Innovative Densification Process of a Fe-Cr-C Powder Metallurgy Steel

In this study, the efficacy of an innovative ultra-fast sintering technique called electro-sinter-forging (ESF) was evaluated in the densification of Fe-Cr-C steel. Although ESF proved to be effective in densifying several different metallic materials and composites, it has not yet been applied to powder metallurgy Fe-Cr-C steels. Pre-alloyed Astaloy CrM powders have been ad-mixed with either graphite or graphene and then processed by ESF. By properly tuning the process parameters, final densities higher than 99% were obtained. Mechanical properties such as hardness and transverse rupture strength (TRS) were tested on samples produced by employing different process parameters and then submitted to different post-treatments (machining, heat treatment). A final transverse rupture strength up to 1340 ± 147 MPa was achieved after heat treatment, corresponding to a hardness of 852 ± 41 HV. The experimental characterization highlighted that porosity is the main factor affecting the samples’ mechanical resistance, correlating linearly with the transverse rupture strength. Conversely, it is not possible to establish a similar interdependency between hardness and mechanical resistance, since porosity has a higher effect on the final properties.

Effect of Natural Weathering Ageing on Mechanical Properties of Virgin and Recycled Polylactic Acid

Due to a huge usage of plastics in the packaging industry, an abundant of plastics wastes is dumped at landfills. The environmental issue has given a serious impact to wildlife and humans. Hence, it is strongly advised to substitute the petroleum-based plastics with other plant-based polymers. One of the most popular bioplastics is polylactic acid (PLA). PLA can be biodegraded and recycled. The aim of this work is to investigate the effect of natural weathering ageing on the mechanical properties of virgin and recycled PLA. The investigation was focused on the tensile strength, transverse rupture strength (TRS) and impact energy. The samples were produced using the injection molding process. The samples were exposed to natural weathering continuously for 150 days. The sample inspection was monitored for every 30 days. The results showed that a decreasing in tensile strength of the recycled PLA was faster as compared to the virgin PLA. After 150 days exposed in natural weathering, there is about 41% reduction in the tensile strength of the recycled PLA. As for virgin PLA, the tensile strength is reduced approximately 11%. Similar trend was observed on the transverse rupture strength. The TRS for virgin PLA has dropped almost 14%. On the other hand, the TRS for recycled PLA declined at 17%. The diminishing in impact energy is more significant where it reduced 56% and 73% for virgin PLA and recycled PLA, respectively.

The Development of WC-Based Composite Tools for Friction Stir Welding of High-Softening-Temperature Materials

This work presents a detailed investigation for the effect of Y2O3 and Ni additions on the densification behavior, microstructural evolution and mechanical properties of a WC-Co-TaC-NbC composite. With the aim of obtaining WC-based composites with improved fracture toughness, to be used in severe conditions of high-temperature deformation, different concentrations of Y2O3 were incorporated with and without 5 wt% Ni addition. The consolidated composites were characterized using density measurement, XRD, SEM, hardness, fracture toughness, transverse rupture strength and compression testing. Fully dense composites were obtained through the applied consolidation regime of cold compaction and sintering at 1450 °C for 1.5 h under vacuum with a relative density up to 97%. The addition of 2.5 wt% Y2O3 to the base WC composite increased the relative density and then slightly decreased with the increase of the Y2O3 content. The addition of 5 wt% Ni to the base composites significantly increased the relative density to 97%. The XRD results indicated the existence of the Co3W3C η-phase after sintering, and the intensity of its peaks was reduced with the addition of 5 wt% Ni. The microstructure of the consolidated composites consisted of three phases: WC, Co3W3C and Y2O3. The area fraction of the Y2O3 phase increased as its weight fraction increased. In terms of the fracture toughness, the transverse-rupture strength (TRS) and the compressive strength were significantly improved by the addition of 5 wt% Ni with the 2.5 wt% Y2O3. Accordingly, this composition was used to manufacture the tools for the friction stir welding of the high-softening-temperature materials, which was successfully used for 25 plunges and about 500 cm of butt joints in nickel-based and carbon–steel alloys.

Evaluation of the Properties of an Electro-Sinter-Forged Bearing Steel

In this study one of the most innovative sintering techniques up to date was evaluated: Electro-Sinter-Forging (ESF). Despite it has been proved to be effective in densifying several different metallic materials and composites, bearing steels such as 100Cr6 have never been processed so far. Pre-alloyed Astaloy CrM powders have been ad-mixed with either graphite or graphene and then processed by ESF to produce a 100Cr6 equivalent composition. Porosity has been evaluated by optical microscopy and compared to that one of 100Cr6 commercial samples. Mechanical properties such as hardness and transverse rupture strength were tested on samples produced by employing different process parameters and then submitted to different treatments (machining, heat treatment). The experimental characterization highlighted that porosity is the factor mostly affecting mechanical resistance of the samples, correlating linearly to the transverse rupture strength. Hardness on the other side does not correlate to the mechanical resistance because process related cracking has a higher effect on the final properties. Promising results were obtained that give room to the sinterability by ESF of materials difficult to sinter by conventional press and sinter techniques.

Effect of Ruthenium on Microstructure and Properties of WC- (W, Ti, Ta) C-Co Cemented Carbide

Two different ruthenium content (0.5%, 1.0%) of WC - (W, Ti, Ta) C - Co cemented carbide were prepared by conventional cemented carbide production process. The results showed that adding ruthenium powder can improve the microstructure of alloy compared with excluding ruthenium carbide. The microstructure of alloy with ruthenium addition had uniform structure with less coarse grain.The addition of ruthenium could obviously promote the transverse rupture strength of WC - (W, Ti, Ta) C-Co cemented carbide, and when the adding amount was 1.0%, transverse rupture strength increased by 30%. The addition of ruthenium slightly increased the hardness of the alloy. The results of Scanning electron microscopy and spectrum analysis showed that ruthenium mainly existed in the bonding phase Co. The cutting test showed that the alloy with Ru had better wear resistance.