Benedikt Müller

Joining by forming of piezoceramic macro-fiber arrays within micro-structured surfaces of aluminum sheets

Functional integration of smart materials in sheet metal enables lightweight composite parts which are enhanced by new functionalities. Locally integrated piezoceramic/metal composites consist of a prefabricated array of ten parallel piezoceramic macro-fibers with dimensions of 0.277 mm by 0.232 mm by 10 mm which are joined in micro-formed cavities within the surface of an aluminum sheet metal. By the use of joining by forming, the interference–fit, preload and form–fit of macro-fiber arrays are achieved in a single process step. The paper describes investigations of the joining by forming process in formal planned experiments using the design of experiments method. The influence of the dimensions and preparation of the joining partners, the maximum forming force and the velocity of the forming stamp are varied. The interference–fit and preload depend on the maximum forming force. In contrast, the quality of the form–fit is primarily related to the geometric dimensions and the forming force. Fiber fractures and incipient cracks are the major failure mechanisms during joining by forming of the macro-fibers. The number of cracks is significantly reduced by the use of lower die velocities, lower maximum joining forces and the introduction of additional geometric elements in the microstructure of the metal surface. Concluding, constraints with regard to the design of parts and the process are derived from the experiments.

Monitoring concept for structural integration of PZT-fiber arrays in metal sheets: a numerical and experimental study

ABSTRACT The technique joining by forming allows the structural integration of piezoceramic fibers into locally microstructured metal sheets without any elastic interlayers. A high-volume production of the joining partners causes in statistical deviations from the nominal dimensions. A numerical simulation on geometric process sensitivity shows that the deviations have a high significant influence on the resulting fiber stresses after the joining by forming operation and demonstrate the necessity of a monitoring concept. On this basis, the electromechanical behavior of piezoceramic array transducers is investigated experimentally before, during and after the joining process. The piezoceramic array transducer consists of an arrangement of five electrical interconnected piezoceramic fibers. The findings show that the impedance spectrum depends on the fiber stresses and can be used for in-process monitoring during the joining process. Based on the impedance values the preload state of the interconnected piezoceramic fibers can be specifically controlled and a fiber overload.

In-Process Monitoring of Joining Operations for Piezoceramic Elements

The direct integration of piezo elements into micro-structured aluminum sheets is a new approach for adaptronics and lightweight constructions. With the integration of the active piezoceramic elements the aluminum sheets gain sensor and actuator functionalities. The mechanical interconnections and the preload of the piezoceramic elements are an important issue for the sensor and actuator capability of the later smart material. Post-process inspection methods to characterize the mechanical interconnection of the joining partners and the performance of the transducer after the joining operation are state of the art. Scope of the paper is the development of a novel in-process monitoring method that utilizes the piezoceramic transducer as inherent sensor for failure mode detection and preload evaluation during the joining by forming operation. Within this study, results of forming experiments with array batches of interconnected piezoceramic elements are presented. The piezoceramic batches are electrically contacted inside the joining tool and utilized as material inherent sensor during joining by forming experiments. Test samples are characterized by impedance spectroscopy during the joining operation. Based on the experimental results, a novel in-process-monitoring method utilizing the piezoceramic joining partners as inherent sensor is outlined. It is shown, that with this method a sufficient preload can be adjust on the basis of the intensity of the resonance peak without an overload. Furthermore, error effects to the transducer can be detected at an early stage.

Evaluation of Tribological Properties of AlMg4.5Mn0.7 in Massive Microforming Using the Barrel Compression Test

Tribology is one of the major issues in forming processes. It is influenced by many factors such as workpiece and tool material, lubrication, process parameters, geometric scale etc. Especially in microforming processes, friction plays an important role due to an increased surface to volume ratio and the domination of open over closed lubricant pockets. A simple and sensitive method to quantify the friction factor under realistic conditions of massive forming is the barrel compression test. The friction factor is calculated out of the friction-dependent barreling of cylinder samples while being compressed between two parallel tool surfaces.In these investigations, the barrel compression test was applied to determine the friction factor between cylindrically shaped samples made of the aluminium alloy AlMg4.5Mn0.7 (EN AW 5083) and polished surfaces made of the tool steel 1.3343. The specimen diameter was varied between 0,5 mm and 10 mm. The focus of investigations was the size-dependence of the friction factor under the variation of the parameters such as forming degree, lubrication conditions, and die velocity. In addition to the calculation of the friction factor, surfaces were evaluated by microscopy and roughness measurements.

Investigation of Tribological Properties of AlMg4,5Mn0,7 in a Warm Forming Process Using the Barrel Compression Test

Within the German Collaborative Research Center 39 PT-PIESA the forming of micro cavities into aluminum sheets is one challenging task. During this forming process high forces and stresses occur which lead to a high tool wear. Hence, the actually applied cold forming process should be replaced by a warm forming process. This paper shows the tribological investigations for the warm forming process. Within the experiments the barrel compression test is used to determine the friction conditions by varying the size of the cylindrically shaped samples of AlMg4,5Mn0,7, the forming degree and the lubrication condition (dry, graphite, forming oil). The flat punches were made from hardened steel 1.3343. The friction factor was calculated, and surface roughness was evaluated by 3D-laser microscopy. The experiments show that the friction factor increases, especially at forming degrees below 1 and for small specimen size, compared to cold forming processes. In addition to that, an influence of the lubrication condition onto the surface roughness was observed. For experiments conducted with graphite, the surface roughness is significantly higher than for samples, which were formed dryly or with forming oil.