171 / 1 Industrial Application of Thixoforming at SAG Josef Wöhrer

Industrial Application of Thixoforming at SAG

Josef Wöhrer and Andreas Kraly

Salzburger Aluminium AG in Lend (Austria)

Abstract SAG is a producer of thixoforming raw material for worldwide use at different producers of Semi Solid Metalforming serial parts. The SAG subsidiary Thixalloy® Components Company produces by itself complex designed Thixoforming parts in Alloy A356/A357 for different applications. The production of weldable structure parts and weldable pressure tight parts plays an important role as well as the production of surface treated parts for industrial use.

The customer benefits are tight dimensional tolerances, superior weldability with high process ability, good mechanical properties in the thixoformed condition with and without heat treatment, pressure tightness up to 80 bar after welding, good surface quality and good behaviour in plating.

Yield strengths of 200 MPa can be reached with a very cost efficient T5 heat treatment on TX 540 AlMg5Si2Mn or TX630 AlSi7Mg0.8. The so called Silicon Spheroidization Treatment (SST) is described which enables real parts to fulfil the high demands and to reach the values which are needed on suspension parts and wheels. Additionally in this paper are shown parts which are produced with alloy type MAXXALLOY TX540 Al5Mg2Mn in Thixoforming. So Thixoforming works out as a more and more cost efficient process for Aluminum parts with high quality requirements.

Key words:

Thixalloy, Thixoforming, SST, Serial parts, Structure parts, Maxxalloy

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Introduction The increasing need of lighter and safer cars is the driving force behind research in the area of high strength light metal safety part applications. This also influences dramatically the technological changes to the automobil and has consequences for manufacturers, component suppliers and equipment manufacturers (Figure 1).

Since 1898 the Salzburger Aluminium AG (SAG) in Lend, Austria has been a competent partner in aluminum processes. SAG has been on the forefront when it comes to introducing new technologies in smelting, casting and treating aluminum. SAG prides itself in continuously building upon experience and innovation in both technological advances as well as human resources. The close cooperation with universities, research centers and partners constantly keeps the company abreast with the latest findings in development and know-how resulting in state-of-the-art applied technologies mirrored in the high standard and range of the products. The plant has an annual capacity of 45,000 tons whereas 25,000 tons are solely based on recycling activities. Furthermore, SAG has a casting line, independend from regular production, equipped with own furnace and horizontal casting machine for training as well as research and development purposes. Process-ability and quality control are of highest importance to the company leading to the following process steps: • Scrap material control by melting tests; • Alloy determination; • Melting in any one of the three hearth furnaces or the closed well furnace; • Metal treatment and stirring; • Metal quality control; • Casting process on the horizontal casting machines with continuous metal treatment (degasser, filter box and rod feeder); • Product quality control by analysis of material density, conductivity, • Stacking and packaging including marking ready for transport. The process leads to the high quality of the products such as foundry alloys, extrusion billets, thixotropic and cast forging billets/slugs, cast plates and bus bars. SAG has a recognized experience in casting special alloys, specifically using billets the standard diameters varying from 67 up to 203 mm, with developed technology to cast 50 or 32 mm diameter. The reasons for the excellent quality and variety of the products are based on the long-time experience of SAG and the advanced constructions and designs of casting moulds. Based on comparing investigations with other casting processes (e.g. vertical die casting) the horizontal casting technology gives SAG further advantages in their production. E.g. to a length of 18 m, enabling cutting billets to length during the casting process according to customer requirements. Other benefits include the well known economical aspects such as low energy consumption, low

171 / 2 investment costs, lower necessity of consumables. The duration of the cast can last up to 120 hours casting time and more. The mould technology with high cooling rate and optimized lubrication assures a high productivity and quality. The mould design furthermore allows a higher number of strands on smaller equipment sizes. Process control automation and documentation, automatic starting programs are standards.

Starting Material – Billets – Slugs The future development of lightweight construction (automotive and transport industry) depend especially on material properties of the used components. The claim of better material properties with simultaneous weight reduction gives rise to the development of technologically advanced casting and forming methods. Horizontal continuous casting (HCC) and thixoforming meet these requirements. SAG has 30 years of experience in HCC and more than 10 years with HCC materials. (One by SAG registered THIXALLOY® material). Starting material for the thixoforming process billets with a uniform and finely globulitic structure are produced by magnetically stirred HCC process according to the MHD technique (Magneto-Hydro-Dynamic Stirring), in diameters from 2.5 up to 6 inches.

Although alternative processes have recently appeared, they havenot yet made any decisive impact in the industrial context because the quality of the starting material is decisive for that of the components produced. In SAG’s case the process is mainly used for weldable structure parts, surface treated parts, suspension parts and wheels in the automotive industries. In-line melt treatment and the continuous control of all the casting parameters ensure that the structure is the producible. Since no melting is involved in the thixoforming process, no metal purification and structure adjustment measures are needed there either.

The main characteristics of billets made by the company’s MHD-process are: • Extremely fine grained billets: max. 130 µm grain size; • Good billet surface: max. class 2 (according to SAG’s internal standard); • High process ability and high metal purity during billet production; • Non-metallic inclusions: max. 40 µm; • Content of hydrogen: DI max. 5 %; • Porosity: max. 100 µm; • Peripherical zone: max. 7 mm.

A list of available diameters of the billets for industrial processing depends on the used thixoforming process and mainly on the locking forces of their machines. (See Table 1)

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Table 1: A list of available diameters of the billets for industrial processing depends on the used thixoforming process and mainly on the locking forces of their machines.

Diameter 2.5 inch 3 inch 3.5 inch 4 inch 5 inch 6 inch

Metric in 63.5 76.2 88.9 101.6 28 152.4 mm Locking 300-700 500-800 700-100 800-1200 100-1600 1600-2700 Force tons tons tons tons tons tons Shot 1.0-2.0 kg 1.5-3.0 kg 2.0-4.0 kg 3.5-7.0 kg 6.0-12.0 kg 10.0-20.0 Weight kg

A list of available alloys mainly used for industrial thixo-processing with respecting reached mechanical properties in raw-parts can be found in Table 2.

Table 2: A list of available alloys mainly used for industrial thixo- processing with respecting reached mechanical properties.

Alloy Alloy Type Condition F:

Thixalloy ® 615 AlSi7Mg0.15 Rp0.2: 100 Mpa Rm: 180 Mpa A: 15% Thixalloy ® 630 AlSi7Mg0.30 Rp0.2: 120 Mpa Rm: 230 Mpa A: 12% Thixalloy ® 640 AlSi7Mg0.40 Rp0.2: 130 Mpa Rm: 240 Mpa A: 10% Thixalloy ® 650 AlSi7Mg0.55 Rp0.2: 140 Mpa Rm: 250 Mpa A: 8% Thixalloy ® 680 AlSi7Mg0.80 Rp0.2: 150 Mpa Rm: 260 Mpa A: 5%

The continuously cast billets are sawn into slugs within close weight tolerances. The slugs are dispatched to the further processors in containers optimally adapted for their machine loading logistics. The customer benefits with MHD stirred billets are mainly given by:

• A homogenous distribution of silicon and accompanied alloyed elements in the cross section of the billet; • A low free liquid phase at the end of the heating cycle; • Saving of induction heating time during the preheating of slugs in the followed thixoforming process; • Reduced investment for induction furnaces; • A low scrap rate; and at least above all; • Better mechanical properties of parts; • High and constant serial quality -> 1 batch = 10,000 slugs and constant properties through the MHD process.

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Thixoforming Process Between molten and solid state, alloys show thixo-tropic behavior. The homological basis of this behavior is due to the fact that with increasing shear forces, the solid-phase skeleton of the metallic structure can be completely dismantled (2) This is the most important precondition to achieve low-viscous flow, which finally allows the production of extremely complex shaped parts with very small forming forces. Under low shear forces, the material shows a solid-state behavior and therefore can be handled like a solid part. SAG heats slugs while positioned horizontally in an inductive heating unit for individual billets. This avoids having to tilt them (as would be necessary in a carousel unit) once they have been softened. The power input is determined and controlled by a computer system. This enables the required homogeneous molten fraction to be produced in the slug with pinpoint accuracy, even without temperature measurements. The final adjustment of the structure is carried out during this heating process.

The α-phase is of globular shape, and the metal will only behave thixotropically if this is so. The benefits of induction billet heating and characteristics of the company’s thixoforming process compared to other processes (e.g. die casting units) lies mainly in: • No liquid metal process (therefore less hydrogen); • No melt house, dross, fumes and losses in process environment; • Horizontal billet heating and sure handling; • Exact heat content billet by billet; • Semi solid forming under low shear stress.

Why is Thixoforming a Practical and Economic Solution for the Industry? Thixoforming is a metal forming process which utilises the thixotropic properties of certain alloys when they are heated into a semi-solid state. In this semi solid-solid state the alloys consist of a fluid phase on a globolitic solid phase. Modern casting processes take advantage of these special characteristics, they are called “Thixoforming” processes. Thixoforming is a new process for producing safety parts for the automotive industry. It permits the near net shape production of complexe shaped components combined with a high degree of productivity and reliability. Apart from these economical advantages, components produced by Thixoforming exhibit outstanding mechanical properties. Thixoforming of aluminium alloys has come to play an increasing role as manufacturing route for complexe components of high quality.

Having regard to the properties described above, SAG’s process experience shows that thixoforming can achieve optimum cost/utility characteristics and can considerably improve customer utility.

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The essential advantages and customer benefits of Thixoforming are: • Weight savings can be achieved through optimised component design. • Good internal quality gives the best possible static and dynamic strength. • Production-line components have excellent dimensional accuracy. • Articles of complex shape with large wall thickness differences can be produced. • Small wall thicknesses, in part down to 1.7 mm and averaging 3 mm can be produced, as well as combined in parts with very large wall thicknesses are also possible. • Pressure-tight components, thanks to low porosity up to 80 bars after welding. • Reliable weldability (laser and MIG welding) thanks to low gas content and a lowporosity structure which is based on inline degassing during the HCC process. • Good surface quality for decorative or functional surface finishing & plating. • Fine uniform microstructures give enhanced component properties. • It is an energy efficient process which can be easily automated to achieve consistency. • Production rates are similar to or better than die casting.

The following table shows examples of dimensional properties of thixoforming serial parts with tolerances for customer needs on length and diameter:

Table 3: Dimension tolerances of thixoforming parts Radius down to R1, some areas with jump, in wall thickness up to higher radius. Draft angle: 1° to 3° depending from die depth and location. Gate system: wall thickness in gate zone: at least 3 mm thick.

Nominal Dimension 30 60 120 240 480 (in mm)

Thixoforming tolerances for space-diagonal of 500 mm No Parting Line Included ± 0.07 ± 0.12 ± 0.18 ± 0.25 ± 0.30 Over Parting Line ± 0.12 ± 0.15 ± 0.25 ± 0.33 ± 0.45 Comparison to: HPDC-tolerances DIN 1688 GTA 13/5 (over 50 up to 500 mm) No Parting Line Included ± 0.2 ± 0.3 ± 0.35 ± 0.45 ± 0.60 Over Parting Line 35 ± 0.45 ± 0.50 ± 0.60 ± 0.75

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SAG is a producer of thixoforming raw material for worldwide use at different producers of semi solid metal forming serial parts. The SAG subsidiary THIXALLOY® Components Company produces weldable structure parts and weldable pressure tight parts. This plays an important role as well as the production of surface treated parts for Industrial use.

Thixoforming Parts in the New Audi A8 and the New AUDI A6 Car The use of thixoforming technology and its advantages for lightweight structures parts is a main reason to use it for structural elements of doors (where they replace parts in sheet and castings) or other functional parts in cars.

The SAG group proves the competence of their processes by delivering more than 34 different parts for the new Audi A 8 car (Figure 1). Of all those parts, 13 are produced in the thixoforming process. SAG uses TX 630 alloy (AlSi7Mg0,3) which fulfils the high demand of the customer needs mainly in the field of mechanical properties, good weldability, low porosity and narrow dimensional tolerances. The need of lighter and safer cars is the driving force behind research in the area of high strength light metal safety part application. The new AUDI A6 will have two new thixoforming parts with high ductile alloy type MAXXALLOY TX540 for the air pressure vessel of the air suspension system (Figure 2).

The prerequisite for the above mentioned developments was the big change and challenge in the abilities of SAG’s entire workforce and management and the installation of a highly automated production process.

Additionally in this Paper are shown development parts which are produced with alloy type MAXXALLOY TX540 Al5Mg2Mn in thixoforming. The parts fulfill the requirements with up to 15% elongation in wallthickness of 3 mm together with higher yield strength of 170 MPa which can be used for structure parts and additionally for suspension parts or motorcycle wheel production. The complete heat treatment can be saved by using alloy, test for mold-war are in progress.

Benefits with Thixoforming Parts by Billet What we have described is a significant example of redefining and optimizing a production process at SAG according to new market needs and customer demands. The evolutionary development, from the production of the primary feed stock to supply producers, weight of light components and systems for the automotive industry was SAG’s mission to increase the quality of parts.

Table 5: Alloy: TX630 corresponds with A356; Alloy: TX650 corresponds with A357.

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Serial Part / Alloy State Rp 0.2 Rm A5 % Wall Thickness (Mpa) (Mpa) in mm

Side door part Thixalloy 630® F 120 230 11-14 Side door part Thixalloy 630® T6 260 310 10-15 Side door part Thixalloy 630® T6x3 229 317 16-20 Rear door Thixalloy 650® T6 270 330 8-12 hinge Rear door Thixalloy 650® T6x3 245 325 13-18 hinge Steering wheel Thixalloy 630® T4x3 118 247 20-26

Conclusion So thixoforming works out as a more and more cost efficient process for aluminum parts with high quality requirements.

Our understanding of quality for thixoforming parts is the summary of: • High static and dynamic mechanical properties. • Very low dimensional tolerances -> tailor made net shape quality. • High complexity design and variability of wall thicknesses of parts is practicable. • Wall thicknesses partial of 1.7 mm to a medium range of 3 mm is possible. • Pore free quality in thick wall areas. • Superior Laser, MIG, WIG e.g. weldability secured through low gas content in the raw material and low gas porosity in parts. • Very long tool life in comparison to (Vacuum) - high pressure die casting. • Competitive price through low process costs for complex thixoforming parts.

The additional use of aluminium in automobiles can be rapidly introduced to reduce energy consumption and pollution. In order to achive cost- effective solutions a close partnership between the car industry and the aluminium industry is necessary. The aluminium and automotive industry have a history of collaboration on developing the use of the most common light weight metal in cars. The aluminium content of cars is predicted to double within the next ten yours. Aluminium is the key to light weighting; aluminium is the preferred material of the 21st century.

Therefore thixoforming is ideal for automotive components; those products that demand a high level of complexity, weight savings and thight dimensional tolerances. This is SAG’s chance!

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References [1] MERCER Management Consulting „Automobile technology 2010, Technological changes to the automobile and their consequences for manufacturers, component suppliers and equipment manufacturers“

[2] Dipl. Ing. H. Luechinger, Dipl. Ing. B. Wendinger, SAG „Thixoforming – der rationelle Weg zur Herstellung von Premium- Bauteilen aus Aluminiumwerkstoffen” Giesserei-Rundschau 49 (2002)

Acknowledgements The work was based thanks to a gradual change in the use of ability of partners within the internal and external group, the adoption of automated production processes, groups dedicated to innovation and to activities aimed at guaranteeing quality of SAG’s products.

Josef Wöhrer Chairman & CEO Salzburger Aluminium AG (Lend / Austria) [email protected]

Andreas Kraly Product and Technology, Development in Continuous Casting SAG Aluminium Lend GmbH & Co KG (Lend / Austria) [email protected]

Figures

Figure 1: Figure 2: A-pillar for Audi A8 front door Air Suspension for Audi C6 – Top of the air tank

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