ABSTRACT
The presented work is focused on the influence of imposed strain on the weldability of Dievar alloy. Two mechanisms affecting the microstructure and thus imparting changes in the mechanical properties were applied-heat treatment (hardening and tempering), and rotary swaging. The processed workpieces were further subjected to welding with various welding currents. In order to characterize the effects of welding on the microstructure, especially in the heat-affected zone, and determine material stability under elevated temperatures, samples for uniaxial hot compression testing at temperatures from 600 to 900 °C, optical and scanning electron microscopy, and microhardness testing were taken. The testing revealed that, although the rotary swaged and heat-treated samples featured comparable microhardness, the strength of the swaged material was approximately twice as high as that of the heat-treated one-specifically 1350 MPa. Furthermore, it was found that the rotary swaged sample exhibited favorable welding behavior when compared to the heat-treated one, when the higher welding current was applied.
ABSTRACT
This paper deals with a study of additively manufactured (by the Selective Laser Melting, SLM, method) and conventionally produced AISI 316L stainless steel and their comparison. With the intention to enhance the performance of the workpieces, each material was post-processed via hot rotary swaging under a temperature of 900 °C. The samples of each particular material were analysed regarding porosity, microhardness, high cycle fatigue, and microstructure. The obtained data has shown a significant reduction in the residual porosity and the microhardness increase to 310 HV in the sample after the hot rotary swaging. Based on the acquired data, the sample produced via SLM and post-processed by hot rotary swaging featured higher fatigue resistance compared to conventionally produced samples where the stress was set to 540 MPa. The structure of the printed samples changed from the characteristic melting pools to a structure with a lower average grain size accompanied by a decrease of a high fraction of high-angle grain boundaries and higher geometrically necessary dislocation density. Specifically, the grain size decreased from the average diameters of more than 20 µm to 3.9 µm and 4.1 µm for the SLM and conventionally prepared samples, respectively. In addition, the presented research has brought in the material constants of the Hensel-Spittel formula adapted to predict the hot flow stress evolution of the studied steel with respect to its 3D printed state.
ABSTRACT
This paper deals with the study of high-strength M300 maraging steel produced using the selective laser melting method. Heat treatment consists of solution annealing and subsequent aging; the influence of the selected aging temperatures on the final mechanical properties-microhardness and compressive yield strength-and the structure of the maraging steel are described in detail. The microstructure of the samples is examined using optical and electron microscopy. The compressive test results show that the compressive yield strength increased after heat treatment up to a treatment temperature of 480 °C and then gradually decreased. The sample aged at 480 °C also exhibited the highest observed microhardness of 562 HV. The structure of this sample changed from the original melt pools to a relatively fine-grained structure with a high fraction of high-angle grain boundaries (72%).
ABSTRACT
The presented work focused on the development of the microstructural and mechanical properties of a AISI 316L stainless steel workpiece prepared through additive manufacturing and subsequently processed by hot rotary swaging. In order to characterize the effects of swaging on the structural development, samples were taken for electron microscopy scanning and microhardness measurements were taken after each swaging reduction. The as-built and final swaged pieces were also subjected to tensile testing at room temperature and at 900 °C. The structural analyses showed that the hot swaging introduced a substructural formation; low angle grain boundaries prevailed over high angle ones after each pass. The swaging also imparted an almost complete elimination of the porosity and significant grain size; the average grain area decreased from the original value of 365.5 µm2 to 4.4 µm2 after the final swaging pass. The changes in the texture between the passes were negligible, however, the grain refinement went hand in hand with the microhardness increase (up to almost 300 HV1). The results of the tensile testing confirmed that the mechanical properties of the swaged pieces which improved dramatically and remained favorable up to high temperatures.
