Microstructural and Mechanical Characterization of Square‐Celled TRIP Steel Honeycomb Structures Produced by Electron Beam Melting

Abstract

Material-and energy-saving lightweight constructions are of particular interest to the industry since decades, especially for vehicle and aircraft production. One of the ways of reducing the weight in components is to replace a solid matrix by periodically recurring cell or truss structure. Cellular materials such as metallic foams, truss, or honeycomb structures are characterized by high specific energy absorption and rigidness. Due to their high specific stiffness and compressive strength, square honeycomb structures are well suited for energy dissipating stiffeners, such as sandwich panels and bumpers. [1][2][3] However, the production of honeycomb structures using conventional manufacturing methods is very complex. Metallic square-celled honeycomb structures are often produced in two ways: either slotted metal strips are inserted into each other and joined together by soldering, or they are produced by metal extrusion using complex dies. [4,5] Another recently developed technology is the extrusion of powders with a binder and subsequent debinding and sintering. [6,7] In contrast, additive manufacturing (AM) allows producing most of the complex lightweight structures in a single manufacturing step. All of the AM methods are based on the computer-aided design (CAD), taking a model of a component and building it up layer by layer. One of the most advanced AM methods for the fabrication of metallic components is the electron beam powder-bed fusion (EB-PBF) technology, which is called electron beam melting (EBM) in the following. This process belongs to the group of powderbed AM technologies in which powder particles are fused layer by layer. [8,9] The EBM process is somewhat similar to the widely used selective laser melting (SLM) process, although there are principal differences between laser and electron beam. [10] The microstructure of EBM-manufactured materials is often characterized by columnar and epitaxial grain morphology. Continuous melt crystallization through the multiple layers results in the formation of a strong texture and anisotropy. [8,[11][12][13] This phenomenon can even be utilized to produce single crystalline superalloys by adapting EBM scanning strategy. [14,15] However, in case of materials which can undergo phase transformation in the solid state during cooling (Ti-6Al-4V, Ti-6Al-4V doped with Cu or La, titanium aluminide, and so on), the mentioned columnar and textured structure can be avoided.

Cyclic Crack Growth in Chemically Tailored Isotropic Austenitic Steel Processed by Electron Beam Powder Bed Fusion

“…Thus, both parameter sets resulted in a fine-grained microstructure of very similar appearance characterized by slightly elongated grains along the building direction. Based on these results and data from the literature [ 10 , 19 , 21 ], it is assumed that all other material states processed by PBF-EB in the present work exhibited similar, narrow grain size distribution and similar grain morphology.…”

“…Alongside the elongated grains, the crack growth is accelerated whereas it is decelerated when cracks are growing perpendicular to the longitudinal axis of the grains. In contrast to the microstructure of AISI 316L, the austenitic steel considered in the present work only exhibits a slightly elongated grain shape as discussed in Section 3.1 and as shown previously [ 10 , 21 ]. Instead, an isotropic microstructure with almost equiaxed grains developed due to the cyclic heat treatment inherent to the PBF-EB-process.…”

“…Different aspects of this PBF-EB processed CrMnNi steel have been investigated and published so far. Besides the influence of different processing strategies [ 10 , 19 ] on microstructure evolution, studies have focused on the quasi-static properties [ 10 , 19 ]; the cyclic deformation behavior, which has been investigated in total strain controlled fatigue tests [ 20 ]; feasibility analyses considering different kinds of square-celled structures and their mechanical behavior during compression testing [ 21 , 22 ]; and the influence of a process-induced variation in manganese fraction on the tensile properties [ 19 ]. The latter was achieved by adjusting the beam parameters and energy input, respectively, and, thereby, varying the extent of evaporation of manganese during PBF-EB processing.…”

Stabilizing post-yielding behavior of a stretching dominated lattice structure through microstructural optimization

“…Damage-tolerant alloys are often materials exhibiting deformation mechanisms such as TRIP (Transformation Induced Plasticity) or TWIP (TWinning Induced Plasticity) because their excellent work-hardening capacity drastically improve their resistance to strain localization. Some authors have recently reported the successful fabrication of TRIP [12,[15][16][17] or TWIP [13,14,18] steels by AM. For lightweight applications, Ti-alloys are good candidates due their good specific strength.…”