The Australian forging industry can be divided into four main categories as follows:
1. Open die forging
2. Closed die forging of steel
3. Non-ferrous forging
4. Cold and warm forging
Open Die Forging.
This section of the industry may be classified as jobbing with quantities generally small varying from one off to perhaps 100 off. Tooling for producing open die work may consist of flat or shaped anvils and tubs supplemented by a variety of hand tools. Skill is required to produce a constant shape with reasonable dimensional accuracy such that the resultant forging will produce a satisfactory machined product without too much excess to be removed during the machining process.
The dimensional accuracy of open die work depends on the size of the part being made and it is difficult to lay down precise tolerances. This matter is best negotiated with the forge making the particular part.
Open Die Forging Equipment
The type of equipment used for open die forging in Australia varies from a 50 kilogram ‘C’ frame pneumatic hammer capable of producing forgings of approximately one to two kilograms in weight to a 5000 tonne forging press capable of forging large shafts weighing many tonnes.
Scope of the Australian Open Die Forging Industry
The majority of open die work consists of maintenance items for heavy machinery including large gear blanks. The industry does have a capability for forging non-ferrous metals, particularly aluminium alloys if required. Apart from forging presses and hammers the open die forging industry has a range of ancillary equipment such as bulldozers and ring rolling machines. The raw material used in the industry may vary from ingots to rolled rounds and billets.
Closed Die Steel Forgings.
As the name implies, closed die forgings are made in cavity dies which, when closed together, trap the metal being forged causing it to flow and fill the cavity shape. Excess metal is exited from the die in the form of flash which is subsequently removed by hot trimming after forging.
Die Life for Closed Die Steel Forging
Depending on the cavity shape, the type of preform being used, the type of metal being forged and a variety of other factors including lubrication, the die life in a closed die situation will vary. This variation may be a low as 1000 parts or as high as 50,000 parts. A figure of 5000-10,000 would be typical for more common types of closed die forgings such as hooks, turnbuckles and certain automotive components.
Ensuring Success with Closed Die Steel Forging
The success of the closed die process depends on a correctly designed preform or blocker shape to be presented to the finisher die. It is the design of these preforms which requires skill and experience on the part of the die designer.
Closed Die Forging Processes
Closed die forgings can be made under a hammer, or press or an upset forging machine.
In the case of a press, the preliminary forging operations usually include a minimum of three forging sequences in separate dies in the same set-up. These operations commonly include a flatten, block and finish utilising the same heat and progressing left to fight across the bottom tool holder or bolster of the press.
With hammer forging the various preform shapes and the finisher cavity are set out in the one die block. Usually with hammer work multiple hits are given in each cavity to produce the desired shapes. Unlike a forging press the hammer can be controlled via a foot pedal to give tight or hard blows. The application of a light blow to bend the metal into a given shape is advantageous and helps to improve die life.
Complex Forging Operations
In many instances with more complicated forgings, additional preforming operations may be carried out prior to hammer or press forging. These include reducer rolling, swaging or bending although loss of forging heat when carrying out this additional work may be a problem and reheating may be necessary.
The copper base alloys constitute the main volume category of non ferrous forgings, the majority of these components being consumed by the building industry in the form of plumbers brassware and, to a lesser extent, with door lock furniture.
Aluminium alloy forgings are not only produced by the conventional non ferrous forgers, with a large percentage being made by the closed die steel section of the industry.
There are many applications for Aluminium alloy forgings including compressor blading for jet engines and automotive componentry where in many cases forgings have replaced die castings due to problems associated with porosity in hydraulic applications.
Titanium and Nickel Forgings
Included in the non ferrous section of the industry are special materials such as titanium alloys and the nickel base alloys used in high temperature applications. These materials require high forging pressures and are forged where larger capacity forging equipment is available.
Production of Copper and Aluminium Forgings
Most copper base and aluminium forgings produced by the non ferrous forgers are produced by single die technique utilising one or two blows in a finisher die. Screw presses are most commonly used with the majority of presses being around 200 tonnes. By contrast, one member of the closed die ferrous forgers operates a 1600 tonne screw press which, apart from steel forgings, produces aluminium forgings for the automotive industry.
Production of Brass Forgings
Most brass forgings are made by the trapped die techniques whereby the metal to be forged is accurately cut to weight and forged in a die which is completely closed by the advancing die or punch before forging commences. Hence the term, trapped die. As the punch enters the bottom die, contact pressure in the slug increases and the metal is displaced to the extremities of the die to produce a forging. Virtually no flash is produced with the technique other than some minor protrusions at split lines. The volume of metal being forged is critical as excess material will cause the press to stall or fracture the dies. With brass forgings, manufacture of cored parts such as tees or elbows is made possible by the use of ancillary core rods activated from the ram of the press. With non ferrous metals maintenance of forging temperature is critical to the process and for this reason, tooling must be adequately preheated and forging temperatures maintained at a consistent figure. Ideally these materials should be forged in one hit or in one die with several hits. In the case of larger aluminium forgings this is not possible and multiple dies must be used. In these instances, accurate heating of slugs and tooting is vitally important.
Heat Treatment of Brass and Copper Forgings
Heat treatment of brass forgings or copper base alloys in general may consist of annealing to promote structural uniformity. In the case of plumbers brassware, water quenching from the forging temperature has been successful in reducing dezincification in subsequent service.
Heat Treatment of Aluminium Forgings
The heat treatment of most aluminium forgings consists of solution treatment and precipitation hardening to the T6 condition. In order to produce optimum mechanical properties, precise control of heat treatment temperatures is critical and most producers of aluminium forgings possess in-house treatment facilities.
Raw Materials for Aluminium and Copper Forging
Raw material used for aluminium and copper base forgings is usually extruded bar stock which is cut to length by sawing as these materials are not generally amenable to cropping.
Due to the high cost of extruded forging stock, most forging companies have investigated the use of cast bar as a less costly alternative. In many instances this has been forged quite successfully with the resulting forging possessing properties similar to those produced from extruded bar.
Trimming and Coining of Aluminium and Copper Base Alloys
It should be pointed out that the aluminium and copper base alloys are generally trimmed cold as hot trimming after forging will cause flash line tearing.
Both of these materials are amenable to cold coining and with aluminium alloys, coining after solution heat treatment and before ageing is practised to take advantage of the excellent formability in this condition.
Cold and Warm Steel Forging
Cold and warm forging techniques are used to produce forgings that are on size and require little or no machining. Often these forgings can be produced economically by conventional machining methods.
Differences between Cold and Warm Forging and Hot Forging
In both cold and warm forging, heavier presses are required for forging than for conventional hot forging. Certain parts are made by a combination of warm and cold forging, a typical example would be gear forgings with on size forged gear teeth.
As the name implies, warm forging is carried out in the range 650-850°C. Trapped die forging techniques are used with constant volume raw materials which may or may not be machined to size prior to forging. Accurate die manufacture and tooling alignment is essential.
Depending on steel composition, material to be cold forged is usually spheroidise-annealed to obtain the most suitable microstructural condition for cold forging. After annealing, the slugs are shot blasted, pickled, phosphate coated and soaped to promote maximum lubricity and metal flow. Accurate control of cut piece weights to within fine tolerances is essential for the success of the process. Most operations are carried out in trapped dies usually on high speed machines. Forgings are most commonly produced by both forward and backward extrusion techniques.
Tooling and Properties of Cold Forged Materials
Due to the high tooling loads in cold forging, the tooling is manufactured using a series of compression rings to support the working parts of the dies. Forgings produced by cold forging have higher tensile properties resulting from the cold work but subsequent heat treatment of cold forged parts may eliminate this advantage.