26 ATZ 10/2005 Jahrgang 107

Authors:Joachim Becker, Peter Bogon, Horst Gers, Armin Kiefer, Christoph Leyens, Fridjof Newiak, Karl Roll, Oliver Straube and Bernd Viehweger

Magnesium-Knetlegierungen für den Automobilbau

You will find the figures mentioned in this article in the German issue of ATZ 10/2005 beginning on page 922.

Mg-Wrought Alloys for Automotive Applications

Although the application of Mg wrought alloys is known to have potentials for their use inautomotive industry their use is still of small importance. In the BMBF-funded researchproject “MIA – Magnesium im Automobilbau” associating named companies such asDaimlerChrysler AG, ThyssenKrupp Umformtechnik GmbH, Otto Fuchs KG, Honsel GmbH &Co. KG, GP innovation GmbH, OSK Kiefer GmbH as well as the Brandenburg University ofTechnology the basic knowledge concerning the prototypical manufacturing of parts ma-de of Mg wrought alloys AZ31 and AZ80 is being acquired.

1 Potentials of Mg Wrought Alloys

The use of Mg wrought alloys for automotiveapplications offers wide potentials forweight-reduction in car-design. The evalua-tion of potentials, concerning only the phys-ical density shows advantages in weight ofabout 75 percent compared to steel and 35percent compared to aluminium. Consider-ing different constructional concepts andspecific material properties that take intoaccount material dependent design for Mg-parts, a comparable density specific young’smodulus and a superior tensile yieldstrength becomes obvious, Figure 1.

Limited use followed by small productionquantities of Mg wrought alloys is mainlycaused by their comparably high purchase

2 Mg-Casting and Melting Process

In order to improve the manufacturingprocess of AZ31- and AZ80-Mg-material a nov-el melting and casting equipment has beeninstalled and started up at the Otto Fuchs KG.For melting of Mg wrought alloys an insula-tion of the melt from atmosphere is requiredand has been realised by means of sophisti-cated constructional details. Hence the con-sumption of atmosphere polluting SF6-basedinert gases has been drastically minimised.The application of insulation salts is not nec-essary anymore. The producible continuous-ly casted Mg-material in form of bars and rodsis characterised by homogeneous surfacequality, minimised oxide-embeddings as wellas an absolutely salt-free quality. The recy-cling of Mg-filings – taken from subsequentprocessing-steps – within the melting processis enabled. Therefore cost saving potentialsduring operating the casting equipment havebeen ensured, the environmentally compati-bility of the casting process could be im-proved, Figure 3.

3 Hot Rolling and Sheet Metal Forming of AZ31-Blanks

The forming of Mg-materials causes particu-lar demands on processing technology. Due

price and a lack of knowledge with respectto the manufacturing of high-quality Mgbase material in form of continuously castedbars as well as their subsequent processingto car components and assemblies. The re-duction of production costs can be achievedby up-scaling the existent production facili-ties as a result of an enhanced demand forMg wrought alloys. Therefore potential ar-eas for the application of Mg wrought alloyshave to be detected, their processing tech-nologies improved and economicallystreamlined [1, 2]. Lacking knowledge in as-sociation to the production of Mg wroughtalloys for high-quality forgings, extrudedMg-products and Mg sheet metal applica-tions for power transmission, chassis andcar body are supplied by various BMBF-fund-ed research projects. Therefore the main ob-jective of the project “MIA – Magnesium imAutomobilbau” is testing of the prototypicalmanufacturing process of parts made of Mgwrought alloys AZ31 and AZ80. Particular at-tention is paid to the continuous castingprocess, the rolling technology, the processof conventional and hydro-mechanical deepdrawing of Mg-blanks, the isothermal forg-ing, the mechanical surface treatment, thestretch bending of profiles as well as suit-able joining techniques in order to enablethe manufacturing of assemblies, Figure 2.

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to the sudden enhancement of formability atforming temperatures above 225°C the tem-perature control whilst forming operations isof special importance [3, 4, 5]. During hot-rolling of Mg wrought alloys an intense cool-ing of roll barrels of the rolling mill directlyin contact with the blank-surface has to beavoided. Therefore for the conduction ofrolling tests, carried out at the Chair of Physi-cal Metallurgy and Materials Technology atthe BTU, a prototypical rolling mill equippedwith a heated pairs of rolling barrels is in use.The technology of hot rolling of AZ31 sheetmetal, directly from continuously casted ma-terial, has been developed. Blanks of variousthicknesses of 1.0 and 1.5 mm and maximumdimensions of 600 mm x 1.000 mm could besupplied to the project partners. The mechan-ical properties of manufactured AZ31-sheet-metal compared to commercially availableAZ31-sheet-metal made of extruded base-ma-terial, are shown in Figure 4.

Remaining lubricant and oxide layershave been removed from AZ31 blanks bymeans of suction blasting. This process doesnot cause any surface work-hardening effects.Suitable blasting parameters and an opti-mised choice of blasting agent enable the pos-sibility of a controlled manipulation of sur-face structure of Mg sheet metal.

The general formability of Mg sheet metal,manufactured at the Brandenburg Universityof Technology, was proven by means of con-ventional cup drawing tests. Additional form-ing characteristics could be determined inform of forming limit diagrams. Characteris-tic limiting draw ratios of BUT sheet metalreach values of 2.5 at a forming temperatureof 225°C. The use of Mg sheet metal for rein-forcing parts of car-bodies can extend the fu-ture application areas of Mg wrought alloys.Therefore research tasks concerning the pro-totypical manufacturing of automotive partsare partially focussed on conventional deepdrawing at elevated temperatures of an interi-or roof segment of a sports car convertible.This enables the possibility of subsequenttesting of hybrid joining techniques applyingexterior roof segments made of aluminiumand interior roof segments made of AZ31.Furthermore the assembling of a joined hy-brid roof to a car body taken from the com-mon production line is easy to realise.

In order to fulfil the technological require-ments the forming process is designed as acombination of stretch-forming and deep-drawing. The necessary variability of the deepdrawing process with respect to the materialflow is ensured by applying a prototypicalheatable deep-drawing-tool, equipped with asegment-elastic blank-holder system [7]. Usinga hydraulic press equipped with multi point

tool has been equipped with an additionalpressure unit. In order to tap the full poten-tials for Mg-forming processes the entire toolcan be heated up to a maximum temperatureof 300°C. Enhancing the complexity of ap-plied tube-cross-section geometries a roof re-inforcement section has been chosen for fur-ther forming tests. The 3-chamber-profilemade of the Mg wrought alloy AZ31 has beenformed without failures in geometry or sur-face cracks, Figure 8.

3.2 Forging of Mg Wrought Alloys AZ31 and AZ80Further developments in Mg forming tech-nology can be described for forgings. Highquality forgings of Mg wrought alloys arethought to perform with increasing futuredemands [8]. By means of cylindrical upset-ting tests flow stress curves have been ac-quired and the evolution of microstructureas a result of warm forming process docu-mented. Suitable parameters for processingthe Mg alloy types AZ31 and AZ80 were deter-mined. The practical evaluation of experi-mentally obtained results has been realisedby forging of a traverse control arm and a mo-tor bike rim. For the motor bike rim the forg-ing of the centre of the rim based on continu-ously casted material as well as the subse-quent rolling processes could be realised suc-cessfully, Figure 9.

An enhancement of fatigue properties ofMg forgings is realisable by means of typicalsurface treatment processes, as there are shotpeening and roller burnishing. This leads toimportant improvements of cycles-to-failurevalues of Mg specimens, Figure 10. For the ex-aminations wide ranges of blasting agentsand Almen-intensities have been tested.

4 Joining Techniques

The development of Mg applications for auto-motive car design is strongly related to an in-creasing demand for suitable joining solu-tions. The implementation of sophisticatedjoining techniques in assembling Mg parts toentire component systems is of particular im-portance. Especially the hybrid technologiesoffer wide potentials (aluminium-magne-sium) for future applications. Therefore theproject partially aims the parameter optimi-sation for various joining techniques. Pointsof interests are the folding operation of Mgblanks, thermal joining techniques such aslaser-welding and -brazing, mechanical join-ing processes like punch riveting and clinch-ing as well as FD-screwing. By means of heatconduction laser-welding process in sequen-tial order and applying a diode-laser-devicefrom both side of the blank smooth surfaces

die cushion the optimised material’s flowcan be realised due to locally differing valuesof surface pressures between blank and blankholder, Figure 5.

Results of substantial FE-calculations, con-cerning the forming process of the interiorroof segment and obtaining local mechanicalund thermal loads of the forming tool duringforming as well as the blank heating process,have been taken into account for dimension-ing the deep drawing tool. The forming tests,using commercially available AZ31 sheetmetal, supplied by the Salzgitter MagnesiumTechnology GmbH, have shown that themanufacturing of mentioned part is possibleat a forming temperature of 225 °C, Figure 6.

The subsequent cutting operations ofdeep-drawn interior roof segments applyingalready existent cutting tools have been re-alised without problems, although a tenden-cy to “flittering” has to be mentioned. Detect-ed small cracks as a result of embossing-oper-ations can be explained by the particular alu-minium-oriented design of used cutting/em-bossing tools and may be avoided by few alter-ations.

In addition to this, the forming limits ofAZ31-blanks were examined by means of hy-dromechanical deep drawing and suitable lu-bricants have been proved. Furthermore themanufacturing of a hydromechanicallyformed exterior roof segment is intended.

3.1 Stretch-Bending of AZ31-ProfilesProfiles of AZ31 wrought alloys are said tohave a wide range of future applications.Therefore examination concerning the suit-ability of Mg profiles of various cross-sectiongeometries for the stretch bending process atroom temperature as well as at elevated tem-peratures is a point of interest. PreliminaryFE-calculations have shown that Mg-profilesexhibit suitable forming properties for a widerange of bending geometries applied in auto-motive industry. In order to avoid large defor-mations of cross section areas during stretchbending geometry-specific internal pressuresare usually applied. Necessary values of opti-mised pressures inside the hollow profile andrequired forces for stretch bending at roomtemperature have been determined. In orderto achieve this goal a profile with a simplifiedquadratic cross-section of 40 mm x 40 mmand a wall thickness of 2 mm has been as-sumed, Figure 7.

The practical verification of numericallyobtained results concerning the stretch-bend-ing of tubes of simple and complex cross-sec-tion geometries is being carried out at theBUT. Therefore a 3D-CNC stretch-bending ma-chine supplying an axial load of 60 kN is ap-plied. Moreover, the installed stretch-bending

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28 ATZ 10/2005 Jahrgang 107

of welding seams without deformations and low porosity weremanufactured. Welded specimens exhibit a slightly reducedstrength (app. 7%), although ductility is considerably reduced,Figure 11.

5 Summary

The examinations carried out in the project “MIA - Magnesiumim Automobilbau“ have demonstrated the widespread varietyof possibilities for the application of Mg wrought alloys in au-tomotive design. By means of a novel technology for the con-tinuously casting process of Mg wrought alloys German indus-try is in the position to supply high-quality base material inform of rods and bars. The subsequent processing of AZ31 basematerial to thin-walled hollow profiles of complex cross-sec-tion geometries is possible and state of the art. In order to pro-duce AZ31 sheet metal, able to fulfil the requirements ongeometry-complexity in automotive car body design and ongood forming properties, hot-rolling is a suitable process. Thedesign of forming tools has to be aligned to the necessities ofMg-specific forming-behaviour. Due to enhanced formabilityof Mg wrought alloys above forming temperatures of 225°Cthe process has to be maintained in a suitable temperaturerange. Furthermore sophisticated tool concepts are needed forthe activation of forming-reserves. On the basis of the manu-facturing of an interior roof-segment of a sports car convert-ible the tests have been successfully concluded. Although atendency toward “flittering” could be determined, necessarycutting and embossing operations were realised on AZ31parts. The stretch-bending of hollow Mg profiles can be re-alised applying internal pressure devices. In dependence ondemanded complexity of profiles of cross section and requiredbending radii this process can be carried out at room temper-ature or heated. Isothermal forging is a suitable way for sup-plying high quality parts of higher strengths which are appro-priate to the type of duty. Their fatigue properties can be sig-nificantly improved by the application of surface treatmentprocesses such as shot peening and roller burnishing.

The authors are very grateful to the German Federal Min-istry of Education and Research (BMBF-PTJ) for funding thisproject (Project-No.: 03N3106K).

References[1] Kainer, F.; von Buch, F.: Moderne Entwicklungen von Legierungen für den

Leichtbau, Materialwissenschaft und Werkstofftechnik, 30 (1999), S. 159-167[2] Diem, W.: Magnesium in Different Applications, AutoTechnology, 1 (2001), S.

40-41[3] Wagener, H.-W.; Hosse-Hartmann, J.: Zum Tiefziehen von Mg-Blech, UTF

science, 1 (2001), S. 28-34[4] Doege, E.; Haller, B.; Janssen, St.: Aufnahme von Fließkurven von Magne-

siumwerkstoffen, UTF science, 2 (2002), S. 13-17[5] Viehweger, B. et al: Hydromechanisches Tiefziehen und Hochdruckblechum-

formung als Verfahren zur Herstellung komplexer Bauteile aus Magnesium-feinblechen des Typs AZ31B-0, Materialwissenschaft und Werkstofftechnik,35 (2004), S. 440-446

[6] Juchmann, P.; Wolff, S.: Mit Magnesium geht´s leichter, MM Maschinen-markt, 109 (2003), S. 64-70

[7] Hengelhaupt, J.; Häusermann, M.: Neues Werkzeugkonzept für das Tief-ziehen. In: Neuere Entwicklungen in der Blechumformung, Fellbach 2002.MAT INFO Werkstoff-Informationsgesellschaft mbH, Frankfurt, 2002

[8] Fischer, G.; Becker, J.; Stich, A.: Gesenkschmieden hochfester Magnesium-Knetlegierungen für Bauteile der Automobil- und Luftfahrtindustrie, Material-kunde und Werkstofftechnik, 31 (2000), S. 993-999

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