Mirko Schaper

Microstructural Characterization and Mechanical Performance of Hot Work Tool Steel Processed by Selective Laser Melting

Microstructural characterization of hot work tool steel processed by selective laser melting was carried out. The findings shed light on the interrelationship between processing parameters and the microstructural evolution. It was found that the microstructure after layer-wise processing partially consists of metastable-retained austenite which transforms to martensite in a subsequent tensile test. This improves the mechanical properties of the hot work tool steel enabling direct application.

Lattice Structures Manufactured by SLM: On the Effect of Geometrical Dimensions on Microstructure Evolution During Processing

Employing selective laser melting direct microstructure manipulation is feasible through adjustment of thermal gradients and solidification velocity. Currently, the exposure strategy and laser energy have to be adapted in order to meet a processing window suited for introducing highly anisotropic microstructures. As selective laser melting allows for production of filigree complex structures, the impact of geometry on the microstructure evolution is investigated in the current study and it is shown that miniaturization of structures as well leads to the evolution of anisotropic microstructure.

Process Analysis and Physical Simulation of Electromagnetic Joining of Thin-Walled parts

To avoid typical problems when connecting different metallic materials as aluminum and titanium as e.g. the formation of intermetallic phases, electromagnetic welding represents an alternative technology to conventional (i.e. usually thermal) joining processes. Although feasibility and potential of this technique are already proved, the fundamental correlations of partand process-parameters have not yet been investigated systematically. As an approach to examine these, the performance of model experiments and supplementary technological tests is suggested. The resulting connection quality is evaluated using metallographic methods.

Processing of New Materials by Additive Manufacturing: Iron-Based Alloys Containing Silver for Biomedical Applications

In the biomedical sector, production of bioresorbable implants remains challenging due to improper dissolution rates or deficient strength of many candidate alloys. Promising materials for overcoming the prevalent drawbacks are iron-based alloys containing silver. However, due to immiscibility of iron and silver these alloys cannot be manufactured based on conventional processing routes. In this study, iron-manganese-silver alloys were for the first time synthesized by means of additive manufacturing. Based on combined mechanical, microscopic, and electrochemical studies, it is shown that silver particles well distributed in the matrix can be obtained, leading to cathodic sites in the composite material. Eventually, this results in an increased dissolution rate of the alloy. Stress–strain curves showed that the incorporation of silver barely affects the mechanical properties.

Simulation of Gas and Spray Quenching during Extrusion of Aluminium Alloys

After the extrusion process most aluminium alloy profiles don´t satisfy the necessary strength requirements. An increase of strength can be obtained by age hardening of hardenable aluminium alloys. Age hardening includes the three steps of solution annealing, quenching and aging and is usually carried out in a separate process after extrusion. The integration of the sub-steps solution annealing and quenching in the extrusion process results in a marked reduction of the complete process chain. The applicability of this integration depends primarily on the quenching power of the cooling module and on the quench sensitivity of the aluminium alloy. Using the finite element method the non-steady-state process of quenching the profiles after leaving the extrusion press has been simulated. The boundary conditions for quenching are varied for a gas nozzle field and a spray cooling using heat transfer coefficients based on experiments. The simulation results support the design of gas nozzle fields or spray cooling for the extrusion process of different aluminium alloys.

Functional encapsulation of laser melted Inconel 718 by Arc-PVD and HVOF for post compacting by hot isostatic pressing

In this study, the Selective Laser Melting (SLM) technology was used to manufacture flat specimens from Inconel 718 powder. The SLM process parameters have a major impact on the microstructure as well as on the mechanical properties of the fabricated specimens. Despite using optimized processing parameters, defects like pores cannot be completely avoided. These pores act as stress raisers and lead to premature crack initiation under cyclic loading, eventually reducing the fatigue strength of the material. Hot Isostatic Pressing (HIP) offers the possibility to eliminate the porosity and thus to increase the fatigue performance of the material. HIP combines high pressure and high temperature to produce materials with superior properties. Unfortunately, open porosity, i.e. open pores on the surface, can prevent full densification. In the present work, SLM flat specimens were encapsulated by means of Cathodic Arc Deposition (Arc-PVD) and High Velocity Oxygen Fuel Spraying (HVOF) to seal open pores. For this purpose, different encapsulation materials were investigated with a focus on materials offering additional functions such as an improved high temperature corrosion resistance or applicability as a bond coat for thermal barrier coatings.

Optimised press-hardening process using spray cooling – process integrated heat treatment of 22MnB5 sheet metal

Abstract Press-hardening is a comparatively time consuming forming process since both forming as well as quenching is performed in the die tool. In order to accelerate this process, an alternative cooling strategy is considered which is based on spray cooling outside the die tool. The object of our investigation is 22MnB5 material. Quenching tests show that sufficient cooling rates greater than 50 K/s can be obtained by means of water-air spray cooling using two-component nozzles. The strength values and fracture strains realised here correspond to the mechanical properties attainable during press-hardening. Furthermore, a plant design is presented which is suitable for cooling vehicle body parts such as tunnels in order to reduce the turnaround time by means of prematurely removing the components from the die-forming tool and subsequently cooling in a spray field.

Influences of interface and surface pretreatment on the mechanical properties of metal-CFRP hybrid structures manufactured by resin transfer moulding

The combination of sheet metal and carbon-fibre-reinforced plastic (CFRP) is a promising approach in the sector of automotive lightweight construction. The hybrid structures allow a symbiotical usage of the specific advantages of each material. First of all, this article specifies the process chain by manufacturing hybrid materials with an intrinsic resin transfer moulding (RTM) process. Subsequently, research results regarding the interface between metal and CFRP component as well as the surface pretreatment of metallic component with laser structuring are illustrated and discussed. By means of four-point-bending tests, it is found that the mechanical properties of metal-CFRP hybrid structures are improved by using a glass fleece or an epoxy-based adhesive film as intermediate layer or due to surface pretreatment of metallic component with laser structuring. Additionally, a finite-element simulation for a four-point-bending test of a hybrid part is compared to an experiment for the linear elastic region, where strain and stress distributions are focused.

Increase the Deformability of NiCo Single Crystals Using of Electrical Pulse-Like Currents

The effect of short-term electrical pulses on metallic single crystals, with and without the application of external mechanical stresses, was investigated with the aid of deformation reliefs. The nickel-cobalt single crystals were subjected to electrical pulses and subsequently microscopically measured. In doing this, it was established that an electrical pulse without a simultaneously applied mechanical stress, has no influence on the deformation relief. It was possible to show that on loading the single crystal with a mechanical stress, the deformation relief significantly changes even when the stress was markedly below the flow stress.

Simulation of Integrated Heat‐treatment of Precision Forged Components

Precision forging with integrated heat-treatment (cooling from the forging temperature by means of spray cooling) is an innovative process for manufacturing high performance components such as surface hardened and tempered gear wheels. Using a process chain which is shortened in comparison with conventional process sequences, production times can be lowered and processing energy can be saved. With respect to the work piece, a numerical computation of the process steps from precision forging, quenching and tempering from the residual heat can be performed to support the process design. Here, the aims are to predict suitable process parameters and mechanical properties of final components. In the following, the modelling approach exemplified using of a gear wheel of hardening and tempering steel 42CrMo4 is to be introduced which is employed for the collaborative research centre 489 “Process chain for manufacturing precision forged high performance components” at the Leibniz Universitat Hannover. Microstructure developments, as e.g. recrystallisation, grain growth and microstructural transformations, as well as mechanical properties due to tempering by the residual heat are considered for this process using the commercial finite element software ANSYS®. For this purpose user specified sub routines were developed to enhance the capabilities of ANSYS® by the application of the Ansys Parametric Design Language (APDL) and User Programmable Features (UPF), respectively. Results of the computation can be verified by micrographs and hardness measurements.