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V prispevku predstavljamo izhodišča in smernice poteka razvoja naprednega orodnega jekla za najzahtevnejše aplikacije v industriji nožev, ki se izvaja v podjetju SIJ Ravne Systems v povezavi z Univerzo v Mariboru Fakulteto za strojništvo. Razvoj je na eni strani povezan z identifikacijo ključnih lastnosti orodnega jekla, ki predstavljajo konkurenčno prednost v primerjavi z ostalimi konvencionalnimi jekli in to so: žilavost, trdota, trdnost in rezilna sposobnost. Po drugi strani pa razvoj predstavlja tudi vpeljavo ustrezne tehnologije izdelave in naknadne toplotne oziroma mehanske obdelave orodnega jekla, kar omogoča doseganje ključnih lastnosti izdelka – nožev.

Heat-affected zones (HAZs) in real welds are usually quite narrow, and consequently most standard mechanical tests are difficult or even impossible. Therefore, simulated microstructures are often used for mechanical tests. However, the most often used weld thermal cycle simulator produces only a few millimeters wide area of simulated microstructure in the middle of specimens. Consequently, these kind of simulated specimen are not suitable for standard tensile tests, and even for Charpy impact tests, the simulated area can be too narrow. Therefore, to investigate the mechanical properties of a fine-grain heat-affected zone in 18CrNiMo7-6 steel, two methods were used for simulation of as-welded microstructures: (a) a weld thermal cycle simulator, and (b) as an alternative, though not yet verified option, austenitizing in a laboratory furnace + water quenching. The microstructures were compared and mechanical properties investigated. The grain sizes of the simulated specimens were 10.9 μm (water-quenched) and 12.6 μm (simulator), whereby the deviations from the real weld were less than 10%. Both types of simulated specimen were used for hardness measurement, Charpy impact tests, and fatigue tests. Water-quenched specimens were large enough to enable standard tensile testing. A hardness of 425 HV, yield strength Rp02 = 1121 MPa, tensile strength Rm = 1475 MPa, impact energy KV = 73.11 J, and crack propagation threshold ΔKthR = 4.33 MPa m0.5 were obtained with the water quenched specimens, and 419 HV, KV = 101.49 J, and ΔKthR = 3.4 MPa m0.5 with the specimens prepared with the simulator. Comparison of the results confirmed that the annealed and quenched specimens were suitable for mechanical tests of FG HAZs, even for standard tensile tests. Due to the use of simulated test specimens, the mechanical properties determined can be linked to the FG HAZ microstructure in 18CrNiMo7-6 steel.

The steel 31Mn4+QT630 is used frequently for mining support arches. The supports are cold strained, first during service, and again by re-rolling prior to reinstallation, which results in strain hardening and a loss of ductility. Consequently, many of the rerolled arch-segments are not suitable for reinstallation because their mechanical properties are inadequate. The objective of this work was to assess the feasibility of restoration of the required mechanical properties by means of a cost-efficient single step heat treatment. Specimens were cold deformed to different degrees in the range 0 % to 45 % to establish the relation between the degree of cold deformation and the hardness. Differently strain hardened specimens were subjected to subcritical annealing at temperatures 450 °C to 700 °C in the duration 0.5 h to 8 h to determine a suitable time-temperature combinations. Microstructures and mechanical properties were investigated of as-received, cold strained and recrystallized specimens. Tests performed were optical microscopy, scanning electron microscopy, tensile tests, hardness measurements and Charpy impact tests. Elongation at break of the as-received material was below the requirements of the applicable standard, and its microstructure contained significant fractions of pre-eutectoid ferrite and pearlite. Upon cold straining, hardness increased by approximately 2 HV per 1 % of strain. After one-hour recrystallization at 600 °C to 620 °C, the microstructure consisted of a ferritic matrix containing evenly dispersed globular carbide particles. The original ductility was restored, while the elongation, yield strength, and hardness were higher than in the as-received condition. These results confirmed that it is feasible to restore the original mechanical properties of heavily strained profiles with an adequate single-step heat treatment. Furthermore, they indicated that it should be possible to recover the required properties of inhomogeneously strained material with the same set of well optimized heat treatment parameters. Consequently, it should be possible to increase the reuse rate and to decrease the costs for new support arches significantly.

Purpose Melting, fusion and solidification are the principal mechanisms used in selective laser melting to produce a product. Several thermal phenomena occur during the fabrication process, such as powder melting, melt pool formation, mixing of materials (fusion), rapid solidification, re-melting, high thermal gradient, reheating and cooling. These phenomena result in several types of pores, defects, irregular surfaces, bending and residual stress. This paper aims to focus on the physical behaviors of Ti-6Al-4V alloy at several scanning speeds and their effect on porosity and metallurgical properties. Design/methodology/approach Seven scanning speeds between 150 and 1000 mm/s were chosen to observe the occurrence of different pores, defects and microstructural formations and their effect on hardness and tensile properties. Findings The various mentioned malformations occur due to the results of possible uncertainties during the melting-fusion-solidification process. Size, shape, number, location and content of the pores varied in different samples. The a cicular a' size changes with different scanning speeds. Eventually, both porosity and microstructure have shown influential consequences on the hardness and tensile properties in the samples manufactured with different scanning speeds. Originality/value This study showed the adverse effects of different physical behaviors that occurred during the fabrication process, leading to the formation of complex pores. The causations and plausible solutions of the pore formation are interpreted in this paper. The authors observe that a circular a' size differed with scanning speeds, and these influence the mechanical properties.

Different energy densities in Selective Laser Melting produce different metallurgical properties of the product surface, which include adhered powder particles, waviness, porosity, and microstructure. In this research, the corrosion behavior of these structural properties was investigated and compared with heat-treated samples. The size and arrays of α'-martensitic varied significantly due to the effects of different cooling rates and the number of effective thermal cycles caused by different scanning speeds and energy densities. The surface particles experienced different thermal effects that significantly changed their microstructure with each other and with respect to the main surface, which remarkably affected the corrosion properties. The densities, hardness, and tensile properties of the samples were also presented and interpreted their relations with corrosion. These correlations between the metallurgical and corrosion properties of SLM products support biomedical engineers in the further development of bespoke medical devices, especially implants.

This study has investigated the causes of pores' formation during fabrication of the first few layers on the support structure and the process of product improvement in Selective Laser Melting. The notable pores are, fundamentally, formation of the void with uneven shape and the voids containing partially melted or stuck particles. The voids spread over a few consecutive layers from the starting one. The porosity occurred due to unsupported areas among supporting points, improper melt pool formation, melt pool's instability, inconsistency of heat transfer, crescent shape formation, uprooting of the lifted edge by the powder re-coater, and materials' spattering. These phenomena can be regulated by controlling the amount of energy input, melting time, amount of re-melting, delaying between melting and re-melting, and reheating and cooling timing using the combinations of laser powers, scanning speeds, and hatch spacings. Eventually, a sample having almost flawless starting layers was fabricated from Ti-6Al-4V powder.

Properties of a product fabricated using Selective Laser Melting (SLM) technology depend upon its building orientation as well as Energy Density (ED) used for its fabrication process. As ED is the key factor among all the process parameters in SLM technology, this study has focused on ED along with building orientation. Seven sets of EDs and four sets of building orientations for each set of ED were selected to investigate the dissimilarities among tensile properties as well as other properties of Ti-6Al-4V alloy specimens. The scanning speed was varied to set the different ED values as it influences the thermal characteristics of the molten pool more than the other processing parameters. Tensile strengths of the specimens have differed significantly with respect to building orientations as well as EDs. The tensile characteristics of the specimens have been explored by tensile testing and analyzed regarding metallurgical properties, which are density, porosity, defect, microstructure, and surface morphology. Specimens built in lengthwise vertical position and the energy density of 65 J/mm3 have shown the best tensile properties in this study.

Weld cladding or weld overlay is a frequently used method for repair welding of damaged surfaces and for production of different surface coatings. The conventional coated electrodes have a circular cross-section. In order to increase the productivity and to decrease dilution and the depth of the heat affected zone (HAZ), the geometry of the electrode core was modified. Experimental weld cladding was carried out with rutile coated electrodes of rectangular cross-sections of 12.56×1 mm2, and for reference, also with a conventional φ 4 mm electrode Rutilen 2000 S. The coating of rectangular electrodes was identical and the core material almost identical to the materials of the standard electrode. The base material was the structural steel 355JR. The goal of investigation was to determine the welding parameters for the rectangular electrodes and to compare geometries and mechanical properties of the welds. Hardness and the dimensions of weld metal and HAZ were measured. Results with the 6.28×2 mm 2 and 6.28×2 mm electrode were similar to the results with the standard electrode. However, with the 12.56×1 mm2 rectangular electrode, significantly lower currents were sufficient to obtain a good quality of the deposition layer. Due to possibility to weld with currents as low as 80-100 A, shallower and smaller HAZs and less dilution can be achieved with the rectangular 12.56×1 mm2 electrode than with standard cylindrical φ 4 mm electrode.

A novel, Nb- and Si-rich and Be-free Ni-based alloy was cast by two methods of investment casting and continuous casting to study the microstructure evolution during solidification and its mechanical properties. The solidification of the alloy started with the primary crystallization of FCC-γ, followed by a binary eutectic reaction, with the formation of a heterogeneous constituent: FCC-γ+G-phase, which replaced the low-melting eutectic (FCC-γ+NiBe) in the Be-bearing alloys. AlNi6Si3 and γ′ formed during the terminal stages of solidification by investment casting, while the formation of AlNi6Si3 was suppressed by continuous casting. The Scheil solidification model agreed very well with the experimental results.

Improving the metallurgical properties of the products fabricated by the Selective laser melting (SLM) process is still a challenge focusing on processing parameters and Energy Density (ED). In this study, the density, porosity and microstructure of cuboid Ti-6Al-4V alloy samples fabricated by the SLM process were investigated, paying particular attention to the manufacturing key factor ED. Seven different EDs between 39 J/mm3 and 260 J/mm3 were achieved by varying the scanning speed from 1000 mm/s to 150 mm/s. The results show the ED, as well as cooling conditions, have a great influence on the above mentioned metallurgical properties. Significantly different densities have been observed, with different Eds where the difference between the sample with the highest relative density and the sample with the lowest relative density was almost 2.45%. The highest relative product density of almost 99.45% was obtained at the ED value of 65 J/mm3. The volume, shape and numbers of pores were different with different EDs. The resultant microstructure consisted of four typical hierarchical α′ martensites, namely, primary, secondary, tertiary and quartic α′ martensites within columnar prior β grains.

In this paper we present research that is connected to the performance of a series of experiments combined with the vacuum-induction melting and continuous vertical casting of a NiTi alloy in order to produce the strand. The theoretical chosen parameters made it possible to obtain a continuously cast strand with a diameter of 11 mm. The strand microstructures were investigated with a light and scanning electron microscope, while the chemical composition of the single phase was identified with the semi-quantitative micro-analysis energy-dispersive X-ray spectroscopy and inductively coupled plasma – optical emission spectrometry. The research showed that the microstructure is dendritic, where in the inter-dendritic region the eutectic is composed of a dark NiTi phase and a bright TiNi3–x phase. In some areas we found Ti carbides and phases rich in Fe. The micro-chemical analysis of the NiTi strand showed that the composition changed over the cross and longitudinal sections, which is proof that the as-cast alloys are inhomogeneous. In the final part, the electrochemical behaviours of NiTi strand samples were compared to a commercially available NiTi cast alloy with the same composition.

The surface morphology of a product plays a crucial role under mechanical loading and chemical environment. Surfaces of Selective Laser Melting (SLM) products often contain high roughness, which varies in different planes as well. The authors have explored the surface characteristics of the SLM samples that are influenced by different combinations of laser processing parameters. The considered processing parameters were Energy Density (ED) and its technological parameters namely laser power, scanning speed and hatch spacing. Additionally, a comparison study has been executed by rescanning effects considering melting with low ED and, thereafter, rescanning by the best possible laser processing parameters. The results evidently showed that the surface morphologies differ significantly due to different laser processing parameters. Eventually, the thermal and physical behavior of materials, such as the viscosity of the melt pool, thermal and physical stability of the melt pool, solidification time, cooling time, shrinkage, capillary effect, surface tension, balling effect, and the amount of melting of a powder particle, influenced the surface properties of the samples, along with unpredictability. The results showed an interesting correlation between the processing parameters and the occurrence of microcracks on the vertical walls of the specimens caused by the partially melted adhered powder particles.

The research focuses on the mechanical properties and fatigue behaviour of individual parts of Heat Affected Zones (HAZ) in a weld joint on 18CrNiMo7-6 steel. In real welds, the HAZ is heterogeneous and narrow. Therefore, specimens having a desired HAZ microstructure uniform over the whole specimens‘ volume were prepared artificially by water quenching: The Coarse Grain HAZ (CG HAZ), with average grain size of 200 µm, Fine Grain HAZ (FG HAZ) with average grain size of 10 µm, and CG-FG HAZ with average grain size of 25 µm. Comparison with a real weld revealed that microstructures of specimens corresponded very well to the microstructures in HAZ in a real weld joint. Fatigue crack growth and the threshold for long crack propagation ΔKthR were determined on HAZ microstructures. A weld toe and its stress concentration due to weld cap were simulated by a U shaped groove on the fatigued specimens. Real HAZ often contains small defects where a crack usually initiates. Therefore, a defect was simulated with a Vickers impress at the bottom of the groove in all three HAZ-microstructures. Investigation revealed that the threshold for long crack propagation ΔKthR of the FG HAZ was the lowest, not much higher was the threshold in CG HAZ, while the threshold ΔKthR in CG-FG HAZ was significantly higher. On the other hand, long crack propagation in CG HAZ is faster in comparison to the long crack propagation in FG HAZ. In all three cases, a Paris curve and its coefficients were determined for R ratios −1 and 0.1. Fatigue tests revealed that some cracks, which had been initiated below the fatigue limit in CG-FG HAZ, stopped propagating due to strong grain boundary barriers and high toughness, but, in the cases of CG HAZ and FG HAZ, the cracks propagated without stopping until the breakage of the specimen. The fatigue limits were also determined for all three HAZ-microstructures in the presence of the defect.

In respect to the cost of constituents, Cu–Al–Ni alloys can be regarded as low-cost shape memory alloys. However, the processing is quite costly due to the brittleness of the ingots. Therefore it would be advantageous to produce semi-finished products directly from the melt via highly productive technique. A laboratory device for vertical continuous casting directly from the vacuum induction melting furnace was experimentally employed for continuous casting of the Cu-13 wt.% Al-4 wt.% Ni shape memory alloy. A water-cooled copper crystalliser with a graphite insert was used to cast ϕ 16 mm strands. Diverse sets of casting parameters were tested, the average casting speeds being within the range from 250 mm min −1 to 625 mm min −1 . The castability of the alloy using this technique was good. Strands were examined in the as-cast condition, and no cracks or other defects were discovered. Medium casting speeds resulted in good surface quality, and a fully martensitic microstructure was obtained even at the lowest average casting speed. The paper presents and discusses the surface quality, microstructures and mechanical properties.

The microstructure of an Al–Mn–Be–Cu alloy was characterised after melt spinning and heat-treatment at different temperatures. The optimised melt-spinning parameters made it possible to obtain ribbons with thicknesses ranging from 30 to 200 μm, having a microstructure composed of a supersaturated Al-rich solid solution, finely dispersed icosahedral quasicrystalline (IQC) particles, and a small fraction of Al 2 Cu. The finest dispersion of the IQC-particles, and consequently the highest microhardness, was obtained on the wheel-side. The initial microstructure started to decompose at temperatures around 400 °C. The IQC-phase was replaced by the intermetallic compounds τ 1 -Al 29 Mn 6 Cu 4 and Be 4 Al(Mn,Cu). The maximum hardness of the ribbons, which was approximately 50% higher than in the as-cast state, was attained after heat treatment at 400 °C, and could be attributed to the fine dispersion of τ 1 -Al 29 Mn 6 Cu 4 and IQC-particles.

In this work we characterized the structure of continuously cast small cross-section rods (O10 mm) of the Ni-based superalloy GMR 235. In the microstructure prevailed dendritic columnar γ-grains with γ′-precipitates. In the interdendritic regions MC-carbide and M3B2-boride were identified. The inverse macrosegregation was very faint, except at the secondary witness marks and natural corrugations. It was found that the alternating drawing mode had much greater influence on microstructure than other casting parameters. Special attention was given to explanation of processes leading to formation of surface marks (primary and secondary witness marks and natural corrugations). Formation of hot tears and appearance of inverse segregation is also discussed.

In this paper the applicability and the efficiency of in-situ electrical resistance measurements with a four-probe method for monitoring the solidification process are presented. For this purpose a special measurement cell was developed. It enables simultaneous in-situ electrical resistance and temperature measurements during solidification to characterize the L → S phase transformation of metals and alloys. The method was experimentally demonstrated with a eutectic Pb – Sn (i. e. Pb-61.9 wt.% Sn) alloy. Additionally, the L → S phase transformation was monitored and compared with conventional differential thermoanalysis. The results of our investigation show that the simultaneous in-situ electrical resistance and temperature measurement method is able to determine the exact position of the start and end points of the L → S phase transformations from the resistance curves R(t) and the corresponding temperatures.

Cu—Al—Ni shape memory alloys (SMAs) have been investigated as materials for medical devices, but little is known about their biocompatibility. The aim of this work was to study the response of rat peritoneal macrophages (PMØ) to a Cu—Al—Ni SMA in vitro, by measuring the functional activity of mitochondria, necrosis, apoptosis, and production of proinflammatory cytokines. Rapidly solidified (RS) thin ribbons were used for the tests. The control alloy was a permanent mold casting of the same composition, but without the shape memory effect. Our results showed that the control alloy was severely cytotoxic, whereas RS ribbons induced neither necrosis nor apoptosis of PMØ. These findings correlated with the data that RS ribbons are significantly more resistant to corrosion compared to the control alloy, as judged by the lesser release of Cu and Ni in the conditioning medium. However, the ribbons generated intracellular reactive oxygen species and upregulated the production of IL-6 by PMØ. These effects were almost completely abolished by conditioning the RS ribbons for 5 weeks. In conclusion, RS significantly improves the corrosion stability and biocompatibility of Cu—Al—Ni SMA. The biocompatibility of this functional material could be additionally enhanced by conditioning the ribbons in cell culture medium.

Cu-Al-Ni shape memory alloys (SMAs) have been investigated as materials for medical devices, but their biomedical application is still limited. The aim of this work was to compare the microstructure, corrosion and cytotoxicity in vitro of a Cu-Al-Ni SMA. Rapidly solidified (RS) thin ribbons, manufactured via melt spinning, were used for the tests. The control alloy was a permanent mould casting of the same composition, but without shape memory effect. The results show that RS ribbons are significantly more resistant to corrosion compared with the control alloy, as judged by the lesser release of Cu and Ni into the conditioning medium. These results correlate with the finding that RS ribbons were not cytotoxic to L929 mouse fibroblasts and rat thymocytes. In addition, the RS ribbon conditioning medium inhibited cellular proliferation and IL-2 production by activated rat splenocytes to a much lesser extent. The inhibitory effects were almost completely abolished by conditioning the RS ribbons in culture medium for 4 weeks. Microstructural analysis showed that RS ribbons are martensitic, with boron particles as a minor phase. In contrast, the control Cu-Al-Ni alloy had a complex multiphase microstructure. Examination of the alloy surfaces after conditioning by energy dispersive X-ray and Auger electron spectroscopy showed the formation of Cu and Al oxide layers and confirmed that the metals in RS ribbons are less susceptible to oxidation and corrosion compared with the control alloy. In conclusion, these results suggest that rapid solidification significantly improves the corrosion stability and biocompatibility in vitro of Cu-Al-Ni SMA ribbons.

The aim of this work was to compare the microstructures of two high noble experimental Au-Pt alloys with similar composition with their corrosion and biocompatibility in vitro. We showed that Au-Pt II alloy, composed of 87.3 wt.% Au, 9.9 wt.% Pt, 1.7 wt.% Zn and 0.5 wt.% Ir + Rh + In, although possessing better mechanical properties than the Au-Pt I alloy (86.9 wt.% Au, 10.4 wt.% Pt, 1.5 wt.% Zn and 0.5 wt.% Ir + Rh + In), exerted higher adverse effects on the viability of L929 cells and the suppression of rat thymocyte functions, such as proliferation activity, the production of Interleukin-2 (IL-2), expression of IL-2 receptor and activation — induced apoptosis after stimulation of the cells with Concanavalin-A. These results correlated with the higher release of Zn ions in the culture medium. As Zn2+, at the concentrations which were detected in the alloy’s culture media, showed a lesser cytotoxic effect than the Au-Pt conditioning media, we concluded that Zn is probably not the only element responsible for alloy cytotoxicity. Microstructural characterization of the alloys, performed by means of scanning electron microscopy in addition to energy dispersive X-ray and X-ray diffraction analyses, showed that Au-Pt I is a two-phase alloy containing a dominant Au-rich α1 phase and a minor Pt-rich α2 phase. On the other hand, the Au-Pt II alloy additionally contained three minor phases: AuZn3, Pt3Zn and Au1.4Zn0.52. The highest content of Zn was identified in the Pt3Zn phase. After conditioning, the Pt3Zn and AuZn3 phases disappeared, suggesting that they are predominantly responsible for Zn loss, lower corrosion stability and subsequent lower biocompatibility of the Au-Pt II alloy.

In this study, the effect of powder spreading direction was investigated on selectively laser-melted specimens. The results showed that the metallurgical properties of the specimens varied during fabrication with respect to their position on the build tray. The density, porosity, and tensile properties of the Co–Cr–W–Mo alloy were investigated on cuboid and tensile specimens fabricated at different locations. Two different significant positions on the tray were selected along the powder spreading direction. One set of specimens was located near the start line of powder spreading, and the other set was located near the end of the building tray. The main role in the consequences of powder layering was played by the distribution of powder particle sizes and the packing density of the layers. As a result, laser penetration, melt pool formation, and fusion characteristics varied. To confirm the occurrence of variations in sample density, an additional experiment was performed with a Ti–6Al–4V alloy. Furthermore, the powders were collected at two different fabricating locations and their size distribution for both materials was investigated.