Jean Pierre Kruth

Additively manufactured porous tantalum implants

The medical device industrys interest in open porous, metallic biomaterials has increased in response to additive manufacturing techniques enabling the production of complex shapes that cannot be produced with conventional techniques. Tantalum is an important metal for medical devices because of its good biocompatibility. In this study selective laser melting technology was used for the first time to manufacture highly porous pure tantalum implants with fully interconnected open pores. The architecture of the porous structure in combination with the material properties of tantalum result in mechanical properties close to those of human bone and allow for bone ingrowth. The bone regeneration performance of the porous tantalum was evaluated in vivo using an orthotopic load-bearing bone defect model in the rat femur. After 12 weeks, substantial bone ingrowth, good quality of the regenerated bone and a strong, functional implant-bone interface connection were observed. Compared to identical porous Ti-6Al-4V structures, laser-melted tantalum shows excellent osteoconductive properties, has a higher normalized fatigue strength and allows for more plastic deformation due to its high ductility. It is therefore concluded that this is a first step towards a new generation of open porous tantalum implants manufactured using selective laser melting.

Processing AlSi10Mg by selective laser melting: parameter optimisation and material characterisation

Abstract Owing to their attractive combination of mechanical properties, high heat conductivity and low weight, the Al–Si alloys found a large number of applications in the Additive Manufacturing field for automotive, aerospace and domestic industries. However, due to their high reflectivity and heat conductivity, they are harder to process by Selective Laser Melting. This work elaborates on both the optimisation of process parameters, in order to get nearly fully dense parts, and the material properties resulting from this specific material process combination. A process parameter window is defined, in which the formed melt pool is stable and meets the set requirements. In this process window, the parameter set for optimal density is defined. It is shown that AlSi10Mg parts produced by SLM have mechanical properties higher or at least comparable to the cast material because of the very fine microstructure.

Methods for the sequencing of sheet metal bending operations

The sequencing of part set-ups, in the context of design verification or process planning activities for sheet metal bending operations, is a rather complex combinatorial problem. The verification of the feasibility and the acceptability of a single bend set-up requires CPU-time consuming operations, e.g. collision checking and a manipulation requirement analysis. For more complex parts, and thus increased numbers of bends, exhaustive search methods can therefore not be applied in a time-economic way. Although the identification of an optimal sequence may not always be possible, a number of techniques can be applied to significantly downscale the problem size. Several complementary approaches have been worked out to proceed in identifying near-optimum feasible bend sequences. Constraint solving and branch-and-bound techniques are used to identify interesting potential solutions. The reported branch-and-bound search method is characterized by a dynamically updated penalty system, that reflects the manufact...

Fatigue behaviour of NiTi shape memory alloy scaffolds produced by SLM, a unit cell design comparison

Selective laser melting (SLM) is an additive manufacturing technique able to produce complex functional parts via successively melting layers of metal powder. This process grants the freedom to design highly complex scaffold components to allow bone ingrowth and aid mechanical anchorage. This paper investigates the compression fatigue behaviour of three different unit cells (octahedron, cellular gyroid and sheet gyroid) of SLM nitinol scaffolds. It was found that triply periodic minimal surfaces display superior static mechanical properties in comparison to conventional octahedron beam lattice structures at identical volume fractions. Fatigue resistance was also found to be highly geometry dependent due to the effects of AM processing techniques on the surface topography and notch sensitivity. Geometries minimising nodal points and the staircase effect displayed the greatest fatigue resistance when normalized to yield strength. Furthermore oxygen analysis showed a large oxygen uptake during SLM processing which must be altered to meet ASTM medical grade standards and may significantly reduce fatigue life. These achieved fatigue properties indicate that NiTi scaffolds produced via SLM can provide sufficient mechanical support over an implants lifetime within stress range values experienced in real life.

Composite wires with high tensile core for wire EDM

Abstract In recent years wire EDM cutting speeds and final surface roughness have been continuously improving. Much higher cutting speeds during roughing and surface roughness values lower than 0.1 μm Ra during finishing can be obtained. These enhancements are mostly attributed to the use of more advanced generator technology, but also to the type of wire. This paper discusses the use of new composite wires comprising a high tensile core and several coatings. Several wire compositions are introduced and experimentally tested. The use of a very high tensile core, the application of a layer that prevents the process heat from weakening the wire core and the presence of a superficial top coating with different possible functions are discussed. Results obtained while cutting with prototype wires show that a significant rise in accuracy especially in corner cutting is attained, while the cutting rate is at a comparable level as commercial reference wires.

Algorithms for the Design Verification and Automatic Process Planning for Bent Sheet Metal Parts

Abstract Sheet metal bending processes require detailed process planning in order to eliminate infeasible set-ups or avoidable economically demanding manipulation requirements. In this paper it is demonstrated how the complexity of a bend sequencing task can be handled by means of an efficient reduction of the search field through well-chosen representation schemes and the identification of geometric constraints. The expertise established by experienced process planners was integrated in search algorithms based on precedence constraint solving and dynamic branch-and-bound techniques. A case study is used to illustrate these procedures. Additionally, the characteristics of a dedicated collision detection algorithm, developed for fast sequence verification during search procedures, are outlined.

A study of degradation of laser‐sintered moulds using wear tests

Purpose – Surface changes like wear or local plastic deformation occurring during service of a mould make it unsuitable for further use and determine its life‐time. The performance of the mould and its design are important aspects that influence its longevity. The purpose of this paper is to study the behaviour of mould materials manufactured by laser sintering, a type of rapid manufacturing process, and to explain their relative in‐service performance.Design/methodology/approach – Mould specimens were made by two laser‐sintering machines (a DTM Sinterstation and an EOSint M250 Xtended equipped with CO2 lasers) using two iron‐based commercial powders, i.e. LaserForm ST‐100 and DirectSteel 20 V1, respectively. They have been subjected to two types of wear tests, i.e. bidirectional fretting and unidirectional pin‐on‐disc tests, under loading conditions leading to wear and/or plastic deformation of the specimens. The results obtained show similar trends to that obtained from the real‐life performance of poly...

Methods for Monitoring of Laser Cutting by Means of Acoustic and Photodiode Sensors

Laser cutting is a well-established sheet metal processing method. Nowadays a trend towards the cutting of thick plates (> 15 mm) can be observed. However for these thick plates the process window in which good cutting results can be obtained is more narrow compared to that for thin sheets due to the very difficult balance to be found between the different process parameters. Even after determination of the process window, a good cutting quality cannot always be guaranteed. Therefore cutting of thick plates is still characterized by a large scrap percentage, which impedes a breakthrough to large scale industrial use. A solution to this problem is to incorporate a sensor system in the laser cutting machine that monitors the cut quality on-line. This monitoring system could then be integrated in a process control system, which adapts the process parameters in function of the observed cut quality in real time. In this way a good cut quality could always be guaranteed. In this study, the first step in this direction, the determination of an appropriate monitoring system, is dealt with. The applicability for monitoring purposes of two types of sensors is investigated: a microphone and a photodiode. For both types, correlation between the sensor output and the cut quality is investigated in a qualitative way. The scope of the reported research was not limited to contour cutting, also piercing is covered in the study.

Acoustic and Optical Monitoring of High-Power CO2 Laser Cutting

In this paper, the development of a monitoring system for high-power CO2 laser cutting of thick steel plates (>15 mm) is reported. The aim of this system is to increase the robustness and autonomy of the laser cutting process of thick plates, which is still characterized by more narrow process windows compared to cutting of thin sheets. The applicability for monitoring purposes of two types of sensors is investigated: the acoustic microphone and the photodiode. For both types, correlation between the sensor output and the cut quality is investigated. Both contour cutting and piercing are covered in the study. The full penetration of the piercing can be monitored by both sensors. Furthermore quantitative relations between cut quality parameters and photodiode signal parameters could be determined: the mean level of the photodiode signal correlates well with the drag of the striations and dross formation, whereas the standard deviation proves to correlate well with the occurrence of burning defects and the cut edge roughness.

Microstructural Investigation of M2 High Speed Steel Produced by Selective Laser Melting: Microstructural Investigation of M2 High Speed Steel

In SLM, a high energy density is applied by a laser to locally melt layers of metallic powder particles in order to create functional parts. This high energy intensity and additive process character create a specific/unique microstructure. The solidified melt pools in HSS M2 SLM parts are observed to consist of two distinct zones next to the heat affected zone. The first zone, at the bottom of the melt pool, consists of very fine cells and columnar dendrites and has a hardness of 655±80Hv0.1. The other zone contains acicular martensite and has a hardness of 859±80Hv0.1 which is already similar to the hardness of conventionally produced and heat treated HSS M2. Furthermore, the relative amount of these two zones is observed to be dependent on the scan velocity of the laser and hence can be controlled during the SLM process.