The Hot Metal Ezine

The slave cylinder project evolved from the need to create a replacement slave cylinder for a 1960s English car in which spares were no longer available. The original slave cylinders were originally pressure die cast by the thousands, but we were faced with the challenge to cast them in green sand in small batches of twenty at a time. The split patterns were constructed from jelutang timber and mounted on a pattern board and carefully matched on both sides for perfect register of the pattern halves, (See image 03) then the gates and runners were set up along with the sprue and feeders. Shellac was used to seal the patterns, and several coats were applied, a light burnish with steel wool between each coat was given. The patterns were then waxed and a good rubbing of graphite powder was applied to make the patterns super smooth, and easier to part from the green sand moulds.

At the time, the castings were limited to three per mould box, but up to ten per box could be done as long as you had the capacity to mould and pour. As you can see from the finished cylinder an internal bore is required, so a baked sand core (5/8" Dia) was used to create the hollow in each of the castings. (See image 01) The hollow allowed the boring/machining operation to proceed quickly with a minimum amount of waste metal. The core also meant that less feed metal was required for the castings. The sand cores were commercially made cores which are dirt cheap to buy, so cheap in fact that it is not worth the trouble to make them... but having said that we have made our own sand cores from time to time with success, but you cant beat a commercially made blown and baked sand core.

One of the biggest problems with this casting job was the painful process of ridding the metal of porosity, the first few cylinders made were so porous that when they were pressure tested the fluid just oozed out through the metal every where, three things were required; A lot of work was required on the feeding of the castings, careful management of the moisture content of the green sand was needed, and the employment of steel chills imbedded in the green sand at the time of moulding was required. The extra feed metal ensured plenty of metal was available during the solidification; the green sand was made with lowest possible amount of moisture. And the coup'de gra was the decision to make a set of steel chills to help cool the metal as rapidly as possible after the pouring operation. The chills were placed along the barrel, and at the end of the casting in both the cope & drag moulds.

Another problem that reared its ugly head was the moisture that gathered on the steel chills for the duration of time before the moulds were poured, a quick fix for that was to smear the chills with engine oil and dip the chills in a tin of graphite powder and then carefully placed onto the pattern along the barrel or at the heavy end, green sand was carefully packed by hand so that the chills would not be dislodged during the ramming process.

The metal used to cast the cylinders was high quality scrap sourced from late model automotive cylinder heads; the usual fluxing & rigorous degassing procedures were carried out before the metal was poured.
The gates and runners (see image 02) were a very simple set up with runners and ingates set up in the cope mould and the cross runner set up in the drag mould and being fed by the sprue, two fairly large feeders were placed between each casting at the heavy end to ensure that the feed metal was as close as possible to avoid any freezing of feed metal. As you can see from the photographs the cylinders cast extremely well, and the reject rate was about 1 in 20, sometimes it is zero.
The gates, runners, feeders etc are removed after cooling, the burnt sand cores are removed and the castings prepared for machining operations. The finished cylinder is shown in image 04, which is ready to pack and despatch to customers.

Projects such as these can be personally & financially rewarding as long as you streamline the operation and you are confident that you can do the job, don’t take on any paying job if you feel you can't handle it.

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DMD Australia.

Welding of Aluminum Alloys.

Aluminum and its alloys can be joined by more methods than any other metal, but aluminum has several chemical and physical properties that need to be understood when using the various joining processes.

The specific properties that affect welding are its oxide characteristics, its thermal, electrical, and nonmagnetic characteristics, lack of color change when heated, and wide range of mechanical properties and melting temperatures that result from alloying with other metals.

Oxide. Aluminum oxide melts at about 2050 ^oC which is much higher than the melting point of the base alloy. If the oxide is not removed or displaced, the result is incomplete fusion. In some joining processes, chlorides and fluorides are used in order to remove the oxide contained. Chlorides and fluorides must be removed after the joining operation to avoid a possible corrosion problem in service.

Hydrogen Solubility. Hydrogen dissolves very rapidly in molten aluminum. However, hydrogen has almost no solubility in solid aluminum and it has been determined to be the primary cause of porosity in aluminum welds. High temperatures of the weld pool allow a large amount of hydrogen to be absorbed, and as the pool solidifies, the solubility of hydrogen is greatly reduced. Hydrogen that exceeds the effective solubility limit forms gas porosity, if it does not escape from the solidifying weld.

Electrical Conductivity. For arc welding, it is important that aluminum alloys possess high electrical conductivity -- pure aluminum has 62% that of pure copper. High electrical conductivity permits the use of long contact tubes guns, because resistance heating of the electrode does not occur, as is experienced with ferrous electrodes.

Thermal Characteristics. The thermal conductivity of aluminum is about 6 times that of steel. Although the melting temperature of aluminum alloys is substantially bellow that of ferrous alloys, higher heat inputs are required to weld aluminum because of its high specific heat.
High thermal conductivity makes aluminum very sensitive to fluctuations in heat input by the welding process.

Forms of Aluminum. Most forms of aluminum can be welded. All the wrought forms (sheet, plate, extrusions, forgings, rod, bar and impact extrusions), as well as sand and permanent mold castings, can be welded. Welding on conventional die-castings produces excessive porosity in both the weld and the base metal adjacent to the weld because of internal gas. Vacuum die-castings, however, have been welded with excellent results. Powder metallurgy (P/M) parts also may suffer from porosity during welding because of internal gas.
The alloy composition is a much more significant factor than the form in determining the weldability of an aluminum alloy.

Filler Alloy Selection Criteria

When choosing the optimum filler alloy, the application (end use) of the welded part and its desired performance must be prime considerations. Many alloys and alloy combinations can be joined using any one of several filler alloys, but only one filler may be optimal for a specific application.

The primary factors commonly considered when selecting a welding filler alloy are:

Ease of welding is the first consideration for most welding applications. In general, the non-heat-treatable aluminum alloys can be welded with a filler alloy of the same basic composition as the base alloy.

The heat-treatable aluminum alloys are somewhat more metallurgically complex and more sensitive to "hot short" cracking, which results from heat - affected zone (HAZ) liquidation during the welding operation. Generally, a dissimilar alloy filler having higher levels of solute (for example, copper or silicon) is used in this case.

Welding Processes

Production of aluminum products (all types of castings exclusive of ingots) has increased over the past 30 years at a fairly steady rate.

Aluminum casting alloys must contain, in addition to strengthening elements, sufficient amounts of eutectic-forming elements (usually silicon) in order to have adequate fluidity to feed the shrinkage that occurs in all but the simplest castings. Required amounts of eutectic formers depend in part on casting process. Alloys for sand casting generally are lower in eutectics than those for casting in metal molds, because sand molds can tolerate a degree of hot shortness that would lead to extensive cracking in non-yielding metal molds.

The range of cooling rates characteristic of the casting process being used controls to some extent the distribution of alloying and impurity elements. For example, the extremely high cooling rates inherent in die casting result in fine dispersion of strengthening and eutectic-forming constituents, and reasonably good castings can be obtained in spite of impurity contents that would render sand or plaster-mold castings unacceptable. However, with these minor exceptions, most aluminum foundry alloys can be cast by all processes, and choice of casting technique usually is controlled by factors other than alloy composition.

A large number of aluminum alloys has been developed for casting, but most of them are varieties of six basic types: aluminum-copper, aluminum-copper-silicon, aluminum-silicon, aluminum-magnesium, aluminum-zinc-magnesium and aluminum-tin alloys.

Aluminum-copper alloys that contain 4 to 5% Cu, with the usual impurities iron and silicon and sometimes with small amounts of magnesium, are heat-treatable and can reach quite high strength and ductility, especially if prepared from ingot containing less than 0.15% iron.

Manganese in small amounts also may be added, mainly to combine with the iron and silicon and reduce their embrittling effect. However, these alloys have poor castability and require very careful gating if sound castings are to be obtained. Such alloys are used mainly in sand casting; when they are cast in metal molds, silicon must be added to increase fluidity and curtail hot shortness, and this addition of silicon substantially reduces ductility.

AI-Cu alloys with somewhat higher copper contents (7 to 8%), formerly the most commonly used aluminum casting alloys, have steadily been replaced by AI-Cu-Si alloys and today are used to a very limited extent. The best attribute of these higher-copper Al-Cu alloys is their insensitivity to impurities, but they have very low strength and only fair castability. Also in limited use are AI-Cu alloys that contain 9 to 11 % Cu, whose high-temperature strength and wear resistance make them suitable for automotive pistons and cylinder blocks. These alloys usually contain manganese as an impurity because wrought metal scrap is used in preparing them. The manganese has little effect.

Very good high-temperature strength is an attribute of alloys containing copper, nickel and magnesium, sometimes with iron in place of part of the nickel.

Aluminum-copper-silicon alloys. The most widely used aluminum casting alloys are those that contain silicon together with copper. The amounts of both additions vary widely, so that the copper predominates in some alloys and the silicon in others. In these alloys, the copper contributes to strength, and the silicon improves castability and reduces hot shortness. Thus, the higher silicon alloys normally are used for more complex castings and for permanent mold and die casting processes, which cannot tolerate hot-short alloys.

Al-Cu-Si alloys with more than 3 to 4% Cu are heat treatable, but usually heat treatment is used only with those alloys that also contain magnesium, which enhances their response to heat treatment. Without magnesium, response is too slow for heat treatment to be economical.

High-silicon alloys (> 10% Si) have low thermal expansion, which makes them suitable for high-temperature operations. When silicon content exceeds 12 to 13% (silicon contents as high as 22% are typical), primary silicon crystals are present and, if properly distributed, cause excellent wear resistance. Automotive engine blocks and pistons are major uses of these alloys.

Aluminum-silicon alloys that do not contain copper additions are used when good castability and good corrosion resistance are needed. If high strength is also needed, magnesium additions make these alloys heat treatable.

Alloys with silicon contents as low as 2% have been used for casting, but silicon content usually is between 5 and 13%. Strength and ductility of these alloys, especially the ones with higher silicon, can be substantially improved by "modification".

Modification of the hypoeutectic alloys is particularly advantageous in sand castings, and can be effectively achieved through the addition of a controlled amount of sodium or strontium, which refines the silicon eutectic. Calcium and antimony additions are also used. Pseudomodification of sand castings, in which the size of the eutectic but not the structure is affected, may be achieved by solidification at high rates, such as occurs when chills are used. With permanent mold castings, modification of the eutectic also is advantageous, but the effect on properties is not as dramatic as with sand castings.

Aluminum-magnesium alloys. High corrosion resistance, especially to seawater and marine atmospheres, is the primary advantage of castings made of Al-Mg alloys. Best corrosion resistance requires low impurity content (both solid and gaseous), and thus alloys must be prepared from high-quality metals and handled with great care in the foundry. The relatively poor castability of Al-Mg alloys and the tendency of the magnesium to oxidize increase handling difficulties and, therefore, cost.

Aluminum-zinc-magnesium alloys have the ability to naturally age, achieving full strength at room temperature 20 to 30 days after casting. This strengthening process can be accelerated by furnace aging.

The high-temperature solution heat treatment and drastic quenching required by other alloys (Al-Cu and AI-Si-Mg alloys, for example) is not necessary for optimum properties in most Al-Zn-Mg alloy castings.

However, microsegregation of Mg-Zn phases can occur in these alloys, which reverses the accepted rule that faster solidification results in higher properties. When it is found in an Al-Zn-Mg alloy casting that the strength of the thin or highly chilled sections are lower than the thick or slowly cooled sections, the weaker sections can be strengthened to the required level by solution heat treatment and quenching, followed by natural or artificial (furnace) aging. Castability of Al-Zn-Mg alloys is poor, but they have good general corrosion resistance despite some susceptibility to stress corrosion.

Aluminum-tin alloys that contain about 6% Sn (and small amounts of copper and nickel for strengthening) are used for cast bearings because of the excellent lubricity imparted by tin. Bearing performance of Al-Sn alloys is strongly affected by casting method. Fine interdendritic distribution of tin, which is necessary for best bearing properties, requires small interdendritic spacing, and small spacing is obtained only with casting methods in which cooling is rapid.

Selection of Casting Alloy

Each casting process requires specific metal characteristics. For example, die and permanent mold casting generally require alloys with good fluidity and resistance to hot tearing, whereas these properties are less critical in sand, plaster and investment casting, where molds and cores offer less resistance to shrinkage. Discussions of required alloy characteristics, and lists of alloys commonly used, are presented for the various casting processes in the section that follows.

The application for which a casting is to be made affects alloy selection by establishing requirements for strength and ductility, as well as special service requirements such as pressure characteristics, corrosion resistance and surface treatments.

Economic considerations also may be important in alloy selection. Total cost of making a casting is affected by required heat treatment and by weldability and machinability, in addition to ingot and melting costs.

Full development of the potential of any casting alloy depends in large part on foundry technique. Foundry personnel should be consulted on alloy selection; use of alloys with which such personnel are familiar often results in better and more economical castings.

Selection of the proper alloy requires careful consideration of all the factors discussed above, which are presented in the brief outline that follows.

Zinc-Aluminum Foundry Alloys.

Foundry Practice

Corrosion resistance and machining

Foundry Project: Clutch Slave Cylinder.

Emachineshop.

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DMD Australia.

Welding of Aluminum Alloys.

Filler Alloy Selection Criteria

Welding Processes

Selection of Casting Alloy

Casting Processes.