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V. M. KEVORKIJAN: PRODUCTION TECHNOLOGY AND USE OF ALUMINUM ...

PRODUCTION TECHNOLOGY AND USE OFALUMINUM DROP FORGING COMPONENTS

PROIZVODNJA IN UPORABA IZKOVKOV IZ Al-ZLITIN

Varu`an M. Kevorkijanzasebni raziskovalec, Lackova 139, 2341 Limbu{, Slovenija

[email protected]

Prejem rokopisa - received: 2000-08-02; sprejem za objavo - accepted for publication: 2000-09-09

In this report the production and use of forged aluminum parts are described and the automotive and other principal markets forforged aluminum components are outlined.

Key words: drop forging, lightening, aluminum alloys, forged automotive components

V delu opisujemo proizvodnjo in podro~ja uporabe aluminijastih odkovkov s poudarkom na njihovi uporabi v avtomobilskiindustriji.

Klju~ne besede: vro~e utopno kovanje, avtomobilske komponente, aluminijeve zlitine, izkovki iz aluminija, avtomobilskaindustrija

1 INTRODUCTION

In attempting to achieve the "three-litre" fuel-consumption target some leading European car makersrecently introduced the "think light" concept based onall-aluminum body and the replacement of steel in manysuspension parts by aluminum.

Global market demands and the growing number ofcar makers who are recognizing aluminum’s potential asa realistic and cost effective alternative to steel forgings,have signaled the start to this kind of business inSlovenia- an associate member of the European Union.

According to the Forging Industry Association"forging is a manufacturing process where metal ispressed, pounded or squeezed under great pressure intohigh-strength parts known as forgings. The process isnormally (but not always) performed hot by preheatingthe metal to a desired temperature before it is worked. Itis important to note that the forging process is entirelydifferent from the casting (or foundry) process, as themetal used to make forged parts is never melted andpoured (as in the casting process). The forging processcan create parts that are stronger than thosemanufactured by any other metalworking process. Thisis why forgings are almost always used where reliabilityand human safety are critical".

The combination of lightness, strength andformability make aluminum the ideal material for anytransport application. The initial investment in energy isrepaid many times over in fuel savings andenvironmental improvements during the life of mostvehicles.

Customers of forged aluminum parts operate innumerous sectors. From the automotive and motorcycle

industries to the hydraulic-pneumatic industry andothers. However, they all have one thing in common: toremain competitive they strive for ever improvingperformance.

These customers are very demanding and they areselective in their choice of suppliers. After all, thesesuppliers have to make a clear contribution to theirultimate performance and must have the will toconstantly improve their performance.

Today the opportunities for aluminum in the car arecentered around the body, the wheels, the suspension anditems of equipment. The suspension componentsrepresent the most important market segment foraluminum forgings in modern cars. The automotiveindustry is one of the main users of forged components,closely followed by the machine tool and apparatusindustry, as indicated in Table 1, which indicates theconsumption of aluminum forgings in Germany’sautomotive sector - one of the most important consumersof aluminum forgings in the European automobileindustry.

Table 1: Market for forged aluminum components in Germany(excluding wheels) 1

Tabela 1: Povpra{evanje po izkovkih iz aluminija v Nem~iji (brezplati{~) 1

Year 1994 1995 1996 1997 1998Tonnage 8884 9166 9902 11603 14283Transport industry(%)

47 45 55 64 67

Mechanical and equip-ment engineering (%)

22 24 18 13 13

The automobile industry forecasts an increase of over300% in forged-aluminum components in cars2. In

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UDK 669.715:621.73.043:629.113 ISSN 1580-2949Strokovni ~lanek MATER. TEHNOL. 35(3-4)191(2001)

comparison with steel, however, forged-aluminumcomponents represent a niche market which, even afterachieving all growth forecasts, will retain a nichecharacter. The increasing demands for aluminum in carsare mostly initiated by the necessity to make moderncars lighter and more environmentally friendly. But thereare also safety, technical, commercial and other reasonswhich are all very influential on increasing theproportion of aluminum forgings in current and futuregenerations of passenger cars and other vehicles.

2 HOT FORGING OF ALUMINUM

The technology applied is a conventionaldrop-forging process with a closed die (hot forging)performed on both screw and mechanical presses. Theselected aluminum alloy is plastically deformed in thehot but solid condition in a shaped die until it takes onthe shape of the die cavity. The starting material is mostoften extruded aluminum bars, but in some cases castbillets are also used for production. During themanufacturing process a work-piece is produced whichis characterized by a homogeneous structure completelyfree of pores and blowholes.

The production process is organized into thefollowing production areas: raw material inspection andstorage; cutting; forging; trimming; heat treatment;chemical and/or mechanical surface cleaning and finalcontrol and delivery. The raw material area consists ofmetal containers piled up in bundles. The mainaluminum alloys used in production are: 6080, 6060,2014, 2017 and 7075.

For cutting the raw material and preparation of theforging stock, different band- and disk- sawing machinesare used. According to 3, using a circular or band sawwith carbide-tipped blades is the fastest and generally themost satisfactory method. Sawing, however, producessharp edges or burrs that may initiate effects when thestock is forged in closed dies. Burrs and sharp edges areusually removed by a radiusing machine. State-of-the-artsaws for cutting aluminum alloys are highly automatedand frequently have an automatic radiusing capabilityand control systems that permit very precise control ofeither stock length or stock volume and therefore stockweight.

Forging is performed on different forging linesconsisting of screw and mechanical presses all equippedwith tunnel furnaces for material preheating. The metaltemperature is a critical element in the aluminum forgingprocess and careful control of temperature duringpreheating is important. The heating equipment haspyrometric controls that can maintain the temperature towithin ±5 °C. The tunnel furnaces have three zones:preheat, high heat, and discharge. All furnaces areequipped with recording/controlling instruments and arefrequently surveyed for temperature uniformity in a

manner similar to that used for solution- treatment andaging furnaces.

Heated aluminum-alloy billets are temperaturechecked using contact pyrometry. The typical capacityfor preheating of aluminum billets is 200-400 kg/h.

The heating time for aluminum alloys variesdepending on the section thickness of the stock and thefurnace capabilities. Generally, 4 to 8 min per 10-2m ofsection thickness is sufficient to ensure that thealuminum alloys have reached the desired temperature ofpreheating.

The heating of dies is the second critical element inthe aluminum-forging process. The die temperature usedfor the closed-die forging of aluminum alloys variesfrom 150 to 260 °C depending on the alloy being forged.On-press die-heating systems are based on gas-firedburners.

Die lubricant is the third critical element in thealuminum forging process 3. The lubricants used inaluminum alloy forging are subject to severe servicedemands. They must be capable of modifying the surfaceof the die to achieve the desired reduction in friction,withstand the high die and metal temperatures andpressures employed, and yet leave the forging surfacesand forging geometry unaffected. Lubricant formulationsare typically highly proprietary and are developed eitherby the lubricant manufacturers or by the forgersthemselves. Lubricant composition varies with thedemands of the forging process and the forging type. Themajor active element in aluminum-alloy forginglubricants is graphite; however, other organic andinorganic compounds are added to colloidal suspensionsin order to achieve the desired results. Carriers foraluminum-alloy forging lubricants vary from mineralspirits to mineral oils and water.

Lubricant application is typically achieved byspraying the lubricant onto the dies while the latter areassembled in the press and just prior to forging. Apressurized-air or airless spraying system is usuallyemployed.

Trimming presses are used for cold and exceptionallyhot trimming operations.

The heat-treatment equipment comprises of the mainelectrical furnace for solution annealing and electricalfurnaces for aging. All aluminum-alloy forgings, except1xxx, 3xxx, and 5xxx series alloys, are heat treated withsolution treatment, quench, and artificial aging processesin order to achieve the final mechanical properties.Because of the shape complexity of aluminum forgings,quench-racking procedures are particularly critical forobtaining the the uniform quench necessary to achievethe required mechanical properties and to minimizedistortion. Furthermore, quenching techniques foraluminum-alloy forging are also critical because of theirconfiguration and often widely varying cross-sectionalthicknesses within the same forging. Depending on thespecific aluminum alloy being processed, quench


192 MATERIALI IN TEHNOLOGIJE 35 (2001) 3-4

techniques for forgings include controlled-temperaturewater from 20 to 100 °C and synthetic quenchants, suchas polyalkylene glycol and others, designed to achievethe required quench rate in order to obtain the requiredmechanical properties without excessive distortion.State-of-the-art aluminum-forging solution-treatmentand aging furnaces have multiple control/recordingsystems, microprocessor furnace control and operationsystems, and quench monitoring and recording systems,that provide very precise control and repeatability of theheat treatment process.

A pickling line with several reaction vats in aprotective cabin is used for chemical etching of thesurface of forged parts. After chemical etching withcaustic soda and nitric acid the surface recovers its shinysilver aluminum appearance. Purely technical parts canthen routinely be made to look decorative. Any damagedparts can be separated out after etching by using a dyepenetrant or better by visual inspection in the finalcontrol area.

Aluminum-alloy forgings are usually cleaned as soonas possible after being forged. The following treatment isa standard cleaning process that removes lubricantresidue and leaves a good surface with a naturalaluminum color:

• Etch in a 4 to 8% (by weight) aqueous solution ofcaustic soda at 70 °C for 0.5 to 5 min;

• Rinse immediately in hot water at 75 °C or higher for0.5 to 5 min;

• Desmut by immersion in a 10% (by volume) aqueoussolution of nitric acid at 88 °C minimum;

• Rinse in hot water.With this procedure the values specified in DIN 1749

can be achieved by drop-forging and subsequent heattreatment with absolute certainty. Moreover, qualifiedmanufacturers declare that the strength values in DINcan be consistently exceeded by 30% and themanufacturing tolerances can be up to 100% less 4.

3 FROM AN IDEA TO THE FINAL FORGEDPRODUCT

Starting with the customer request, specialistsworking hand-in-hand with the customer prepare the besttechnical and commercial offer.

The technical offer is based on well-knownforgeability facts valid for aluminum alloys 3; theexperience accumulated in the technical office; 3-Dmodelling (CAD/CAM); simulation calculations of thefilling and solidifying processes in the dies andinnovative technology that results in high quality at acompetitive cost.

The main purpose of all of the above-listed efforts isto define the optimal forgeable design of a future forgedpart which is closely matched to the customer’s needs atthe minimum requested cost. This is the art of aluminumforging.

For the closed-die forging of aluminum alloys, diematerial selection, die design, and manufacturing arecritical elements in the overall aluminum forgingprocess. Dies are a major element in the final cost ofsuch forgings but less than the cost of the aluminumalloy which is the dominant cost in the breakdown of thetotal cost of production.

A key element in the cost control of dies foraluminum forging and in the successful fabrication ofaluminum-alloy forgings is die design and die systemengineering. Moreover, the design of aluminum forgingdies is highly intensive in engineering skills and is basedupon extensive empirical knowledge and experience.Because of this, the advent of computer-aided design(CAD) hardware and software has had an extensiveimpact on aluminum-alloy die design.

Most aluminum-alloy forging dies for currentindustrial needs are produced by CAM-driven CNCdirect sinking with very close tolerances (e.g. ±0.07 mmor less).

Once the offered forgeable design is fully confirmedby the customer, the rest is the routine production offorging.

4 QUALITY CONTROL

The inspection of aluminum-alloy forgings takes twoforms: in-process inspection and final inspection.In-process inspection, using such techniques as statisticalprocess control and/or statistical quality control, is usedto determine if the product being manufactured meetscritical characteristics and that the forging process isunder control. Final inspection, including mechanicalproperty testing, is used to verify that the completedforging product conforms with all drawings andspecification criteria. Typical final inspection proceduresused for aluminum-alloy forgings include dimensionaland heat-treatment verification and non-destructiveevaluation.

All final forgings are subject to dimensionalverification. For closed-die forgings, conformance of thedie cavities to the drawing requirements, a criticalelement in dimensional control, is accomplished prior toplacing the dies in service by using layout inspection ofplaster or plastic casts of the cavities. With theavailability of a CAD data base on forgings, such layoutinspections can be accomplished more expediently withCAM-driven equipment, such as coordinate-measuringmachines or other automated inspection techniques.With verification of the die-cavity dimensions prior touse, the final part of the dimensional inspection may belimited to verifying the critical dimensions controlled bythe process (such as die closure) and monitoring thechanges in the die cavity.

Proper heat treatment of aluminum alloy forgings isverified by hardness measurements. In addition to theseinspections, mechanical property tests are conducted on


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forgings to verify conformance to specifications.Mechanical property tests vary from destruction offorgings to testing of extensions and/or prolongationsforged integrally with the parts.

Finally, aluminum alloy forgings are frequentlysubject to non-destructive evaluation to verify surface orinternal quality.

To arrive at a perfect finished product engineers needto take care during the process from the very beginning.The functional requirements and, above all, the dynamicstresses to which the product is exposed are the startingpoint for well-conceived design and a choice of alloywhich guarantees the required structural and mechanicalcharacteristics.

5 THE FUTURE PROSPECTS OF ALUMINUMDROP FORGED COMPONENTS PRODUCED INSLOVENIA

All around the world, automotive producers willcontinue to "think more and more light" demanding

highly competitive light metal forgings for futuregenerations of passenger cars. To be capable to "thinklight and competitive" in automotive forgings for theglobal satisfaction of customers, one needs at least threethings - knowledge for thinking, aluminum for forgingand technology for running. And in Slovenia wealreadyhave all of them in-house!

6 REFERENCES

1 R. Leiber, Aluminium, 75 (1999) 10, 8932 R. Leiber, Aluminum-Praxis, (1998) 2, 33 Metals Handbook, Vol. 14, Forming and Forging, 9. Ed., ASMInternational, Metals Park, Ohio, 1988, 241

4 Sistermans, H. D., Neuere Entwicklungen in der Massivumformung,IFU, Stuttgart, 1991, 357


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