding.
Gravity pouring is often the process of choice, particularly in automated, high-volume production of castings with relatively thick sections. Gas absorption and oxide formation leading to porosity during solidification are common problems with this process. S. C. Kwok and A. A. Wirch from Acustar Canada's Etobicoke Casting Plant list the sources of hydrogen contamination as: * furnace atmosphere; * moisture from refractories, dirty skimmers; * hydrated corrosion products in the charge, such as weathered ingot and scrap; * oil contaminated turnings, chips or scrap; * damp fluxes.
Etobicoke follows the common practice of bubbling an inert gas (dry nitrogen) through the melt. Monoatomic hydrogen is removed in a mass transfer operation as [H.sub.2].
Besides a degassing flux to remove entrapped gas, Kwok and Wirch recommend three other types of fluxes: * cover flux--a chloride mixture that prevents gas pickup and reduces oxide dross formation but not capable of wetting coalescing nonmetallic impurities suspended in the molten alloy; * clean flux--or fluorides which may be added to the covering flux, combine with suspended particles (or [Al.sub.2][O.sub.3] skin) through a wetting action, and float to the surface; * drossing-off fluxes--exothermic fluxes that react with dross, releasing entrapped metal.
Another frequently encountered problem results when metal enters the gating system too quickly, entrapping air.
Gravity Pour Solutions--One individual, with 11 years' experience in troubleshooting permanent mold problems, advocates re-examining traditional permanent mold systems which he terms "overgated." Peter Petto III of Arrow Aluminum believes in starting with what he terms a "minimum" gating system and then refining it. What some disparagingly refer to as a "bandage" approach can become, in Petto's view, the framework for a well-designed production process.
In dealing with oxide inclusions, Petto recommends experimenting with the mold incline--with the usual caveats concerning pouring speed and positioning risers above the mold cavity. Alternately, he recommends what have been termed as "exceptions" to the ideal system: enlarged sprue bottom, runner extensions and vents beyond the gate(s); changing the sprue geometry or riser placement. Repositioning the riser may allow inclusions to float into the riser. Filters may also be useful, he said.
Petto identifies localized shrink as the other most common major permanent mold defect, stemming from three causes: an inadequate gating system; an isolated heavy section fed by a riser through thinner casting sections; or a hot spot.
Foundries experiencing localized shrink are usually in a quandry because the "holy trinity" of process parameters--metal temperature, average mold temperature and pouring time--are dependent; the effects of any changes are difficult to analyze. If any one of the three must be varied, Petto advocates changing pouring time: "It seems easy to experiment with this, especially if a sprue and runner system have already grown `too big.' In this case, metal can be poured in at various rates, not filling the system. Often a particular rate will yield good castings.
"The paradox becomes apparent once this ideal pouring time is discovered and the system is resized to achieve this rate when filled. Suddenly the casting has misruns and coldshots! Here's what has happened. The insulating effect of a stream of aluminum in an oversized sprue and runner is difficult to duplicate consistently with mold coatings. The air in the big runner insulates the stream tremendously.
"To have enough insulation and that same proper diameter at the point of restriction in the gating system is difficult. It is far easier to create a spot restriction in the sprue and carefully monitor pouring time to detect erosion of this restriction point."
Low Pressure Casting
Low pressure permanent mold casting--in the range of 3-15 psi--is one of the more recent permanent mold processes. Production output equals that of the gravity process, but when fully automated, may actually exceed gravity pouring for castings weighing more than 3-6 lb.
"It is a process, both technically and economically, that bridges the gulf between the gravity (permanent mold) and high pressure diecasting process," said A. W. Plume, Twickenham, England. "All alloys that can be gravity diecast can also be low pressure cast. Other alloys can be low pressure cast that are at present utilized on the high pressure process.
"This alloy characteristic enables the designer to greatly increase the scope of application of the smaller quantity output casting, and consider these for the low pressure process very economical in comparison to the other processes normally available."
Plume believes the process warrants attention because of its: * high metal yield--90-98% compared to 50-60% for gravity casting and reduced remelting costs; * reduction in fettling or trimming costs--mainly labor--and improved appearance; * high quality and denser grain structure; * good surface definition compared to gravity diecasting--less pronounced flow lines; * consistent dimensional accuracy on production runs.
The main disadvantage according to Dr. Heinrich Fuchs, Kolbenschmidt AG, is that the casting must remain on the furnace until completely solidified, lowering yield. However, Fuchs says, "Applying low pressure die casting to its full potential [at Kolbenschmidt AG] results in scrap return figures of around 20% for a typical cylinder head casting."
In the basic low pressure process air or inert gas is used to feed the metal, against gravity, into a die and hold it until solidification is complete. Air is forced out ahead of the molten metal and through vents or parting lines in the die. Bottom filling lends itself to progressive solidification, and pressure and flow rates can be adjusted according to the intricacies of the particular die or thickness of different sections.
Low pressure die casting is capable of producing castings to tolerances equal to gravity permanent mold casting. The dies are usually made of cast iron, wear parts or complex sections of H13 or equivalent tool steel.
Roger Reamer of Progressive Castings, Plymouth, MN, has had considerable experience in converting castings to low pressure aluminum permanent mold production. In assessing operating costs, Reamer advises looking at all phases: manpower, metal loss and finishing supplies. Commenting on the cost of remelting, he said, "While metal loss is a large consideration in the typical foundry operation from the first melt to each subsequent remelt, energy to remelt unnecessary gates and risers from returns because of poor yields must be a cost consideration in any operation." He considers a realistic yield of 65% for gravity perm mold compared to 95% for low pressure.
At the Novi conference, Reamer asked his audience to consider other cost factors of remelt besides labor, including unnecessary use of equipment because of poor yields, furnace maintenance--bricks, insulation, heating elements and crucibles--are items that have substantial price tags and finite lives. Supplies such as saw blades, grinding belts and discs are reduced. This can add up to a substantial savings when looked at from the standpoint of inches of metal cut or ground with each supply item.
Melt Metallurgy--Controlling the hydrogen content and melt cleanliness is critical to maintaining low casting porosity. Strontium is commonly used, but as Fuchs pointed out at the Novi conference, "Where solidification rates approach those of sand casting, modification will not be sufficient, even at high strontium levels. On top of that, high strontium contents will support hydrogen pickup."
Research at Kolbenschmidt AG (and elsewhere) shows that phosphorus can be effectively used to increase the performance of aluminum-silicon alloys at higher temperatures, according to Fuchs. "Phosphorus additions of approximately 40-60 ppm lead to a `grainy' structure of the silicon in the eutectic," he said.
Proponents also note that progressive solidification is enhanced because the top of the casting solidifies first and the bottom last.
Programmed pouring, in which the rate is automatically changed during each pour, means solidification can be controlled for differences in section thickness and contour. Programming also permits "burning-in" of large castings, according to Glenn Stahl, Stahl Specialty Co. Varying the rate of metal entry "also can prevent flash at the bottom of the mold, since the pressure can be reduced to what is required to fill the cavity without producing flash," he said.
Tilt pouring also allows the foundry to select where the metal will enter the mold--tailoring the gating system to avoid hot spots.
Tilt pouring lends itself to high volume production. Two or more pour cups may be filled by one operator before the casting itself is poured automatically. Two or more runners also may be poured from a single cup. Since the metal is not poured directly into the mold, it can be transferred more quickly from the ladle.
Castings produced when molten metal solidifies between die halves under pressure are said to be squeeze cast. The molten metal is forged in the heated die after solidification begins. The high pressure, typically 8-10 ksi, greatly increases the contact of the solidifying casting and the die, leading to significantly greater heat transfer than in other permanent mold operations.
The castings produced typically have a fine grain microstructure. Gas porosity is lowered by superheating the melt slightly during pouring. Absence of interior porosity is a hallmark of the process.
In a paper presented at the Novi conference, Jack Dorcic of IIT Research Institute notes that squeeze casting also permits filling of heavy sections, minimizing hot spots and shrinkage pores. Because of the relatively low melt temperature, certain alloys with wide freezing ranges can be cast, with tensile properties comparable to forgings but using less pressure.
He points out that because the pressure required is less than in forging, larger parts can be produced using the same press capacity.
Yield for squeeze casting is typically about 80%, and because it is a near-net shape process, less finishing is required than for certain competing processes.
Squeeze Cast Composites
Interest and research involving Metal Matrix Composites (MMCs) and cast composites has been increasing over the past ten years, despite relatively few applications, according to Milton W. Toaz from the Piston & Sleeve Div, JP Industries, Inc.
He believes that squeeze cast preforms such as those used in Toyota's diesel piston are extremely competitive, both from an engineering and a production standpoint.
Different fibers and particulates are used, including: [Al.sub.2][O.sub.3], [Al.sub.2][O.sub.3]-Si[O.sub.2], Gr, SiC and [Si.sub.3][N.sub.4]. Several different metals have been used, although aluminum alloys have received the most attention. Toaz notes that alumina appears to be the most popular choice to date, alumino-silicates a distant second.
Composite preforms are made from a water-based slurry containing the bulk fiber, organic and inorganic binders and dispersants. Vacuum is applied through a porous forming tool, creating the preformed mat. Fiber orientation is determined by the tool design itself; density can be controlled either during forming or subsequent operations after drying and firing.
Toyota's cast composite piston, introduced about 1984, incorporated low cost, discontinuous ceramic fibers into a headland ring belt using pressure infiltration. Because of the casting integrity attainable with squeeze casting, Toaz believes the process is ideal for selectively reinforcing high wear areas.
Cast composites, like most MMCs, are anisotropic--their properties can be varied by changing the fibers' orientation. Selective reinforcement of castings can be controlled by adjusting only two properties: fiber content and fiber orientation. In fact, the two are interrelated.
Taking Al-Si alloys used in heavy duty aluminum diesel pistons as an example, it was found that wear resistance is improved by increasing the fiber content. In addition, as Toaz notes: "The addition of ceramic fibers to aluminum alloys has a pronounced anisotropic influence on thermal expansion. Increasing fiber content decreases the thermal expansion in a direction parallel to the fiber axis but has little effect in the direction transverse to the fiber axis."
Both hardness and modulus rise with increasing fiber content in conformance to the rule of mixtures. Increased strength, unchanged at room temperature, rises significantly with higher temperature.
The effect on thermal conductivity, however, is just the opposite: fiber content has less effect in the direction parallel to the fiber axis than transverse to the fibers.
Selective reinforcement permits increasing the thermal fatigue in the crown area, wear resistance in the ring belt and improving the scuffing resistance of the skirt. By choosing the appropriate alloy matrix and casting process, Toaz believes an exceptionally long-lived piston can be cast.
Modeling and Finite Element Analysis (FEA) have provided the advanced engineering design techniques required to implement selective reinforcement. The FEA model undergoes a computer simulation of engine operating conditions in order to predict the maximum principal stresses.
The selectively reinforced piston that is developed is then subjected to the same computer simulation. Results of extended engine tests have been very favorable, according to Toaz.
He predicts that the heavy duty diesel, rather than the automotive market, holds more promise for squeeze cast composites. Two factors mitigate against its use in the later market for perhaps ten years, says Toaz: the lack of reliable NDT techniques, and more importantly, cost. He says the industry also is concerned with the lack of manufacturing capability to meet high-volume demands.
These factors notwithstanding, Toaz predicts that the advanced diesel engines now under development will use MMC pistons simply because there will be no other cost competitive alternatives capable of delivering the same performance.
PHOTO : Micrographs showing the effect of phosphorous treatment and sodium modification on the
PHOTO : eutectic structure of alloy A356.
PHOTO : The castings shown were tilt poured in a three-cavity "stacked" mold.
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|Author:||Burditt, Michael F.|
|Date:||Jul 1, 1989|
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