Characterization of AMAG AL6-CHA sheet material for Chassis application in the automotive industry

Transcription

1 Characterization of AMAG AL6-CHA sheet material for Chassis application in the automotive industry Josef Berneder 1, Ramona Prillhofer 1, Josef Enser 1, Gunther Rank 1 and Torsten Grohmann 1 1 AMAG rolling GmbH, Postfach 32, Ranshofen, 5282, Austria Josef.berneder@amag.at Keywords: chassis, automotive sheet, 6xxx, heat treatable, corrosion resistance, rear and front axles Abstract. Aluminium is already extensively used in car production to reduce the CO 2 emissions by weight reduction. A further beneficial effect of lightweight design can be generated in components of the chassis by reducing the weight of unsprung mass thereby enhancing the driving comfort and reducing the noise level. The medium strength alloys of the type AlMg3Mn (EN AW-5754) and AlMg3.5Mn (EN AW-5454) are currently the aluminium sheet material choice for application in chassis components. The newly developed alloy AMAG AL6-CHA was optimized with regard to chassis applications and shows the potential of significant increase of the mechanical properties compared to state-ofthe-art 5xxx series alloys. AMAG AL6-CHA, which is a 6xxx series alloy with balanced Mg/Siratio, is characterized with regard to mechanical properties and intergranular corrosion resistance in delivery temper and after artificial aging with the typical heat treatment cycle 25 C/6 min in peak aged temper. Furthermore we will show the results of the Charpy-V-notch impact test and the formability is described per bend test and grain size analysis. Introduction Aluminium is already extensively used in car production to reduce the CO 2 emissions by weight reduction. A further beneficial effect of lightweight design can be generated in components of the chassis by reducing the weight of unsprung mass and thereby enhancing the driving comfort and reducing the noise level [1]. The medium strength alloys of the type AlMg3Mn (EN AW-5754) and AlMg3.5Mn (EN AW-5454) are currently the aluminium sheet material choice for application in chassis components [1, 2, 3]. These non-heat treatable alloys mainly allow the following options in the production of the chassis component based on sheet material: (1) Forming/stamping of complex parts in soft temper O/H111 with the consequence that there is no possibility of an additional increase in mechanical properties of the formed component compared to the delivery temper O/H111 (with the exception of strain hardening in the deformed areas). (2) The forming/stamping is done from sheet material in a strainhardened temper (e.g. H24) which allows only the forming of less complex parts, due to the high level of mechanical properties combined with low elongation values. Heat treatable 6xxx series alloys provide the major benefit that the forming is done in the soft delivery temper with a subsequent significant increase of the mechanical properties after aging of the component at elevated temperatures to peak aged temper. The newly developed alloy AMAG AL6-CHA was optimized with regard to chassis applications and shows the potential of a significant increase of the mechanical properties compared to state-ofthe-art 5xxx series alloys.

2 Experimental Microstructure: The microstructure was characterized by using optical microscopy (OM). OM was performed on metallographic cross sections in L, T and S direction. Grain size measurements were performed after Barker-etching for 6 s. Bend test: To characterize the bending performance a plane strain bending test according to VDA was carried out. The bend test samples with the dimensions 6 x 6 mm² were cut out from the sheet material in and 9 direction so that the bending edge is aligned in rolling direction ( ) as well perpendicular to the RD (9 ). Characterization of the mechanical properties: Tensile tests were carried out according to DIN EN 12 (Lo = 8 mm). The tensile test was carried out in three different testing directions referred to the rolling direction (, 45 and 9 ). Charpy-V-notch Impact Test: The testing apparatus consists of a pendulum of known mass and length that is dropped from a known height to impact a notched specimen of material. The energy transferred to the material can be inferred by comparing the difference in the height of the hammer before and after the fracture (energy absorbed by the fracture event). The tests were carried out according to DIN Corrosion testing: Corrosion Testing was performed according to ISO 11846, method B. The maximum corrosion depth was evaluated on metallographic cross sections longitudinal to the rolling direction. The 6xxx series aluminium alloys are characterized in general through their main alloying elements Si and Mg and additions of Cu, Mn and Fe. The additions of Mg and Si are made either in amounts such that quasi-binary Al-Mg 2 Si alloys would form (Mg:Si, 1.73:1) or with excess Si above that needed to form Mg 2 Si [4]. As it can be seen in Table 1, AMAG AL6-CHA shows a Si/Mg-ratio of 1.. AMAG AL6-CHA Table 1: Actual chemical composition of the investigated material in wt.% Si Mg Fe Cu Mn Si/Mg [wt.%] [wt.%] [wt.%] [wt.%] [wt.%] [-] AMAG AL6-CHA sheet material was solution heat treated on a continuous heat treatment line (CHTL) with a soaking time of several minutes and quenched to room temperature with cold water. The very fast heating-up rate to the solution annealing temperature (>4 C/min) which can be realized in a CHTL, contributes to a fine-grained and equiaxed grain structure which is favorable for forming operations as well important in order to avoid the orange peel effect on the surface of heavy deformed areas of a component. Following the solution annealing the hereby produced coils were cut-to-length and all samples for the investigations presented in this paper were machined from these blanks which have been leveled in the course of the cut-to-length process. This leveling operation introduces cold work into the material and increases the mechanical properties in delivery temper (in particular the Yield Tensile Strength) hence decreasing the formability. All investigations were performed with gauge 4. mm sheet material as this is a typical thickness for the application of AMAG AL6-CHA in the chassis of cars. The final strength of the manufactured component will be achieved after the forming operations through strain hardening and by artificial aging to peak aged temper.

3 Bending Angle [ ] Results and Discussions Microstructure Grain size and grain shape: A three-dimensional micrograph showing the grain size and shape of AMAG AL6-CHA is depicted in Figure 1. S LT L Figure 1: Three-dimensional micrograph of AMAG AL6-CHA material, 5-times magnified The average grain size was recorded in L, T and S direction (which refers to the grain length, width and grain thickness) and the results are summarized in Table 2. Table 2: Average grain size of AMAG AL6-CHA in L, T and S direction grain size [μm] AMAG AL6-CHA L T S AMAG AL6-CHA shows due to an optimized heat treatment and rolling sequence a grain structure with only minor differences between grain length and grain width which is again an indication for uniform forming characteristics without any pronounced preferred direction of plastic flow. Bend test performance in temper and with optical evaluation of bend line: Bending is an important parameter for evaluating the forming capabilities of thick gauge sheet material. The results of the bend test for AMAG AL-CHA are shown in Figure 2 for temper as well with the bend line parallel ( ) and perpendicular to rolling direction (9 ) AL6-CHA, AL6-CHA, Figure 2: Bending angle of AMAG AL6-CHA in delivery temper and after artificial aging to temper with 25 C/6 min, tested in direction and 9

4 Ultimate Tensile Strength R m [MPa] Yield Tensile Strength R P,2 [MPa] Bending with the bend line parallel ( ) to the direction of rolling shows better results compared to bending in 9. The results can generally be considered satisfying for thick gauge material with small differences between testing direction and 9. As expected, artificial aging to temper reduces the attainable bending angle. The bend line was also optically evaluated just before and after fracture of the sample both in testing direction and 9 (see Table 3). Table 3: Optical evaluation of the bend line just before (No Fracture) and after (Fracture) Bend Line parallel to RD ( ) Bend Line perpendicular to RD (9 ) No Fracture Fracture No Fracture Fracture It can be seen that the first flaws are formed due to orange peeling on the heavy bent areas. These flaws are the starter for the formation of cracks which cause the fracture of the sample. Mechanical properties in temper and : Figure 3 shows the Ultimate Tensile Strength R m (UTS) in delivery temper and after artificial aging to temper. The results show only small differences dependent on the testing direction but as expected with the highest values in rolling direction ( ) and with the lowest perpendicular to the RD (9 ). The same is valid for the Yield Tensile Strength Rp.2 (YTS) also depicted in Figure 3. Figure 4 shows the Elongation A8. Especially the values for the delivery temper are on a high level for thick gauge material which contributes to a good formability AL6-CHA, AL6-CHA, AL6-CHA, AL6-CHA, AL6-CHA, AL6-CHA, Figure 3: UTS R m and YTS Rp,2 of AMAG AL6-CHA in delivery temper and after artificial aging to temper with 25 C/6 min, tested in direction, 45 and 9

5 Charpy-V-notched toughness [J/cm²] Elongation A 8 [%] AL6-CHA, AL6-CHA, AL6-CHA, Figure 4: Elongation A 8 of AMAG AL6-CHA in delivery temper and after artificial aging to temper with 25 C/6 min, tested in direction, 45 and 9 Charpy-V-notch impact test: The Charpy-V-notch impact test is very practical in order to evaluate with a comparatively simple test the toughness of thick gauge sheet material. The results for temper and for testing direction and 9 are depicted in Figure AL6-CHA, AL6-CHA, Figure 5: Charpy-V-notch toughness of AMAG AL6-CHA in delivery temper and after artificial aging to temper with 25 C/6 min, tested in direction and 9 As expected the delivery temper shows high values compared to the peak aged temper. Intergranular corrosion resistance in temper and : Table 4 illustrates the typical appearance of pitting and intergranular corrosion of AMAG AL6-CHA in delivery temper and after artificial aging to peak aged temper. Table 4: Typical appearance and maximum corrosion depth of AMAG AL6-CHA in delivery temper and after artificial aging to temper with 25 C/6 min max. IGC max. IGC AMAG AL6-CHA, AMAG AL6-CHA, Depth Depth delivery temper Use condition [µm] [µm] 14 15

6 Delivery temper shows mainly intergranular corrosion attack whereas the appearance after artificial aging to the use condition changes significantly as pitting dominates with only sporadic formation of intergranular corrosion attack. This improvement in corrosion resistance due to artificial aging is also visible in the maximum corrosion depth which is smaller in temper. Conclusions The actual material choice for chassis parts are the medium strength alloys of the type AlMg3Mn (EN AW-5754) and AlMg3.5Mn (EN AW-5454) [1, 2, 3]. The typical mechanical properties of these alloys in comparison to AMAG AL6-CHA in use condition are depicted in Table 5. The forming operation of the chassis components (subframes for rear and front axles) is performed with AMAG AL6-CHA in the well formable delivery condition followed by an artificial aging of the part to peak aged temper with a typical heat treatment cycle of 25 C/6 min. Thus it appears that by using AMAG AL6-CHA the YTS Rp,2 can be increased compared to state-of-theart 5xxx series alloys by more than 2 % and the UTS R m by 3 %. Table 5: Comparison of the typical mechanical properties of state-of-the-art 5xxx series aluminium alloys for chassis application compared to AMAG AL6-CHA [5] UTS YTS UTS YTS Temper A8 Temper A8 EN AW EN AW O/H111 (Use condition) O/H111 (Use condition) R m Rp,2 R m Rp,2 [MPa] [MPa] [%] [MPa] [MPa] [%] AMAG > 19 > 8 > 18 AL6- > 2 < 17 > 2 (delivery) CHA > 215 > 85 > 17 AMAG AL6- CHA * (Use condition) *...Delivery temper + artificial aging with 25 C/6 min > 28 > 26 > 1 AMAG AL6-CHA optimized for chassis application can be produced up to a thickness of 6.5 mm. Acknowledgements The authors gratefully acknowledge Austria Metall AG (AMAG), whose support has made this study possible. References [1] J. Hirsch, Automotive Trends in Aluminium The european perspective, Materials Forum Volume 28, 15-23, 24 [2] W.S. Miller, L. Zhuang, J. Bottema, A.J. Wittebrood, P.De Smet, A. Haszler, A. Vieregge, Recent development in aluminium alloys for the automotive industry, Materials Science and Engineering A28, 37-49, 2 [3] F. Ostermann, Anwendungstechnologie Aluminium, Berlin Heidelberg, Springer Verlag, , 27 [4] I.J. Polmear, Aluminium Alloys A century of age hardening, Materials Forum, Volume 28, 1-14, 24 [5] Aluminium und Aluminiumlegierungen Bänder, Bleche und Platten, Teil 2: Mechanische Eigenschaften, Europäische Norm EN 485-2, Oktober 28