Frank Vollertsen

A manifold variety of additive manufacturing techniques has a significant positive impact on many industry sectors. Large components are often manufactured via directed energy deposition (DED) instead of using powder bed fusion processes (PBF). The advantages of the DED process are a high build-up rate with values up to 300 cm³/h and a nearly limitless build-up volume. In combination with the lightweight material aluminum it is possible to manufacture large lightweight components with geometries adapted to customer requirements in small batches. This contributes the pursuit of higher efficiency of machines through lightweight materials as well as lightweight design. A low-defect additive manufacturing of high strength aluminum EN AW-7075 powder via DED is an important challenge. The laser power has a significant influence on the remaining porosity. By increasing the laser power from 2 kW to 4 kW the porosity in single welding tracks can be lowered from 2.1% to only (0.09 ± 0.07)% (n = 3). However, when manufacturing larger specimens; the remaining porosity is higher than in single tracks; which can be attributed to the oxide skin on the preceding welding tracks. Further investigations regarding the mechanical properties were carried out. In tensile tests an ultimate tensile strength of (222 ± 17) MPa (n = 6) was measured. The DED processed EN AW-7075 shows comparable mechanical properties to PBF processed EN AW-7075.

In microsheet metal forming, the lengths of the contact zones between blank and die are important parameters to describe the contact behavior, but they can hardly be detected experimentally. Thus, a finite element simulation enables access to these zones and especially, their length. This length and its development during a metal forming process are analyzed in this work to establish its efficient measurement method. The part at the die called drawing edge is defined as a deformable body with material properties. Since every node of such a body can report pressure, the contact length is the product of the number of n – 1 nodes under contact pressure with the equidistant distance of lE = 1 μm between each node. It is shown, that with this method the direct determination of the contact zones position and length is enabled. Additionally, the simulative results show that the contact length is influenced by the geometrical conditions of the punch and die.

In the automotive industry, the seam edge quality of laser brazed seams is a crucial factor for the required high optical quality standards of customer-visible connections of zinc-coated car body parts. The type of zinc coating is decisive for the resulting geometry of the seam edges, which are aspired to be straight. Especially brazing of hot-dip galvanised steel with conventional brazing setup is not resulting in seam edges which fulfil the required optical quality. In this study, the wetting of copper-based filler wire on electrogalvanised and hot-dip galvanised steel sheets is analysed to increase the understanding of the influence of the wetting fronts on the seam edge quality. For this, the wetting fronts are determined in high-speed camera images and the resulting seam edges are captured by microscopic pictures. The results show that two major wetting regimes occur on both types of zinc coatings but with a different occurrence probability: one with a steep wetting front and zinc evaporation in front of wetting (“steep-angle-wetting”) and one where liquid zinc connects with the wetting front and creates a flat wetting angle (“flat-angle-wetting”). It is found that the resulting seam edge quality is mainly depending on the present wetting regime. However, for flat-angle-wetting, in the regime with insufficient seam edge quality, a self-stabilising effect is identified, which is responsible for a missing process window for brazing of hot-dip galvanised steel with sufficient seam edge quality.

The development of novel structural materials with increasing mechanical requirements is a very resource-intense process if conventional methods are used. While there are high-throughput methods for the development of functional materials, this is not the case for structural materials. Their mechanical properties are determined by their microstructure, so that increased sample volumes are needed. Furthermore, new short-time characterization techniques are required for individual samples which do not necessarily measure the desired material properties, but descriptors which can later be mapped on material properties. While universal micro-hardness testing is being commonly used, it is limited in its capability to measure sample volumes which contain a characteristic microstructure. We propose to use alternative and fast deformation techniques for spherical micro-samples in combination with classical characterization techniques such as XRD, DSC or micro magnetic methods, which deliver descriptors for the microstructural state.

As a widely used additive manufacturing technique, the laser metal deposition process (LMD) also known as direct energy deposition (DED) is often used to manufacture large-scale parts. Advantages of the LMD process are the high build-up rate as well as its nearly limitless build-up volume. To manufacture large-scale parts in lightweight design with high strength aluminium alloy EN AW-7075, the LMD process has a disadvantage that must be considered. During the process, the aluminium alloy is melted and has therefore a high solubility for hydrogen. As soon as the melt pool solidifies again, the hydrogen cannot escape the melt and hydrogen pores are formed which weakens the mechanical properties of the manufactured part. To counter this disadvantage, the hydrogen must be successfully kept away from the process zone. Therefore, the covering of the process zone with shielding gas can be improved by an additional shielding gas shroud. Furthermore, the process parameters energy input per unit length as well as the horizontal overlapping between two single tracks can be varied to minimize the pore volume. Best results can be achieved in single tracks with an elevated energy input per unit length from 3000 to 6000 J/cm. To manufacture layers, a minimal horizontal overlapping will lead to lowest pore volume, although this results in a very wavy surface, as a compromise of low pore volume and a nearly even surface a horizontal overlapping of 30 to 37% leads to a pore volume of 0.95% ± 0.50%.

The demand to implement more functionality on limited amount of space promotes miniaturization. This makes hybrid joints under various conditions and also in the micro range necessary. Existing solutions often have restrictions due to the principle of joining. A plastic forming process based on laser induced shockwaves is used to realize sheet‐sheet connections. The knowledge of joining of metallic sheets by laser‐induced shock waves should be used to enable the joining of aluminum with dissimilar materials such as glass and plastics. For industrial applications the dimensioning is important in matters of strength and feasibility of a joint. Therefore an upper bond estimation for the joining strength is developed. The design of the joint can be described on the basis of material properties. Moreover, the stiffness of the joining partners can be used as the decisive criterion for the joining possibility in the joining area. The advantages of this process are applied to different material combinations.

The material diversity in micro products is increasing and against this background, the assembly of micro components is one of the major challenges. It is necessary to implement new joining technologies, which are capable of joining different material combinations. Laser shock joining is a novel mechanical joining process. This process is based on a forming process using TEA-CO₂–laser induced shock waves. The laser irradiates directly on the forming sheet and creates a shock wave above the surface. The pressure of the shock wave leads to a forming of the sheet material. The material is formed into the joining area and creates an undercut which presents the joint. The form closure between the two materials enables a joint compared to conventional clinching processes. So far, investigations were performed to identify the process window and the joining strength for aluminum / steel joints and aluminum / glass joints. One of the most used material category in micro products are plastics. Thus, in this study the suitability of joining aluminum with plastics is investigated. The design of the joining process for the geometrical characteristics of the joining tool is explained. The geometrical process window is described and the suitability of this joining process for aluminum and plastic is shown.