Thorsten Becker

RESIDUAL STRESS MEASUREMENTS AND STRUCTURAL INTEGRITY IMPLICATIONS FOR SELECTIVE LASER MELTED TI-6AL-4V

Selective laser melting (SLM) of Ti-6Al-4V has significant potential in the aerospace and biotechnology industries. SLM employs a focused laser beam to melt successive layers of metallic powder into complex components. This process can result in the generation of high thermally-induced residual stresses. These residual stresses, together with micro-flaws/ pores from the inherent fabrication process, may lead to premature fatigue crack initiation and propagation at relatively low cyclic stresses. The hole-drilling strain gauge method was used to evaluate residual stresses within SLM Ti-6Al-4V specimens, with the intention of understanding the associated mechanisms for the successful application of SLM Ti-6Al-4V in industry.

The achievable mechanical properties of SLM produced Maraging Steel 300 components

Purpose Selective laser melting (SLM) is a process that produces near net shape parts from metallic powders. A concern with SLM-produced metals is the achievable materials performance with respect to mechanical properties. Particularly, three important aspects strongly affect the mechanical properties of the material: internal stresses resulting from steep temperature gradients and high cooling rates, the resulting microstructure and the occurrence of pores and flaws. Design/methodology/approach This paper presents SLM-produced maraging steel 300 (18Ni-300), an iron-nickel steel alloy often used in applications where high fracture toughness and strength are required. The steel’s achievable tensile, crack growth and hardness properties and the manner in which these compare to the wrought counterpart are reported. In addition, this paper investigates the porosity distribution and achievable density, residual stress levels and post-processing procedures using heat-treatments. Findings It is found that tensile properties, hardness and microstructure compare well to its wrought counterpart. Fatigue growth rates are also comparable, though they are influenced by residual stresses and microstructure. Originality/value The investigation into the mechanical performance addresses two issues: the achievable mechanical properties and the understanding of the link between the manufacturing process and the achievable material performance.

Microstructure and mechanical properties of direct metal laser sintered TI-6AL-4V

Direct metal laser sintering (DMLS) is a selective laser melting (SLM) manufacturing process that can produce near net shape parts from metallic powders. A range of materials are suitable for SLM; they include various metals such as titanium, steel, aluminium, and cobalt-chrome alloys. This paper forms part of a research drive that aims to evaluate the material performance of the SLM-manufactured metals. It presents DMLS-produced Ti-6Al-4V, a titanium alloy often used in biomedical and aerospace applications. This paper also studies the effect of several heat treatments on the microstructure and mechanical properties of Ti-6Al-4V processed by SLM. It reports the achievable mechanical properties of the alloy, including quasi-static, crack growth behaviour, density and porosity distribution, and post-processing using various heat-treatment conditions.

Heat treatment of Ti-6Al-4V produced by laserCUSING

LaserCUSING® is a selective laser melting (SLM) process that is capable of manufacturing parts by melting powder with heat input from a laser beam. LaserCUSING demonstrates potential for producing the intricate geometries specifically required for biomedical implants and aerospace applications. One main limitation to this form of rapid prototyping is the lack of published studies on the material performance of the resulting material. Studies of the material’s performance are often complicated by dependence on several factors, including starting powder properties, laser parameters, and post-processing heat treatments. This study aims to investigate the mechanical properties of LaserCUSING-produced Ti-6Al-4V and its performance relative to the conventional wrought counterpart. A combination of conventional and LaserCUSING-tailored heat treatments is performed. The resulting microstructures are studied and linked to the properties obtained from hardness tests. The findings highlight that LaserCused Ti-6Al-4V is competitive with traditional materials, provided that optimal parameters are chosen and parts are subject to tailored post-processing. In the as-built condition, LaserCused Ti-6Al-4V displays superior strength and hardness as a result of a martensitic microstructure, and a poorer performance in ductility. However, the material performance can be improved using tailored heat treatments. Careful consideration must be given to suitable post-processing before application in critical components in the aerospace or biomedical industry can occur

Selective Laser Melting Produced Ti-6Al-4V: Post-Process Heat Treatments to Achieve Superior Tensile Properties

Current post-process heat treatments applied to selective laser melting produced Ti-6Al-4V do not achieve the same microstructure and therefore superior tensile behaviour of thermomechanical processed wrought Ti-6Al-4V. Due to the growing demand for selective laser melting produced parts in industry, research and development towards improved mechanical properties is ongoing. This study is aimed at developing post-process annealing strategies to improve tensile behaviour of selective laser melting produced Ti-6Al-4V parts. Optical and electron microscopy was used to study α grain morphology as a function of annealing temperature, hold time and cooling rate. Quasi-static uniaxial tensile tests were used to measure tensile behaviour of different annealed parts. It was found that elongated α’/α grains can be fragmented into equiaxial grains through applying a high temperature annealing strategy. It is shown that bi-modal microstructures achieve a superior tensile ductility to current heat treated selective laser melting produced Ti-6Al-4V samples.

INFLUENCE OF HEAT TREATMENTS ON THE MICROSTRUCTURE AND TENSILE BEHAVIOUR OF SELECTIVE LASER MELTING-PRODUCED TI-6AL-4V PARTS

In industry, post-process heat treatments of Ti-6Al-4V are performed with the aim of improving its tensile behaviour. While heat treatments of wrought Ti6Al4V have been standardised (e.g., Aerospace Material Specification H-81200), heat treatments of selective laser melting (SLM)-produced Ti-6Al-4V lacks research and understanding. Significant concern exists about SLM Ti6-Al-4V’s achievable ductility attributed to its martensitic (α’) phase. In this research, heat treatments at a range of temperatures are applied to SLM-produced Ti-6Al-4V tensile samples. Microstructural analysis (both optically and through electron backscatter diffraction) was used to identify links between heat treatments and microstructure. Subsequently, uniaxial tensile tests were performed to determine the respective tensile properties of all samples. Correlations in the data show a significant loss in strength with respect to an increase in annealing temperature due to grain growth, while no noticeable trend was observed for fracture strain with regard to annealing temperatures.

On the impact of different system strategies on the material performance of selective laser melting- manufactured TI6AL4V components

Selective Laser Melting (SLM) is a powder-based additive manufacturing process that has gained substantial interest in recent years due to its feasibility of producing geometrically- complex metallic components for end-use in various industries, with or without post-treatment procedures. This paper presents recent research undertaken on different scanning strategies and process parameters with the purpose of providing an overview of the achievable material performance of Ti6Al4V components, and comparing its properties with the conventionally-produced parts. In order to understand their output, differences in the building strategies of the systems studied are analysed, and their influence on the resulting mechanical and metallurgical properties is highlighted.

High-temperature tensile property measurements using digital image correlation over a non-uniform temperature field:

Standard test methods for measuring temperature-specific mechanical properties are typically specimen and time intensive when the influence of varying parameters is taken into consideration. Often, multiple specimens and test runs are required to build a complete database of constitutive parameters. This article presents a numerical–experimental methodology that employs digital image correlation to measure temperature-specific tensile properties of a 12% Cr steel. The methodology presents the unique ability to obtain temperature-specific properties from a single sample: the sample is subjected to a longitudinal thermal gradient through resistive heating in a thermomechanical loading system, while the full-field capabilities of digital image correlation and thermal imaging are used to measure strain and temperature surface maps, respectively. A study on the robustness of the methodology with regard to image saturation control and stress state found the setup capable of deformation measurement up to 900 °C. Elastic moduli and Poisson’s ratios were extracted in the 480 °C–600 °C range from a single specimen. The presented metrology sets the framework for investigations that are aimed at extracting temperature-specific mechanical properties for 12% Cr steels.

The Application of Full-Field Techniques to Estimate both Tensile and Fracture Properties: an Investigation into Modifications to Standard Sample Geometries

This paper investigates modifications to the Brazilian disk and compact tension C(T) standard test geometries to allow full-field extraction of elastic tensile and toughness properties. A central notch has been introduced in the Brazilian disk to allow for the extraction of fracture properties, while the C(T) sample has been lengthened to allow for the extraction of tensile properties. Full-field displacements, measured on samples prepared from PMMA (Polymethyl methacrylate) using digital image correlation are analysed under the assumption of linear elasticity. First, the Virtual Fields Method is applied to estimate Young’s Modulus (E) and Poisson’s Ratio (v). These tensile properties are then used to determine estimates for the peak stress intensity factor (SIF) prior to fracture through a non-linear least squares field fitting approach. Test results show that a modified disk sample performed well in both analysis approaches with relative errors obtained as: 2.1% in E, 12.9% in v and 1% in SIF. A modified C(T) sample obtained relative errors of: 1.5% in E, 40.2% in v and 1% in SIF. Furthermore it is shown that field fitting approaches are less sensitive to error in v than error in E. The presented evidence aims to guide future experiments utilizing full-field techniques to study material properties in the framework of fracture mechanics, in particular in applications where limited material for obtaining tensile properties is available.

From Rapid Prototyping to Rapid Manufacturing — An Industrial and Academic Perspective

Using 3D Printing and Selective Laser Melting as representatives for the whole bunch of additive technologies the aim of this paper is to give a snap shot of its current state. Based on a brief analysis of their main characteristics the progress of this most innovative class manufacturing processes is highlighted. While selected applications from the past demonstrate their significance mainly for accelerated product development, most recent experiences illustrate their huge potential for improvement of industrial performance. Related challenges for academic and industrial research are pointed out. The paper concludes with a consolidated picture of the AM technologies within the work environment.