Mechanical and Physical Properties of Differently Alloyed Sintered Steels as a Function of the Sintering Temperature

In this paper, the effect of processes occurring during the sintering of four powder metallurgy steel grades on the resulting properties were investigated. This included three grades prepared from plain iron powder with admixed graphite, one grade alloyed also with elemental copper and another with Fe-Mn-Si masteralloy. One further grade was prepared from Cr-Mo pre-alloyed powder with admixed graphite. The effect of the sintering processes was examined in the temperature range of 700–1300 °C in an inert atmosphere (Ar). In order to study oxygen removal, DTA/TG runs linked with mass spectrometry (MS) as well as C/O elemental analysis were performed. Charpy impact tests and fractography studies were performed to study the effect of the temperature on the formation and growth of sintering contacts. Characterization also included metallography, dimensional change, sintered density, and hardness measurements to describe the dissolution of carbon and alloying elements during the process. Physical properties that were measured were electrical conductivity and coercive force. The results showed that, in all steels, the reduction of oxides that occur during the heating stage plays a key role in the formation and growth of the sintering contacts as well as in the completion of alloying processes. In the chromium alloy steel, the presence of the stable chromium oxides delays these processes up to higher temperatures, while in the other steels that are based on plain iron powder, these processes take place earlier in the heating stage, at lower temperatures. Compared to the standard Fe-C and Fe-Cu-C grades, the Cr-Mo steel requires more sophisticated sintering to ensure oxygen removal, but on the other hand it offers the best properties. The masteralloy variant, finally, can be regarded as a highly attractive compromise between manufacturing requirements, alloy element content, and product properties.

Laser-Sintered Mg-Zn Supersaturated Solid Solution with High Corrosion Resistance

Solid solutions of Zn as an alloy element in Mg matrixes are expected to show improved corrosion resistance due to the electrode potential being positively shifted. In this study, a supersaturated solid solution of Mg-Zn alloy was achieved using mechanical alloying (MA) combined with laser sintering. In detail, supersaturated solid solution Mg-Zn powders were firstly prepared using MA, as it was able to break through the limit of phase diagram under the action of forced mechanical impact. Then, the alloyed Mg-Zn powders were shaped into parts using laser sintering, during which the limited liquid phase and short cooling time maintained the supersaturated solid solution. The Mg-Zn alloy derived from the as-milled powders for 30 h presented enhanced corrosion potential and consequently a reduced corrosion rate of 0.54 mm/year. Cell toxicity tests confirmed that the Mg-Zn solid solution possessed good cytocompatibility for potential clinical applications. This study offers a new strategy for fabricating Mg-Zn solid solutions using laser sintering with MA.

Modeling the Effect of Chemistry Changes on Phase Transformation Timing, Hardness, and Distortion in Carburized 8620 Gear Steel

AISI 8620 low carbon steel is widely used due to its relatively low cost and excellent case hardening properties. The nominal chemistry of AISI 8620 can have a large range, affecting the phase transformation timing and final hardness of a carburized case. Different vendors and different heats of steel can have different chemistries under the same AISI 8620 range which will change the result of a well-established heat treatment process. Modeling the effects of alloy element variation can save countless hours and scrap costs while providing assurance that mechanical requirements are met. The DANTE model was validated using data from a previous publication and was used to study the effect of chemistry variations on hardness and phase transformation timing. Finally, a model of high and low chemistries was executed to observe the changes in hardness, retained austenite and residual stress caused by alloy variation within the validated heat treatment process.

Multilayer Maraging/CoCrNi Composites With Synergistic Strengthening-Toughening Behavior

A novel multilayer maraging/CoCrNi composite with good mechanical properties was successfully fabricated by a vacuum hot-rolling and aging treatment. The yield strength, tensile strength, uniform elongation, and fracture elongation reached 1,151, 1,380 MPa, 15.7, and 24% respectively, realizing the aim of synergistic strengthening–toughening by effectively improving the yield strength of the CoCrNi alloy and strain-hardening capacity of the maraging steel. The vacuum state, high rolling reduction ratio, and alloy element diffusion are beneficial in strengthening the clad interface. The good work-hardening capacity of the CoCrNi alloy compensates for the poor strain-softening behavior of the maraging steel, effectively delaying the premature localized necking of the multilayer composites. The strengthening–toughening mechanism of the multilayer maraging/CoCrNi composites is mainly attributed to the strong interface, nanoscale precipitation, and strain-induced twinning.

Microstructure and alloy element distribution near the fusion line of aluminum alloy 6061 in laser wire-filling welding

Aluminum alloy 6061(AA6061) sheets of 4 mm in thickness are joined by laser wire-filling welding (LWFW) using the ER4047 welding wire. Microstructure and alloy element distributions near the fusion line are characterized and are investigated by optical microscope, scanning electron microscope, energy dispersive spectrometer. The results showed that the well-formed welded joints are obtained with a few thermal cracks near the fusion line. The coarse grain and a reduction in the weight ratio of magnesium to silicon can be observed, when the welding speed decreases under constant laser power. The thermal crack is caused by the decrease of the weight fraction of magnesium and the proportion of silicon content has an effect on the microhardness of welded joints. By properly controlling the welding speed, the various properties of AA6061 LWFW joints can be balanced.