Industrial mould-casting processes

Sand Casting

In sand casting, reusable, permanent patterns are used to make the sand moulds that are destroyed in removing the solidified casting.

There are two basic kinds of molding material - natural and syn­thetic. The former is called «green sand» and consists of mixtures of sand, clay, and moisture. The latter is called «dry sand» and consists of sand and synthetic binders cured thermally or chemically. The advan­tage of synthetics is that their composition can be controlled within tight limits. The sand cores used for forming the inside shape of hollow parts of the casting are made using dry sand components.

The extra metal needed to fill and feed the casting generates the scrap and causes relatively low metal yields. Sand casting requires cleaning, which involves removing molding sand and cutting off the extra metal in the attached runners and risers.

Of all the casting methods, sand casting is the most versatile. It is not only suitable for small-scale production but can be cost-effective where castings are required in larger numbers.

Low-pressure sand casting

Low-pressure sand casting is an innovative of sand casting. The casting cavity is filled from below via a fill tube - the riser stack - and a nozzle from a crucible under low gas pressure (up to 0.7 bar). The pressure is maintained during solidification. With microprocessor con­trol, the pressure during injection can be varied as required to ensure that the mould fills in the optimal way for its particular shape. Thanks to this technique, casting can be made with wall thicknesses as low as 2.5 mm, which is difficult with conventional sand casting. By means of this process, other important requirements can be satisfied. These in-

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elude close dimensional tolerances, good surface quality, cleaning time lower, since the conventional sand casting arrangements for gating and risering are not needed.

It is a landing-flap mounting in A356.0-T6 alloy, which has the required high yield and ultimate tensile strength in the load-bearing zones. Apart from such regular mass-production, low-pressure sand casting is especially well-suited to making prototypes of casting being developed for permanent mould.

Pressure die casting belongs to the family of permanent-mould casting. In pressure die casting, a molten charge is injected into a wa­ter-cooled die of heat-resistant steel by a casting piston under high pressure and at high speed. Pressure is applied throughout solidifica­tion. The process causes turbulent filling of the die, leading to gas in­clusions in the casting origination from entrapped air and from the die-lubricating and parting compounds. These inclusions generally rule out subsequent heat treatment or welding because they would cause blis­tering.

Aluminum casting alloys are mainly pressure die-cast using horizontal cold-chamber machines. In these machines, the casting chamber into which a measured amount of melt is introduced, is hori­zontal and is arranged at right angles to the die. The melt quantity is regulated automatically. Pressure die casting is suited to the production of intricate castings of large surface area and accurate dimensions and yields an outstandingly good surface quality.

Both me machine and its dies are very expensive, and for this reason pressure die casting is economical only for high-volume pro­duction. Complex cores cannot be used to form re-entrant inner details, limiting the range of shapes that can be cast. The chief objective of the newest variants of the pressure die casting process is to avoid the com­pressed gas inclusions that are a feature of conventional pressure die casting and which cause porosity in the cast structure and blistering during heat treatment. Avoiding entrapped gas makes the castings heat-treatable and weldable. This is achieved either by evacuating both the die and the casting chamber (vacuum die casting) or by slow filling while evacuating the die. The purpose of filling more slowly and with much reduced pressure is to minimize turbulence. Such processes need suitably designed injection systems with special controls to program the speed of the casting piston.

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Vacuum die casting

By the first method, the melt is poured into the casting chamber in the traditional way (i.e. from a ladle). The die is evacuated after the casting piston has advanced past the pouring hole.

The melt is drawn into the casting cavity by the vacuum itself via a ceramic tube that is attached to the casting chamber and dips into the liquid melt. This variant has the advantages that more time is avail­able to evacuate the die and transfer is less turbulent. The pressure in the casting chamber and the casting cavity must be less than 50 milli­bars to ensure a satisfactory casting. Casting made in this way can be welded or undergo solution heat treatment at temperatures above 500 °C with no risk of gas blisters. Depending on the choice of alloy and heat treatment, they can have high mechanical strength with rela­tively high ductility. The technique has made it possible to penetrate new application areas replacing, for example, impact-extruded, sand-cast, permanent mould cast, and forged parts, as well as, in case of steel. This is especially important for mass-produced parts, such as used in making motor vehicles. Vacuum die-cast and heat-treated parts, are often chosen for safety critical components because of their favourable combination of properties.

Роге-free die casting (PFDC)

In this method of pressure die casting, developed and used in Ja­pan, the air in the casting cavity is displaced by oxygen. This oxygen reacts with the melt and forms aluminum oxides. The main purpose of the process is to deplete gas within the mould. Hydrogen plays no role. The process requires special gating to fragment the injected stream. The AI2O3 reaction product is very finely dispersed in the cast structure where it is said to have no negative effects. PFDC is used in Japan for producing wheel rims for cars and, recently, for utility vehicles.

Squeeze casting

This is another innovative die casting process that takes two forms: direct and indirect.

In direct squeeze casting, the die is filled with a defined amount of liquid metal via a trough. The die, which may have two or more parts, is closed from above by hydraulic pressure, forcing the metal to conform to the die surfaces. The pressure is maintained during solidifi-

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cation and be from 500 to 1500 bar, depending on the size and wall thickness of the casting.

Unlike direct squeeze casting, the indirect version has proved it­self in production and has obtained a foothold in foundries producing casting for special applications, chiefly automotive wheels and pistons. It differs from the direct process in that the melt is not poured directly into the die, but into a casting chamber situated below it. The bottom of the chamber is sealed by a piston whose actuating cylinder is fixed to a pivot. After filling, the chamber is swung into position under the die and the piston is raised to fill the die. Once this is accomplished, the cylinder pressure is increased to 1000-1200 bar and held at that level until solidification is complete. The advantages of indirect over direct squeeze casting are that the die can be filled with the least possi­ble turbulence and that horizontally or vertically closed dies can be used as well as multiple dies. The high pressures used in squeeze cast­ing do not merely ensure optimal filling of the die; they prevent the premature separation of the solidifying casting. This ensures better heat transfer, resulting in a greater rate of solidification, which, in turn, leads to a finer cast structure and improved mechanical properties, in particular to higher yield and ultimate tensile strengths.

Indirect squeeze casting is especially suitable for making fibre-reinforced castings. These have fibre-shaped elements {fibre cake per­forms) cast into them in order to increase their strength at elevated temperatures among other properties. The fibres are commonly made of silicon carbide or alumina. If the cast component is to be usable, it is essential for the melt to infiltrate between the fibrers. This requires a low filling speed and relatively high pressure during solidification. In­creased strength at high working temperatures is especially important for pistons. At temperatures above 300 °C a casting reinforced with alumina fibres has double rotating-beam fatigue strength of the same casting without reinforcement.Temperatures as high as this occur, for example, at the edges of the piston head of a diesel engine heavy load and in high specific output automobile engines.

Under severe temperature cycling, castings with fibre rein­forcement can have three to four times the strength of parts made with­out reinforcing additions. One of the first applications in mass produc­tion is a lorry engine piston head with fibre-reinforced edges.

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Thixocasting

A novel casting process that has lately been the subject of much attention is thixocasting. The technique takes advantage of the thixotropic properties of metal alloys in the partly solidified state. Thixotropic behavior means that the material behaves as a solid, when at rest, but flows like a liquid when rapidly deformed, its viscosity fal­ling as the stress increases. To bring this about, the metal has to be heated to a point in its freezing range between the solidus and liquidus temperatures. This point should be chosen so that, for example, 40 % of the volume is liquid, the rest remaining solid.

Aluminum alloys having a long freezing range are suitable for thixocasting (e.g. 356.0 and 357.0). To attain a good thixotropic state, the a-crystals in the solid solution must be equiaxed in order to ensure that the liquid and solid phases flow uniformly and do not separate. Such a structure can be created by so-called «rheocasting», whereby, the melt is electromagnetically stirred during solidification. This breaks off or melts off the dendrite arms, which condense to globular shapes when held just above the solidus temperature, resulting in in­digenous growth of free-floating crystals with the desired equiaxed structure. This method is used, for example, to cast logs up to 150 mm diameter for semifabricated products. Another technique is strain-induced melt activation (SIMA), in which the material is given pre­liminary cold-working, during subsequent heat treatment, the stored strain energy drives recrystalization to form equiaxed grains of a solid solution. The SIMA process is so far only suitable for produc­ing small samples. Yet, another method is mechanical agitation dur­ing solidification.

Current practice in thixocasting is,as follows. The log is first sawn into slugs having sufficient weight for*the part to be cast. The slugs are then preheated slugs are then preheated to a predetermined temperature in their melting range. This is done automatically using temperature sensors to attain a definite proportion of liquid metal often between 35 % and 50 %. At this temperature, the slug is still behaving as a solid. It is placed in the casting chamber of the casting machine and is injected into the mould cavity by the casting piston. During pre­heating and casting, precautions must be taken to ensure that oxide skins from the surface are kept out of the casting.

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A number of advantages over conventional pressure die casting methods are claimed for thixocasting, provided that key process pa­rameters are kept under control. These advantages contain a significant energy saving, including a large part of the heat of fusion and the whole of the additional heat, as well as, energy for holding the melt at temperature. A further advantage is better dimensional tolerances ow­ing to the reduced shrinkage. Since the cast part is closer to final di­mensions («near net shape»), it needs less further working and gener­ates less scrap to be recycled. Productivity ^сал be higher than with tra­ditional methods of pressure die casting. Because the process tempera­ture is about 100 °C lower, the temperature cycling is less severe, lengthening the tool life. This also makes it possible to use low-iron al­loys because the cooler melt has less tendency to attack the die by dis­solving iron. A vital advantage is that the mould fills without air inclu­sions. By using a parting vacuum in the die cavity, leak-proof castings can be produced, which can be welded and heat-treated. The present-day demand for highly ductile, safety-critical components can be satis­fied by rapidly solidified thixocasting alloys containing less than 0.15% iron.

The peculiar characteristics of the thixocasting process make it suitable for casting variants of the usual wrought alloys. Thixocasting is a good way of producing particle-reinforced materials because the particles have less tendency to separate out of the partially solidi­fied melt.

In addition to the classical methods of mould casting, a variety of specialized processes are also used. These include investment cast­ing, also known as lost wax.

Lost-wax and lost-foam casting

In lost-wax casting, both mould and pattern are destroyed at each cast. The ceramic mould is made as a single piece. A number of patterns are prepared using injection-moulding and are assembled to­gether in a «tree», the patterns being the branches and the feed channel the trunk. The whole is coated by dipping in several baths of ceramic slurry. After these layers of slurry have hardened, the wax is melted out and the ceramic mould is fired, ready for casting, usually under vacuum, into the preheated mould. Very intricate castings can be made to close dimensional tolerances using this technique.

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In lost-foam casting, also, both mould and pattern are destroyed during each cast. The pattern is made of polystyrene foam, which is easily, vapourized while the mould is of sand without binder. The pat­terns are made by blowing the foam into specially-designed moulds. After washing with parting compound, the pattern is embedded in the sand which is then compacted under vibration. The foam pattern re­mains in the mould and is vapourized as the melt fills the mould. The foam pattern remains in the mould and is vapourized as the melt fills the mould. The wash serves mainly to control the escape pressure of the vapourized foam and to support the sand in the gap between the melt and the still solid foam. The gas permeability of the wash coating is critically important: it must be regulated so as to provide a vapour pressure in the gas high enough to support the coating.

ALUMINIUM EXTRUSION -METAL FORMING PROCESS

Aluminium extruding had its beginnings in the last century. The process is fundamentally one of pressing a billet of aluminium through a profile cut into a die.

The die maker is able to cut almost any profile into die steel. This versatility enables the extruder to provide the end user with an almost unlimited range of shapes. These shapes can be provided in lengths of up to 15 metres.

Other methods of forming metal into the shapes required by fab­ricators are extremely labour intensive and often require special skills. Processes such as bending and machining may have a labour compo­nent of as much as 80 % of the final cost of the article. By comparison the cost of labour in an aluminium extrusion would be around 15 %. In many cases, therefore, significant savings can be shown in the fabrica­tion process through the use of extrusions. Examples will be given to justify this view.

The paper is designed to give a brief insight into the extrusion process, followed by the advantages the extrusion process has for the manufacturing industry.

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Aluminium is one of the most abundant elements on earth and known economically viable reserves are expected to last approximately 1000 years at 1979 consumption rates.

The reduction of aluminium from ore was first commercially achieved in 1855 and, therefore, this metal is a relative newcomer to the metals industry. Subsequent to the commercial production of alu­minium, the metal was adopted for use in the extrusion process which had been invented earlier for the purpose of producing lead pipe.

Principle

One dictionary defines extrusion as «the act of expulsion by me­chanical force». Aluminium extrusion is achieved by subjecting a con­fined billet to pressure and forcing the billet to flow through a profile opening which has been cut into a steel die. The principle of extruding metals can be likened to squeezing toothpaste from a tube or putting decorative icing on a cake.

In essence, the metal is sheared through an opening in a die which is shaped to the required profile. The flow of metal through the die is controlled by frictional forces in the die itself. These are termed «bearing surfaces». For instance, where the metal must move relatively slowly, a long bearing surface is used and, conversely, where a rela­tively fast metal flow is required the bearing surface is shorter.

To produce a hollow section or profile a different type of die is used. The most common type of hollow die used is known as the «porthole» die. To obtain the hollow shape the billet is first split up by the ports of the die, forced over the mandrel (which controls the inside shape) and then, fusion welded with itself before being extruded through the die plate.

Die making

Profiles are cut into the extrusion die using the spark erosion technique. To explain briefly, the spark erosion technique works like this. Firstly, an electrode must be made. This is done by machining a copper or carbon bar to the shape of the required profile. This is a skilled operation and various aids, such as pantographs, are sometimes used. Once completed, the electrode is positioned on the workpiece (die steel) and then, by electrical discharge the shape of the electrode is eroded into the die steel. The method described above is being super­seded by more modern techniques which make use of computer tech-

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nology. However, the «electrode» illustrates very clearly the almost in­finite versatility of shape that the extrusion forming process offers. This facility is extremely significant to any designer as it is obvious that many cost saving features can be designed into the shape.

Profile

The extrusion process does, however, dictate that the profile be produced in length. The length that any particular profile can be pro­duced in is limited only by the size of the profile and the capacity of the press that it is extruded from. However, for practical purposes, namely heat treatment, handling, and transportation, the lengths sel­dom exceed 15 m., but longer lengths are available subject to consulta­tion with the supplier. Conversely, the extrusion can also be cut to ex­tremely short.

Aluminium extrusion

Subsequent to the extrusion process there are a number of heat treatment practices that are designed to give varying mechanical prop­erties to the extruded product, therefore, adding another dimension to the flexibility of design that the aluminium extrusion offers. It is obvi­ous that to machine the features shown in the profiles displayed would be prohibitive due to the labour costs alone, especially if the items are required in length. However, as complexity of shape is only of minor cost consideration in the extrusion process the very high labour costs are eliminated enabling the extrusion formed product to be available to a large proportion of industry.

Strength

There are two other aspects of extruded product to consider, the first being that of strength. There are probably at least 50 different ex­trusion alloys available, but only between'7-M) different alloys in com­mon use. Aluminium can be alloyed to vary in strength from very soft through to stronger than mild steel. However, the ratios which apply to the common alloys are that the extruded product is approximately one third the weight of steel and between a half and two thirds of the strength.

The other advantage that extrusions have is the variety of fin­ishes available that enhance the aesthetic value of the metal. The most common finish to aluminium extrusions is anodising. This process im-

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plants an extremely durable film of aluminium oxide into the extrusion thereby improving the corrosion resistance inherent in the metal.

The aluminium extrusion can also be mechanically polished, scratch finished, or chemically polished, a more recent, development being the electrostatic painting or coating of the extruded product.

Conclusion

The extrusion process is flexible within very broad parameters and allows for «tailor-made» metal shapes to be available to any indus­try at a relatively low cost.

PRECIPITATION AGING

The objectives of precipitation aging is to raise the mechanical strength of heat-treatable alloys. A prerequisite of the process is retention of solute atoms in supersaturated solid solution. The metallurgical changes which occur during ageing are;

• diffusion of solute atoms

• clustering of solute atoms at preferred lattice positions

• ordering of solute atoms at these points in the stiochemical ra­tios and atomic arrangements necessary to form stable intermetallic compounds, thus, forming coherent precipitates

• growth of coherent precipitates to a critical size so that sepa­ration of the coherent precipitate from the lattice can occur with a new release of free energy, thus,, forming an incoherent, or true, precipitate

-growth of incoherent precipitates.

The process driving force is the degree of supersaturation, which increases as an inverse function of temperature. Diffusion being de­pendent, the reaction rate increases exponentially with the temperature increase.

In the 6000 series of alloys, which represent approximately 80 % of North American aluminum extrusion production, the solute atoms contributing to the ageing process are magnesium and silicon with the final precipitate being MgjSi. With these alloys the diffusion rates at room temperature are low, thus, the aging process does not proceed beyond cluster formation. At temperatures of the order of 170 °C,

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(338 °F) coherent precipitates are formed within a few hours, and pre­cipitation of significant quantities of M&Si occur within one day. Be­cause of the exponential dependency of diffusion rate on the tempera­ture, the use of an aging temperature of 200 °C (375 °F) causes consid­erable coherent precipitate formation within one hour with significant Mg2Si precipitation after five hours.

The mechnical strength of metals is governed by dislocation movement; the impairment of this movement increases the energy re­quired for deformation and hence, the strength of the material.

Energy is required for dislocations to cut through clusters and coherent precipitates, causing an increase in mechanical strength. Dis­locations do not cut through incoherent precipitates, but bend round them, the energy required being a function of the diameter of the pre­cipitate. Since the chemical composition of an alloy determines the maximum quantity of precipitate which can occur, it follows that in a state of complete precipitation the distance between precipitated phases increases as a function of precipitate diameter, thus, the number of ob­stacles to dislocation movement is reduced, leading to the conclusion that the strength of an alloy of a given composition decreases with in­creased precipitate diameter.

Since increasing aging temperature causes more rapid diffusion rates, the distance over which solute atoms migrate to incoherent pre­cipitates increases with temperature, leading to the observation that at higher aging temperatures there is more-rapid decline in mechanical properties.

The Aging Oven

For maximum productivity overall time for the precipitation process should be cut to a minimum, through rapid heating rate and short time, higher temperature aging. For many aluminum alloys rapid heating to aging temperature drastically reduces the peak mechanical properties. This weakening effect is not, however, significant with the AA-6000 series of alloys.In these series a rapid heating system (e.g. fluidized beds) can be used. They are generally precluded because of high capital cost or the adverse effects on surface finish. Air furnaces are, therefore, traditional.

Aging ovens consist of a heat-energy source - electrical or com­bustion of fossil fuels. In the latter, combustion products must be a

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part of the furnace atmosphere when clean fuels are employed, or may be directed through a heat exchanger when the combustion products might contaminate the furnace load.

Heat transfer energy source to load is facilitated by forced air. Circulation may be cross-flow or longitudinal flow.

Cross-flow furnaces are more expensive, employing several in. along the oven length, permitting more uniform longitudinal tempera­ture distribution. This type of oven is particularly suited to electric-resistance heating with heating elements mounted along the furnace length between the inner and outer shell.

Longitudinal flow furnaces are less costly, have a single fan with less complex ducting. They do, however, result in large longitudinal thermal gradients during the early aging stages.

Furnace Landing

Heat transfer is accomplished, primarily, by conductivity con­trolled by surface area exposed to hot air and air velocity.

To ensure maximum exposure of the load to hot air, material should be spaced to create air passages permitting maximum contact between extrusion surface and air. Remove, if possible, all continuous obstructions perpendicular to air flow.

Also, see that the load occupies as much of the furnace space as possible perpendicular to the air flow. Otherwise, the air will take the line of least resistance and skip over rather than be forced through the passage among extrusions.

Conduction of heat from air to metal is impeded by a boundary film, or transition layer. Increasing air turbulence decreases the film thickness, thus, reducing resistance to heat transfer. Two general statements provide guidance:

• for a given air velocity, turbulence increases as the air passage decreases

• for a given size and configuration of the air passage, turbu­lence increases with increased air velocity.

Thus, the most efficient heat transfer is achieved by forcing high-velocity air through passages permitting contact with the maxi­mum perimeter of the load extrusions. This condition is more readily achieved in a longitudinal flow aging oven.

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Since the size and configuration of air passages affect hear trans­fer, the heating patterns will vary from load to load depending upon the configuration of the extrusions and loading arrangements.

Aging Practices

A spectrum of practices can produce the desired level of me­chanical properties. Selection of the «best» practice depends upon con­trol exercised over the rate of heat input into all parts of the aging load.

Thermal gradients and differences in heating rates result in variation of the rate of aging of different parts of a load. Incorrect se­lection of aging practice will result in unacceptable variation in me­chanical properties data on the expected time delay between the fastest and slowest heating portion of the load reaching aging temperature which assists a practitioner in selecting the optimum practice.

At. any given aging temperature, there is a range of time during which properties are at or close to the peak. The duration of this prop­erty plateau increases with decrease in aging temperature.

Matching the time span of the property plateau with the ex­pected time delay between the fastest and slowest heating portion of the load permits selection of the highest aging temperature, compatible with the achievement of satisfactory mechanical properties. This repre­sents the temperature at which maximum throughput may be achieved from the furnace. Lower aging temperatures can also be employed giv­ing a safety margin at the expense of longer cycle times.

With aging temperature selected various operating strategies are available.

• Operate on a fixed time cycle. This permits the accurate scheduling of furnace loads. The rate at which the furnace recovers to aging temperature is variable, and the use of this practice in conjunc­tion with maximum aging temperature can lead to underaging or over-aging the parts of some loads. Thus, this practice is best employed in conjunction with lower than maximum temperature, with the cycle time adjusted to compensate for the slower aging response.

• Establish practices based on ingoing air temperature. Nor­mally, aging ovens are equipped with a thermocouple in the ingoing air stream. The temperature of the hottest part of the load lags behind the air temperature, so that the hottest part of the load reaches aging tem-

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perature approximately five minutes after the ingoing air has reached this level. The soak time at temperature can be calculated by adding the minimum time to reach the property plateau to the expected delay be­tween the fastest and coolest part of the furnace reaching temperature.

- Establish time from a load couple placed in the part of the load that will take the longest time to reach temperature. This is the safest practice since it does not depend on estimating temperature gra­dients within the furnace. In this case, aging practice is determined by holding temperature for some period after the load couple has reached aging temperature. Monitoring both ingoing air temperature and tem­perature at the lowest heating part of the load, there is an automatic check that temperature gradients do not exceed expectations and, thereby, act to signal problems if the time delay is excessive.

The mechanical property level attained by precipitation aging depends also on the delay between solution treatment and the precipita­tion process. This effect is particularly marked in press-quenched al­loys in which the mechanical properties are depressed by the order of 5,000 psi (34.5 MPa). The effect is not significant with the 6000 series of alloys air-cooled at the press. These observations support the con­clusions that solute diffusion is enhanced by the high density of vacan­cies quenched-in from solution temperature. As clustering takes place during storage, the supersaturation of solute remaining in solution is diminished. When supersaturation is further reduced as a result of rais­ing temperature to the precipitation aging temperature, the size of the stable nucleus is raised.The longer such material is stored at room tem­perature, the fewer will be the number of stable nuclei and the coarser the precipitate structure.

Air cooling at the press traditional with AA6063 results in fewer quenchedin vacancies. Thus, the mechanical properties of this alloy are affected less by delay between extrusion and aging.

Thermal Efficiency

Precipitation-hardening utilizes thermal energy to strengthen aluminium alloys. Only the energy input required to achieve the metal­lurgical changes is useful. The difference between total energy input and useful energy must be minimized. The following discussion analy­ses the sources of waste energy.

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Thermal efficiency can be divided into conversion efficiency and furnace efficiency. The former relates to the efficiency with which the energy source is converted to heat. Regular inspection and mainte­nance of heat sources by qualified personnel are essential to conserve energy. Furnace efficiency relates to the mode of consumption of the heat generated by elements, or burners, and is divided into three com­ponents.

• Useful energy, that is required to accomplish the metallurgical transformation.

• Skid losses, when heat is absorbed by furnace cars, racks, etc. Furnace cars and supports are usually manufactured from mild steel from economic considerations. Thus, specific heat of the material is fixed. The aging practice determines temperature rise. Therefore, to minimize energy waste, use the lightest structure that will withstand the loads.

• Shell losses is the difference between heat generated by the energy source and energy consumption in raising the skip and the load from ambient temperature to the aging temperature. This energy is used in heating the duct work and inside the shell of the furnace.The losses from the outside wall of the furnace to the surroundings and air leaks take place. Heat losses from the shell to the atmosphere indicate inadequate wall insulation. It is readily detectable from wall tempera­ture during furnace operation. Air leaks, occurring most frequently around furnace doors, are also readily detected and corrected.

Common Problems

The most common problem is obtaining soft material. Fre­quently, this is due to deficiencies in prior operations unrelated to the aging/hardening process. Prime causes of low properties are inade­quate homogenizing practices holding billets, in a reheat furnace for ex­tended periods in a temperature range where rapid precipitation occurs, e.g. from 350° to 450 °C (662-842 °F), or slow cooling from extrusion temperature.

Low mechanical properties can occur as a result of underaging or overaging, even though, all prior practices are carried out correctly. Unsatisfactory aging practice occurs most frequently when based on either fixed-time cycle or where only ingoing air temperatures are monitored. Neither of these procedures give any indication of tempera-

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ture gradients within a load. Monitoring the fastest and slowest heating portions of die furnace and establishing maximum recovery time de­lays is a wise practice.

Time delay in temperature recovery in many aging ovens is ex­cessive, requiring relatively low-aging temperatures with consequent increase in soaking time to ensure that uniform properties are obtained. This is particularly true for end-flow ovens.

Inadequate heat source or, most frequently, low air-velocity, causes these problems. It is simple to distinguish between the two cases.

- If heat source is inadequate, temperature rise will be low and there will be a continual demand for energy when the air couple has reached operating temperature.

-If velocity is inadequate, the air will reach operating tempera­ture rapidly, energy demand will then drop off. Burner or electrical contacts will cut in for short periods to compensate for small heat losses from the furnace atmosphere, but the rate of heat input to the coolest part of the load will be slow.

To increase air velocity, fan speeds can be increased by chang­ing the sheave size on the fan, or the motor, provided that the fan can accommodate the increased speeds and mat the motor has sufficient power.

Summary

Thermal energy causes diffusion of solute atoms in a su­persaturated solid solution resulting in statistical fluctuations in a num­ber of solute atoms in the alloy matrix. Clustering occurs in areas of high density of solute atoms resulting in zone formation followed by precipitation of a stable nucleus. Precipitate growth occurs by diffusion causing annihilation of adjacent potential nuclei sites. Diffusion rate is temperature-dependent; therefore, as temperature increases the annihi­lation distance from stable nuclei increases.

It follows, therefore, that as precipitation treatment temperature increases,, the time to form stable nuclei decreases, and the distance between stable nuclei is larger.

Dislocation movement is impeded by zones, requiring energy to cut through the clusters, thus, mechanical strength increases during

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cluster formation. Dislocations bend round stable precipitates; the en­ergy required being a function of the radius of curvature, therefore, in­creases as the distance between precipitates decreases.

Those mechanical properties will rise during cluster and coher­ent particle formation and then, decrease as separate precipitates occur. Time to reach peak properties decreases with increase in aging tem­perature. The duration of peak properties is also reduced, and the rate of property decline accelerates with increase in temperature.

Selection of optimum aging practice requires recognition of temperature gradients within the furnace load. The time temperature combination is chosen to ensure that all parts of the load are at peak properties.

Temperature gradients are reduced by:

• air passages, with minimum obstruction to each face of the ex­trusion in the load

• filling the cross section of the furnace perpendicular to the air­flow direction to eliminate large channels with low resistance to air flow.

• high air velocity.

Thermal efficiency is achieved by regular maintenance of heat source minimizing the weight of the load skip.

TENSILE STRENGTH INFORMATION AS A CRITERION FOR SELECTING FERROUS METAL POWDERS