Heterogeneous macro-micro and nanostructures were formed in the CoCrMo alloys prepared by SLM.
The interlocking of “weld lines” and elongated grains generated 3D textures resembling weaved fabrics.
The formed textures compensated the micro-faults making the SLM CoCrMo alloys “defects-tolerant”.
CrCoMo alloy specimens were successfully fabricated using selective laser melting (SLM). The aim of this study was to carefully investigate microstructure of the SLM specimens in order to understand the influence of their structural features inter-grown on different length scales ranging from nano- to macro-levels on their mechanical properties.
Two different sets of processing parameters developed for building the inner part (core) and the surface (skin) of dental prostheses were tested. Microstructures were characterized by SEM, EBSD and XRD analysis. The elemental distribution was assessed by EDS line profile analysis under TEM. The mechanical properties of the specimens were measured.
The microstructures of both specimens were characterized showing formation of grains comprised of columnar sub-grains with Mo-enrichment at the sub-grain boundaries. Clusters of columnar sub-grains grew coherently along one common crystallographic direction forming much larger single crystal grains which are intercrossing in different directions forming an overall dendrite-like microstructure. Three types of microstructural defects were occasionally observed; small voids (<10 μm), fine cracks at grain boundaries (<10 μm) and cracks at weld line boundaries (>10 μm). Despite the presence of these defects, the yield and the ultimate tensile strength (UTS) were 870 and 430 MPa and 1300 MPa and 1160 MPa, respectively, for the skin and core specimens which are higher than casted dental alloy.
Although the formation of microstructural defects is hard to be avoided during the SLM process, the SLM CoCrMo alloys can achieve improved mechanical properties than their casted counterparts, implying they are “defect-tolerant”.
Cobalt-based alloys have been extensively used in cast and hard facing forms over the past decades because of their excellent corrosion resistance, biocompatibility and strength . Over the years cobalt-chromium-molybdenum (CoCrMo) alloy has demonstrated a remarkable level of versatility and durability as an orthopedic implant material . CrCoMo alloys are adopted also for dental restorations such as customized abutments, crown and bridges in the anterior and posterior region, telescope and conical crowns, and screw-retained restorations. Lately they have been employed for body joints and fracture fixation applications as well . The use of these alloys was, however, for a long time suppressed due to rapidly emerging full ceramic restoration concepts adopted for esthetic reasons and concern toward biocompatibility and metal release aspects . Nevertheless, since machining, cutting, and plastic works of the fabricated dental components are usually difficult due to their high melting point (1623–1723 K), high hardness, and limited ductility , the CrCo dental alloys are still desired.
Processing of these dental alloys can markedly affect the mechanical properties of the resulting components; therefore it should be carefully controlled to attain the desired quality . Although investment casting and closed die forging remain the traditional processes used to manufacture CoCrMo alloy implants nowadays additive manufacturing (AM) is being accepted as the novel candidate for the fabrication of customized dental prostheses . AM can easily fabricate complex 3D parts that are undoubtedly impossible to make by other conventional methods or otherwise are very time consuming to machine and shape into desired geometry. Therefore, AM has attracted increasing attention in this respect. As mentioned before the function and long life expectancy of dental restorations and implant components are of high priority. These put high demands on their corrosion resistance and biocompatibility and also mechanical properties.
Selective laser melting (SLM), as one of the techniques in the AM family, has shown to be a promising approach for preparing dental restorations with good corrosion resistance and biocompatibility but with not so high metal release. Yolanda et al. tested different processing parameters for fabricating the CrCoMo alloy parts and then carefully examined their biocompatibility, metal release and corrosion resistance. The mechanical properties along with corrosion resistance of the SLM CrCo alloy parts have been studied by Takaichi et al. . By correlating the pore formation to the laser energy density (LED) applied they managed to achieve higher yield strength, UTS, and elongation than those of the as-cast alloy and satisfied type 5 criteria in ISO22764. Al Jabbari et al. investigated the metallurgical and interfacial characters of the porcelain fused on metal (PFM) CrCo alloy restorations fabricated by milling, cast and SLM processes. Barucca et al. studied the structural evolution of the CrCo dental restorations made by SLM and confirmed the phase transformation from γ (fcc) to ɛ (hcp) and the formation of carbides by means of electron microscopy characterization. Xiang et al. studied the bond strength between SLM CoCr alloy and ceramics system and achieved bond strength exceeding the requirement of ISO 9691:1999(E). In their work a better behavior in porcelain adherence on SLM CoCr alloy was revealed. Although all these reports seem to prove that SLM is indeed a promising method for fabricating geometrically customized CrCo dental restorations, it should be noted that many structural and microstructural defects are introduced during the process that may deteriorate the mechanical and chemical properties.
In this work selective laser melting of Co-Cr-Mo alloy was performed by applying two sets of recommended laser scanning parameters suitable, respectively, for the manufacture of densely smooth surface layer and less dense inner core of the dental restorations. The microstructures of the prepared specimens at different length scales were characterized and related to their mechanical properties and thus the underlying mechanism for their unusual defects-tolerance was investigated.
It is well understood that the extremely rapid heating induced by the laser melting of the CoCrMo alloy and the subsequent quenching to low temperatures will introduce non-equilibrium products. A technique that could possibly follow the relaxation and other changes during re-heating of the as-prepared sample is the use of mechanical loss spectrum measurements at rising temperature. The heating and following cooling cycle can be done at different heating rates, but it is important to run the same sample several cycles. In this way, the spectrum of the first run can be compared with the spectra of the following runs. A reaction peak that only occurs during the first run is the sign of a non-reversible response. Reactions that show up consequently during several runs are reversible; like a dissolution/precipitation process activated at a certain temperature. Also in this work the mechanical loss spectrum measurement in association with the followed careful microstructure characterization after each heat treatment was employed to elucidate detailed microstructure changes, particular those related to the welding pool boundaries, grain and sub-grain boundaries.