Wilhelm Meiners

Laser additive manufacturing of metallic components: materials, processes and mechanisms

Abstract Unlike conventional materials removal methods, additive manufacturing (AM) is based on a novel materials incremental manufacturing philosophy. Additive manufacturing implies layer by layer shaping and consolidation of powder feedstock to arbitrary configurations, normally using a computer controlled laser. The current development focus of AM is to produce complex shaped functional metallic components, including metals, alloys and metal matrix composites (MMCs), to meet demanding requirements from aerospace, defence, automotive and biomedical industries. Laser sintering (LS), laser melting (LM) and laser metal deposition (LMD) are presently regarded as the three most versatile AM processes. Laser based AM processes generally have a complex non-equilibrium physical and chemical metallurgical nature, which is material and process dependent. The influence of material characteristics and processing conditions on metallurgical mechanisms and resultant microstructural and mechanical properties of AM processed components needs to be clarified. The present review initially defines LS/LM/LMD processes and operative consolidation mechanisms for metallic components. Powder materials used for AM, in the categories of pure metal powder, prealloyed powder and multicomponent metals/alloys/MMCs powder, and associated densification mechanisms during AM are addressed. An in depth review is then presented of material and process aspects of AM, including physical aspects of materials for AM and microstructural and mechanical properties of AM processed components. The overall objective is to establish a relationship between material, process, and metallurgical mechanism for laser based AM of metallic components.

Additive manufacturing of ZrO2-Al2O3 ceramic components by selective laser melting

Purpose – The purpose this paper is to develop an additive manufacturing (AM) technique for high‐strength oxide ceramics. The process development aims at directly manufacturing fully dense ceramic freeform‐components with good mechanical properties.Design/methodology/approach – The selective laser melting of the ceramic materials zirconia and alumina has been investigated experimentally. The approach followed up is to completely melt ZrO2/Al2O3 powder mixtures by a focused laser beam. In order to reduce thermally induced stresses, the ceramic is preheated to a temperature of at least 1,600°C during the build up process.Findings – It is possible to manufacture ceramic objects with almost 100 percent density, without any sintering processes or any post‐processing. Crack‐free specimens have been manufactured that have a flexural strength of more than 500 MPa. Manufactured objects have a fine‐grained two‐phase microstructure consisting of tetragonal zirconia and alpha‐alumina.Research limitations/implications...

Investigation on reducing distortion by preheating during manufacture of aluminum components using selective laser melting

The additive manufacturing process selective laser melting (SLM) can be used to directly produce functional components made out of metal. During the construction process, however, thermally induced residual stress occurs due to the layered build-up and the local input of energy by means of a focused laser beam, which can lead to distortion of the component or sections of the component itself. Normally, distortion is prevented due to supporting structures between the component and the substrate plate. It is not always possible, however, to provide all the areas of a component with supporting structures or to remove them later, depending on how complex the geometry or how accessible the structures are. When the substrate plate is heated during the construction process, the distortion can be reduced or eliminated entirely. Nonetheless, a systematic investigation of the extent to which preheating influences distortion of aluminum components has not yet been conducted. This works aims at systematically investigating the effects of preheating during SLM of aluminum components and determining an appropriate preheating temperature at which distortion practically no longer occurs. A significant reduction in distortion compared to the distortion without preheating can be seen beginning at a preheating temperature of 150 °C. At a preheating temperature of 250 °C, distortion can no longer be detected within the scope of the measuring accuracy independent of the twin cantilever test geometry investigated. In addition to reducing distortion, the preheating avoids the stress-related cracks in the component, which can lead to tearing of the parts of the test geometry. With 90 HV 0.1 at a preheating temperature of 250 °C, the hardness is greater than the required minimum hardness according to DIN EN 1706 of die-cast parts from the material AlSi10Mg. From these results, it can be concluded that a preheating temperature of 250 °C is suitable for reliably manufacturing components made out of the material AlSi10Mg using SLM free of defects and for preventing distortion completely.

Selective laser melting of aluminum die-cast alloy—Correlations between process parameters, solidification conditions, and resulting mechanical properties

The additive production technology selective laser melting (SLM) is used for direct fabrication of metal-based functional components. SLM is one of the powder-bed based AM technologies. SLM is well established in serial production for dental restoration as well as for tooling. Main concern for industrial application remains the scope of processible materials and resulting mechanical properties. Toward processing of aluminum alloys commercially available systems exist with comparability in terms of applied process parameters and resulting mechanical properties remaining a challenge. Often no data are available concerning process parameters and mechanical properties. This holds especially for high-power SLM systems with increased build rates as a result of extended laser powers of up to 1 kW. Especially when processing aluminum alloys, the solidification conditions significantly affect the resulting microstructure in terms of size of dendrites and grains. Consequently, the present paper systematically inves...

Manufacturing of individual biodegradable bone substitute implants using selective laser melting technique

The additive manufacturing technique selective laser melting (SLM) has been successfully proved to be suitable for applications in implant manufacturing. SLM is well known for metal parts and offers direct manufacturing of three-dimensional (3D) parts with high bulk density on the base of individual 3D data, including computer tomography models of anatomical structures. Furthermore, an interconnecting porous structure with defined and reproducible pore size can be integrated during the design of the 3D virtual model of the implant. The objective of this study was to develop the SLM processes for a biodegradable composite material made of β-tricalcium phosphate (β-TCP) and poly(D, L)-lactide (PDLLA). The development of a powder composite material (β-TCP/PDLLA) suitable for the SLM process was successfully performed. The microstructure of the manufactured samples exhibit a homogeneous arrangement of ceramic and polymer. The four-point bending strength was up to 23 MPa. The X-ray diffraction (XRD) analysis of the samples confirmed β-TCP as the only present crystalline phase and the gel permeations chromatography (GPC) analysis documented a degradation of the polymer caused by the laser process less than conventional manufacturing processes. We conclude that SLM presents a new possibility to manufacture individual biodegradable implants made of β-TCP/PDLLA.

Densification behavior, microstructure evolution, and wear property of TiC nanoparticle reinforced AlSi10Mg bulk-form nanocomposites prepared by selective laser melting

Selective laser melting (SLM), due to its unique additive manufacturing processing philosophy, demonstrates a high potential in producing bulk-form nanocomposites with novel nanostructures and enhanced properties. In this study, the nanoscale TiC particle reinforced AlSi10Mg nanocomposite parts were produced by SLM process. The influence of “laser energy per unit length” (LEPUL) on densification behavior, microstructural evolution, and wear property of SLM-processed nanocomposites was studied. It showed that using an insufficient LEPUL of 250 J/m lowered the SLM densification due to the balling effect and the formation of residual pores. The highest densification level (>98% theoretical density) was achieved for SLM-processed parts processed at the LEPUL of 700 J/m. The TiC reinforcement in SLM-processed parts experienced a structural change from the standard nanoscale particle morphology (the average size 75–92 nm) to the relatively coarsened submicron structure (the mean particle size 161 nm) as the app...

Development and characterization of a coronary polylactic acid stent prototype generated by selective laser melting

In-stent restenosis is still an important issue and stent thrombosis is an unresolved risk after coronary intervention. Biodegradable stents would provide initial scaffolding of the stenosed segment and disappear subsequently. The additive manufacturing technology Selective Laser Melting (SLM) enables rapid, parallel, and raw material saving generation of complex 3- dimensional structures with extensive geometric freedom and is currently in use in orthopedic or dental applications. Here, SLM process parameters were adapted for poly-l-lactid acid (PLLA) and PLLA-co-poly-ε-caprolactone (PCL) powders to generate degradable coronary stent prototypes. Biocompatibility of both polymers was evidenced by assessment of cell morphology and of metabolic and adhesive activity at direct and indirect contact with human coronary artery smooth muscle cells, umbilical vein endothelial cells, and endothelial progenitor cells. γ-sterilization was demonstrated to guarantee safety of SLM-processed parts. From PLLA and PCL, stent prototypes were successfully generated and post-processing by spray- and dip-coating proved to thoroughly smoothen stent surfaces. In conclusion, for the first time, biodegradable polymers and the SLM technique were combined for the manufacturing of customized biodegradable coronary artery stent prototypes. SLM is advocated for the development of biodegradable coronary PLLA and PCL stents, potentially optimized for future bifurcation applications.

Structural evolution and formation mechanisms of TiC/Ti nanocomposites prepared by high-energy mechanical alloying

In this work, high-energy ball milling of a micrometre-scaled Ti and TiC powder mixture was performed to prepare TiC/Ti nanocomposites. The constituent phases and microstructural characteristics of the milled powders were studied by an x-ray diffractometer, a scanning electron microscope, an energy dispersive x-ray spectroscope and a transmission electron microscope. Formation mechanisms and theoretical basis of the microstructural development were elucidated. It showed that on increasing the applied milling time, the structures of the Ti constituent experienced a successive change from hcp (5 h) to fcc (10 h) and finally to an amorphous state (≥15 h). The hydrostatic stresses caused by the excess free volume at grain boundaries were calculated to be 3.96 and 5.59 GPa for the Ti constituent in 5 and 10 h milled powders, which was responsible for the hcp to fcc polymorphic change. The amorphization of Ti constituent was due to the large defect concentration induced by severe plastic deformation during milling. The milled powder particles underwent two stages of significant refinement at 10 and 20 h during milling. For a higher milling time above 25 h, powder characteristics and chemical compositions became stable. The competitive action and the final equilibrium between the mechanisms of fracturing and cold welding accounted for the microstructural evolution. The ball milled products were typically nanocomposite powders featured by a nanocrystalline/amorphous Ti matrix reinforced with uniformly dispersed TiC nanoparticles. The finest crystalline sizes of the Ti and TiC constituents were 17.2 nm (after 10 h milling) and 13.5 nm (after 20 h milling), respectively.

Bulk-form TiCx/Ti nanocomposites with controlled nanostructure prepared by a new method: selective laser melting

A novel selective laser melting method was applied to consolidate the high-energy ball-milled nanostructured TiCP/Ti composite powder to prepare TiCx/Ti nanocomposites in bulk form. The substoichiometric TiC0.625 with a hexagonal crystal structure acted as the reinforcing phase, having a lamellar nanostructure with an average thickness of ~48 nm. The nanostructure of TiC0.625 was coarsened and finally disappeared on decreasing the applied linear laser energy density. Reasonable physical mechanisms and conditions for the formation of nanostructured TiCx during laser processing were proposed. It was revealed that the formation of nanoscale hexagonal structured TiC0.625 was due to the action of microscopic pressure induced by evaporative recoil and surface tension on (1 1 1) plane of the lamellar TiCx crystals. The disappearance of nanostructure of TiC0.625 at a lower laser energy input was ascribed to a decreased microscopic pressure and an elevated carbon activity in the molten pool.

Manufacturing of bone substitute implants using Selective Laser Melting

Selective Laser Melting (SLM) is a Rapid Manufacturing technique which enables direct manufacturing of three-dimensional parts with high bulk density on the base of individual three-dimensional data, including computer tomography models of anatomical structures. A new approach follows a regenerative strategy for manufacturing of bone substitute implants. The SLM process is adapted for bioresorbable materials (biopolymer, bioceramic) to build individual implants which will dissolve in the body and be replaced by new bone. The pore structure of the implant is essential for bony ingrowth especially for critical size defects. With the additive technique SLM it is possible to manufacture implants with a defined internal porous structure. Depending on the field of application different materials will gain different specifications concerning for example the degradation time in the body or the mechanical properties of the implant. Using Selective Laser Melting the manufacturing of individual bioresorbable implants will be possible. This work describes the new approach for the qualification of SLM to process polylactide (PLLA) and a composite material polylactide and β-tricalcium phosphate (PLLA/β-TCP). To achieve this the materials morphology has to be prepared for the SLM process and a processing window for the above mentioned materials has to be identified varying the process parameters such as laser power, scanning velocity and laser beam diameter. The process temperature is kept low to prevent degradation of the polymer during melting and to prevent a phase transition of the bioceramic β-TCP.