Wolfgang Böhm

Tailored Heat Treated Accumulative Roll Bonded Aluminum Blanks: Microstructure and Mechanical Behavior

Aluminum alloy AA6016 was accumulative roll bonded up to eight cycles and investigated regarding formability by bending tests. Due to the limited bendability of accumulative roll bonding (ARB) processed materials, a tailored laser heat treatment was performed along the bending edge before forming. This tailored laser heat treatment causes a local recrystallization and recovery of the bending samples at the deformation zone, which locally increases ductility and allows higher bending angles achievable with lower forming forces. Between the recrystallized heat treated zone and the unaffected ultrafine-grained (UFG) base material, a gradient in grain size with a bimodal region is formed. This observed microstructural profile is confirmed by local mechanical testing measuring the hardness and strain rate sensitivity by nanoindentation techniques.

Einfluss von germanium-zusätzen zu kupfer-zinnmischkristallen auf die diffusionsvorgänge bei der bildung der A15-phase Nb3Sn und deren supraleitende eigenschaften

Zusammenfassung Die Kupferecke des ternaren Systems Kupfer-Germanium-Zinn wurde untersucht und festgestellt, dass sich bei Temperaturen zwischen 700 und 750 °C das β-Phasengebiet des Randsystems Kupfer-Zinn durch Germaniumzusatz in das ternare Gebiet ausdehnt. Eine Warmumformung bei 700 °C ist nur in dem Phasengebiet moglich, in dem ausser der β-Phase auch die α-Phase im Gleichgewicht vorhanden ist. Die reine β-Phase wird bei der Warmumformung durch Abkuhlung durch die Verformungswerkzeuge an der Oberflache zur Umwandlung gebracht. Dadurch konnen infolge der sich dann bildenden sproden, plattenformigen Ausscheidungen Mikrorisse entstehen, die sich schnell zu Makrorissen ausweiten. Erste Untersuchungen an Verbundwerkstoffen zwischen diesen Kupfermischkristallen als Mantelwerkstoff und einem Niobkern nach Warm- und/oder Kaltumformung erfolgten an bei 700 – 790 °C zur Erzeugung einer A 15-Diffusionsschicht warmebehandelten Proben. Der Beginn des Ubergangs vom supraleitenden in den normalleitenden Zustand der Diffusionsschicht kann durch Germaniumgehalte von 0,5% erhoht werden. Durch einen Germaniumzusatz von ⩾ 1% wird die A 15Phase Nb 4 Ge bevorzugt ausgebildet. Sie hemmt die Wachstumsgeschwindigkeit der Diffusionsschicht. Die Abhangigkeit der kritischen Stromstarke vom ausseren Feld zeigt, dass bei den Verbundleitern mit fallendem Drahtdurchmesser bei gleicher Vorbehandlung und Diffusionsgluhung die kritische Stromdichte zunimmt. Durch einen Germaniumzusatz von 0,5% kann die kritische Stromdichte der Diffusionsschicht der intermetallischen Verbindung Nb 3 Sn gesteigert werden, so dass gegenuber dem kritischen Strom von aus binaren Legierungen hergestellten Verbundleitern eine deutliche Erhohung festgestellt wird.

Formability of Ultrafine-Grained AA6016 Sheets Processed by Accumulative Roll Bonding

Aluminium alloy AA6016 was accumulative roll bonded up to eight cycles in order to produce an ultrafine-grained microstructure. The formability of these sheets was investigated by means of bending tests. Furthermore the influence of a local laser heat treatment at the bending edge is observed. The strength of the UFG samples is increased by a factor of around two compared to the conventionally grained T4 condition which also results in up to 50 % higher punch forces needed for bending of ARB processed samples. An anisotropic bending behaviour is observed. By applying a tailored laser heat treatment along the bending edge prior to the bending tests a local recrystallization and recovery at the deformation zone of the samples is achieved. Thus, ductility is increased locally whereby bending to an angle of 80° is possible with lower forming forces compared to the non-heat treated specimens. The results are compared to previous studies on mechanical properties and formability investigations of ARB processed AA6016.

Tailored heat treated accumulative roll bonded aluminum blanks: failure under bending stresses

Ultrafine-grained accumulative roll bonded (ARB) sheet metals of aluminum alloys have a high potential for lightweight construction. The mechanical properties can be enhanced regarding strength and ductility by the combination of ARB and a local heat treatment according to the Tailor Heat Treated Blanks technology. The present investigation focuses on the failure behavior of ultrafine-grained ARB blanks. Due to the low formability of these high-strength ARB metals, a detailed understanding of the failure mechanisms is essential. For this purpose, an established approach to determine the different stages of damage of the material for conventional materials is now applied to multilayered aluminum in the as-received and heat-treated condition. In this context, air bending tests are used to qualify and quantify the bending and forming behavior of ARB sheets of AA1050A and AA6016 aluminum alloys.

Modeling of the Plastic Characteristics of AA6082 for the Friction Stir Welding Process

Focus of this paper is to model the plastic forming behavior of AA6082, in order to develop the numerical FE analysis of the friction stir welding processes and the simulation of subsequent forming processes. During the friction stir welding process, the temperatures reached are until 500 °C and have a fundamental role for the correct performance of the process so the material data has to show a temperature dependency. Because of the tool rotation a strain rate sensitivity of the material has to be respected as well. In this context, the general material characteristics of AA6082 were first identified for different stress states. For the uniaxial state the standard PuD-Al used in the automotive industry was applied, for the shear state the ASTM B831-05 was used and for biaxial states the ISO 16842 was exploited. To characterize the plastic flow behavior of the AA6082 at elevated temperatures tensile tests were performed according to DIN EN ISO 6892-2 from 25 °C until 500 °C with a strain rate from 0.1 s-1 up to 6.5 s-1.

Accumulative Roll Bonding: Forming Behavior, Tailored Properties and Upscaling Approach

The Accumulative Roll Bonding (ARB) process enables the manufacturing of high strength sheet metals with outstanding mechanical properties by repeated rolling. However, the significant increase in strength leads to loss in ductility, especially regarding aluminum alloys of the 6000 series. The low formability obviously limits the implementation of these sheet products for formed components in automotive applications. To enhance formability, a local short term heat treatment according to the Tailored Heat Treated Blanks technology is used. For the finite element based design of forming operations accurate information about the plastic behavior of these tailored materials is required. Therefore, different stress - strain paths are considered using the tensile test and the layer compression test. In this context, heat treated and non-heat treated specimens out of ARB processed AA6016 were tested at room temperature. With the experimental results true stress strain curves and yield loci determined from different criteria and represented in a principal stress state were established. Regarding the experimental setup of the ARB process, an upscaling is essential for the production of sufficiently large strips to cut out blanks for the forming of components such as B-pillars. However, this requires the adaptation of the different process steps of the ARB process. In this context, the surface treatment before rolling of such large sheets is investigated, since it is particularly relevant for obtaining a strong bonding between the sheets. Another aspect is the investigation of the rolling process using the finite element analysis. In this regard, a thermal mechanical coupled simulation model of the roll bonding operation will be developed for the evaluation of different material combinations, different process temperatures and varying roller geometries. These investigations will enable the production of lightweight automotive components made of ARB processed high strength aluminum sheet metal with tailored properties.