Forging Aluminium Alloys

Kitmondo 02 May 2014

This article first appeared in Practical Engineering 1940 Vol1 No18. The information within this article is therefore correct as of 1940. The publication attempts to provide historical insight into methods employed in Engineering during this period.

Forging when referring to aluminium, is the operation of rapidly deforming a bar or billet above the recrystallization temperature within the soft range of the metal by hammering or pressing, to result in a semi-manufactured product of a shape quite close to that of the finished product. In this operation both the form and the section are affected, in contrast to stamping, in which the section only is modified.

Aluminium Alloys and Steel Compared

Forging by hammering or pressing is mainly restricted to structural metals, such as steels and strong aluminium alloys, whereas stamping and upsetting is used for most industrial metals, iron, alloy steel, brass, bronze, copper, aluminium, etc. The forging of aluminium alloys does not differ substantially from that of steel in so far as it takes place at an elevated temperature using similar forging equipment. However, the temperature at which aluminium alloys are to be forged is appreciably lower, the temperature range narrower, and the energy required greater than for common steels. At the optimum forging temperature the resistance to deformation of the most common aluminium forging alloy (of the dural type) is about twice that required for soft steel and 1.5 that for nickel-chromium steel.

In other respects of secondary importance aluminium alloys differ from steel: the heating rate for aluminium alloys does not need to be especially slow, as these alloys have not to pass through a dangerous heating zone, as is the case with steel. Prolonged heating times are not objectionable for aluminium alloys, whereas for steel, to avoid scaling, the heating time should be kept to a minimum just long enough to bring the centre of the billet to the forging temperature. Scaling, the troublesome problem in steel since it leads to inclusion, machining difficulties, and sticking on the dies, does not take place with aluminium alloys, as at the forging temperature these alloys remain bright and are practically unaffected.

The finishing forging temperature in aluminium alloys is less critical. It rarely happens that this temperature is kept too high, and normalising is not necessary should too high a finishing temperature have been used; while for steel, too high a finishing forging temperature is not always compatible with a fine-grained structure throughout the forging. The danger of internal rupture originating from slow cooling in large forgings does not seem to be such a problem in aluminium alloys, where the forging temperatures are appreciably lower.

The Metallurgical Factors

A drastic heat-treatment, objectionable for large steel forgings, is standard for aluminium alloys. Cold straightening operations,which can hardly be performed on steel after forging, are practicable, and frequently resorted to in the case of aluminium alloy forgings.

It has been stated that steel forging is 90 per cent, mechanical and only 10 per cent, metallurgical, while in aluminium alloys the proportions are reversed. The metallurgical factors ( which control the forging of aluminium alloys are : (1) Pre-heating; (2) grain refining in roughing; (3) forging temperature range ; (4) heat-treatment; (5) straightening after quenching. The pre-heating time should be sufficiently prolonged to allow for the slow heating rate of aluminium alloys, as insufficient preheating may be responsible for cracks. Over-heating of aluminium alloys is of a different nature from that in steel, and is less to be- feared, as aluminium alloys require an upper forging temperature well below the burning point—i.e., the temperature at which partial melting occurs. Such a partial melting, which would result in a number of small cracks, would take place only if the alloy were accidentally heated to some 50 deg. or 100 deg. C. above the upper forging temperature. The pre-heating medium for aluminium alloys is either air or salt baths. The first, most commonly used, may lead to greater oxidation of the surface of some aluminium alloys, whereas salt bath heating is more likely to cause blisters than the air furnace.

The grain refining by preliminary forging of aluminium alloys should be more pronounced than for steel in order to reduce preferential direction of properties and to result in the highest possible resistance to alternating stresses. The upper forging temperature in aluminium alloys is not limited by scaling, but rather by hot shortness, as these alloys all possess a brittle range below the point of complete solidification; the upper forging temperature should thus be kept reasonably below the critical temperature of this brittleness. However, no hard-and-fast rule can be laid down, and the optimum upper forging temperature is better arrived at by trial. The die should be kept heated above 100 deg. C. in order to prevent any loss of heat when the fight metal billet comes in contact with the die. This heating is all the more important, as aluminium alloys are to be forged at relatively low temperatures and within narrow limits. The finishing forging temperature in the case of aluminium alloys is to be kept within the critical range of the alloy to produce a fine grain and thus enable the forging to respond readily to heat-treatment. This finishing temperature is limited simply by the stress to which the equipment and die may be submitted with safety.

The solution treatment temperature will vary with the type of 31105/ and the size of the forging, and is usually to be adhered to within 5 deg. C. The quenching medium may be hot or cold water, depending upon whether the best resistance to corrosion or the minimum amount of internal stresses is to be obtained. Quenching medium at room temperature affords the best resistance to corrosion, and is preferable for small forgings; hot water, giving lower internal stresses, is preferable for larger or complicated forgings. Immediately after quenching, aluminium alloys are relatively soft, and any straightening operation may be performed before further hardening takes place. The ageing time and temperature depend upon the particular alloy, since some aluminium alloys used for forging harden spontaneously at room temperature after a few hours and are characterised by a low-proof stress to tensile strength ratio, while others receive an additional heat-treatment at between 150 deg. to 200 deg. C, and are characterised by higher elastic properties.

Forging Operations

The sequence of operations in hammer and press forging aluminium alloys is roughly as follows:

(1) Pre-heating the bar or billet to the upper limit of the forging temperature. (2) Roughing by means of hammering, pressing, or extruding to break down the dendritic cast Structure, to reduce the grain size and to ensure a uniform fine structure in all directions of the bar or billet. This roughing requires more care and experience for aluminium alloys than for steel, in view of their reduced plasticity. (3) Preliminary shaping by hammering, pressing or rolling, resulting in a shape closer to that of the semi-manufactured product. This operation may be performed in several stages. (4) Die-forging by hammering, pressing or upsetting between a set of dies to produce a shape as close as possible to that of the finished product. This operation also may be achieved in successive stages. (5) Trimming to eliminate the flash, i.e., the cast metal forced out between the die in completely filling the impression. This trimming is done either hot or cold, depending upon the forging- shop facilities, as aluminium alloys are not likely to fracture if cold-trimmed. (6) Heat-treating, and, if necessary, straightening. (7) Finishing by sandblasting, caustic dipping, tumbling, anodising or similar operations.

Forging Technique

For forging aluminium alloys the following rules should be adhered to:

(1) Billets with uniform grain size throughout should be used for forging material, as differences in grain size may lead to cracks. (2) The breaking down should be performed preferably by rolling or extruding. This roughing should be sufficient to break down completely the cast structure. (3) The die impression must have a polished surface to prevent sticking of the alloy. (4) The die radii must be more liberal than for steel, to promote an easy flow of the light metal. (5) Die shrinkage must be about |in. per foot as is usual for steel, since although aluminium alloys have a higher coefficient of  expansion than steel, the forging temperature is kept appreciably lower. (6) The draft must be slightly larger than for steel, in order to facilitate the extraction of the forging from the die; a draft of about 10 deg. or even 15 deg. must be allowed for average and large-size forgings, whereas 7 deg. may be satisfactory for small forgings.

In view of the narrow range of forging temperatures, the die must be accurately pre-heated to avoid any unnecessary cooling of the stock on the die. Both dies should be abundantly lubricated with thick oil, although some forging shops are working without any lubricants.

Equipment and Dies

The equipment used for the forging of aluminium alloys consists of the hammer, the press, and the upsetting machine. The hammering differs from the pressing, in that pressure is applied by means of successive short blows and is relieved before the metal has fully yielded, whereas the press acts by applying a static pressure on two sides of the billet. The upsetting machine applies a static pressure on the billets in three directions. The means by which the hammer is lifted is used to designate the various type of hammer ; the steam hammer operates by means of steam, whereas the air hammer operates by means of compressed air, and in the board hammer the hammer is lifted by means of a board passing between two rollers and placed at the top of the hammer. The press is either hydraulic or mechanical, depending upon the way it is operated. The upsetting machine in the case of aluminium alloys is used only by a limited extent, and mainly for pistons.

The die requirements for aluminium alloys differ as to whether forging is done by hammering or pressing, since in the case of hammer forgings the die has to resist impact, whereas in pressing, the aluminium alloy flows gradually as a plastic mass, having more time to feed the die cavity. This is advantageous, not only for the metal to be forged, but also for the die itself, and has permitted in the recent past the adoption of magnesium dies. The advantage of this material for dies is to be found in low material and machining costs, and in easier handling at the forging shop.

The Main Requirements

The gradual demand for aluminium alloy forgings permitted a parallel development of the aluminium forging alloys and aluminium forging technique. The first alloys developed and still in use were copper-bearing alloys, of the dural type. For the production of intricate forgings, maximum plasticity under the hammer led to the adoption of low-alloyed metals. The requirements for tough forgings resistant to high-temperature exposure introduced special alloys of the Y-alloy and Lo-Ex type for cylinder heads and pistons. Recently aluminium forgings have found increasing favour in food and chemical industries, where corrosion-resisting alloys of the magnesium silicide type have been introduced.

The main requirements for aluminium forging alloys may be enumerated as follows:

(1) plasticity at the working temperature; (2) wide temperature range for forging; (3) amenability to heat treatment; (4) high resistance to various forms of stresses ; (5) stability of dimensions; (6) good resistance to corrosion. The secondary requirements are : (1) easy machining; (2) strength at elevated temperatures; (3) adaptability to various finishing operations.

No alurninium alloy complies with all these requirements, and this explains why at present the five various types of aluminium alloys or their derivatives are still in use for the production of forgings.

The importance of fibre flow is even greater in alurninium alloys than in other metals, and special attention should be given to this factor in preparing the die design and checking the flow of the fibre on trial forgings. The adoption of the upsetting machine in the production of some aluminium alloy forgings is mainly due to the necessity of achieving the best fibre flow.

The present forging equipment for aluminium alloys compares already in size with that used for steel, and this illustrates how far the forging technique for aluminium alloys has been developed within the past two decades.

[Acknowledgement is made to the Aluminium Union, Ltd., for the information contained in this article, and for the loan of illustration.']