Study of complex carbides obtained after solidification and thermal treatment in High Speed Steels

High-speed steels (HSS) rolls are used in front finishing stands of hot strip mills. Good wear resistance and hardness at high temperature are defining characteristics of HSS. Many carbides are present in these alloys, each having different effects upon the final properties of HSS. As a result, the nature, the morphology and the amount of these carbides are factors of important concern. Optical microscopy combined with electron microscopy lead to quicker identification and characterization of HSS carbides

Prediction of Phase-transformations of Ti6Al4V additively manufactured during Directed Energy Deposition

editorial reviewedDirected Energy Deposition (DED) has been highly used to fabricate and repair components made from different metallic alloys. The characteristics of this process enhances high cooling and heating rates, which results in an out-of-equilibrium microstructures of the build parts. Ti6Al4V additively manufactured alloy is widely used in a variety of applications such as aerospace, automotive, marine equipment and biomechanical applications. However, the improvement of its mechanical properties is still a challenge because they are influenced by the process conditions. The Mechanics of Solid and Materials (MSM) and Metallic Solid Materials (MMS) of ULiège have been focused to correlate DED process parameters and the obtained microstructures. The approach has been based on a strong combination between experimental investigations and numerical modelling [1, 2, 3]. First investigations on this topic consisted in the implementation of a numerical model to predict phase transformations of Ti6Al4V [1]. This approach is based on discretization of the thermal histories (heating and cooling) which have been obtained by Finite Element simulations [3] to capture temperature oscillations in the clad parts during Manufacturing. An advanced approach, consisting in the segmentation of the temperature history in different Time-phase – Temperature – Blocks (TTB) [2], allows the correlation between the thermal histories and the final microstructure. In this context, in a first step, we proceed by summarizing the previous results. Then, a new achievement consisting in modelling a “meso-clad” of Ti6Al4V with mutli-validations (multiple thermocouples and microscopic measurements) is presented. Finally, we present our perspectives by exploiting promising tools in particular machine learning.9. Industry, innovation and infrastructur

Improvement in HSS grade for early stands of hot strip mills - Metallurgical features and mechanical properties assessment

peer reviewedAurora and Kosmos grades are HSS alloys belonging to the complex Fe-Cr-C-X system, where X is a strong carbide former element of the V, Mo or W type. Both alloys were metallurgically characterised prior to their comparison. Metallurgical analyses involved phases identification and carbides quantification by using Scanning Electron Microscopy and Energy Dispersive X rays. Differential Thermal Analysis was performed to allow a better understanding of the solidification sequence of studied alloys while mechanical tests performed were compressive at room temperature and bulk hardness at usual service temperatures. An attempt was made in order to connect experimental results to the good behaviour in operation of Aurora grade. In fact Aurora grade appeared to exhibit strong metallurgical differences when compared to Kosmos grade, especially as concern in nature and amount of carbides

Segregation network structure investigation on 316L stainless steel processed by Laser Cladding

peer reviewedAdditive manufacturing (AM) techniques are promising net-shape manufacturing technologies, producing complex 3D solid parts from powders according to computer-aided design (CAD) models. The one-step building process, the versatility, fast process speed and simplified post-treatment make AM a realistic candidate for fabrication of various components. One important member among the AM technologies is Laser Cladding (LC). The hierarchical new microstructures, different from the conventional ones, and the peculiar segregations caused by this technique have attracted the attention of both academic and industrial world [1]–[3]. In that respect, the structures formed on the common stainless steel (SS) 316L due to its combination of good mechanical properties at high temperatures, good machinability and excellent corrosion resistance appear particularly interesting. Indeed, this work considers a SS316L laser clad deposit produced using a parallel deposition strategy. Microstructural characterization reveals the classical hierarchical macro-, micro- and nano-structures in as-built samples. The extremely high cooling rates lead to specific segregations for each hierarchical structure scale, leading potentially to complex phase formations and enhanced mechanical properties due to improved strengthening mechanisms. Nano-indentations grids, macro-hardness tests and Electron Back Scattered Diffraction (EBSD) observations were carried out in order to evaluate the mechanical properties at different scales, both inside melt pools and in heat affected zones (HAZ). A particular attention was given on the impact that every hierarchical structure may have on the macro-properties at different positions inside the deposits, in order to obtain new insights and to tailor their properties following a bottom-up approach.IAWATH

Development and processability of AISI S2 tool steel by Laser Powder Bed Fusion

peer reviewedNowadays, the advantages of Laser Powder Bed Fusion (LPBF) technology attract both industry and researchers. Indeed, it is possible to build up complex geometrical parts with higher mechanical properties than those obtained by conventional methods. However, LPBF involves complex phenomena due to the high heating and cooling rates that lead to out-of-equilibrium conditions. For this reason, few metal alloys are easily processable up to now. Nevertheless, research on new steels by LPBF has been growing in recent years, in particular, regarding the development of tool steels. This work thus focuses on the development of the tool steel AISI S2 by LPBF. The process map has been investigated by varying the laser power from 100 to 250 W and the scan speed from 400 to 2000 mm/s. By combining surface analysis by means of profilometer observations, density measurements by pycnometry, defects characterization and quantification and investigations on the melt pool morphology, the best process window is selected to have fully dense, defect-free parts. Furthermore, this study allows to have comprehensive insights on the effect of the parameters on the type of defects generated during the manufacturing

Development and processability of AISI S2 tool steel by laser powder bed fusion

Nowadays, the advantages of Laser Powder Bed Fusion (LPBF) technology attract both industry and researchers. Indeed, it is possible to build up complex geometrical parts with higher mechanical properties than those obtained by conventional methods. However, LPBF involves complex phenomena due to the high heating and cooling rates that lead to out-of-equilibrium conditions. For this reason, few metal alloys are easily processable up to now. Nevertheless, research on new steels by LPBF has been growing in recent years, in particular, regarding the development of tool steels. This work thus focuses on the development of the tool steel AISI S2 by LPBF. The process map has been investigated by varying the laser power from 100 to 250 W and the scan speed from 400 to 2000 mm/s. By combining surface analysis by means of profilometer observations, density measurements by pycnometry, defects characterization and quantification and investigations on the melt pool morphology, the best process window is selected to have fully dense, defect-free parts. Furthermore, this study allows to have comprehensive insights on the effect of the parameters on the type of defects generated during the manufacturing

Evaluation under near-equilibrium conditions of the powders mixture AISI S2 Tool Steel and Silicon Carbide for Laser Powder Bed Fusion applications

Forecast the microstructures obtainable in alloys printed through Laser Powder Bed Fusion (LPBF) is a challenge in Additive Manufacturing (AM). The out-of-equilibrium condition of the solidification during the printing increases the difficulty to predict the final microstructure. Techniques such as thermal analysis in near-equilibrium conditions can be adopted to overcome the challenge. With thermal analysis, it is also possible to evaluate the influence of the elements of the alloy on the microstructure. The main aim of this research is to fabricate an innovative alloy with good wear and tribological properties, in particular, an alloy with self-lubricating properties provided by the presence of graphite into the final microstructure. In this work, the evaluation of AISI S2 tool steel + silicon carbide (SiC) is performed in term of microstructural characterization after a Differential Thermal Analysis (DTA) at 5 and 20°C/min of cooling rates. 5, 10, 15 and 20% in volume of SiC were added to S2 tool steel. The influence of SiC on the final microstructure was investigated through optical and scanning electron microscopy with EDS and EBSD analysis that allow the evaluation and comparison of the different phases of the microstructures. The amount of SiC influence the final microstructure, passing from a fully pearlitic matrix, for the lowest amount, to a fully ferritic matrix with Iron/Molybdenum eutectic carbides and precipitates, and several types of graphite increasing the SiC amoun

Microstructure and properties of SLM AlSi10Mg: Understanding the influence of the local thermal history

peer reviewedSelective Laser Melting (SLM) is an Additive Manufacturing technique that is widely used to produce AlSi10Mg parts with a good strength-to-weight ratio. Indeed, strongly refined microstructures are obtained due to the ultra-fast cooling rates reached in this process, conferring high strength to the parts, even in the as-built state. However, microstructural heterogeneities at the scale of the melt pool may exert a detrimental influence on the mechanical properties e.g. by causing a loss in ductility. This study thus aims at a better understanding of the influence of the local thermal history on local variations of microstructure and mechanical properties. Microscopy (i.e. SEM+EDS) and nanoindentation have been combined to reach a detailed knowledge of the local microstructure and properties. In particular, the solute Si content in the -Al matrix, the volume fraction and the size of Si precipitates have been quantified by microscopy analysis. These local microstructural parameters are correlated with the matrix hardness as revealed by nanoindentation. Finally, the results of this detailed characterization are linked with the local thermal history that is approached in two different ways i.e. (i) an analytical description of thermal gradients inside the melt pool based on Rosenthal’s and Matyja’s equations and (ii) a simple Finite Element model for the deposition of a few layers in the SLM process

Preparation of powders mixture of AISI S2 Tool Steel and Silicon Carbide for use in Laser Powder Bed Fusion

Mixing different powders is a promising way to broaden the choice of materials for Laser Powder Bed Fusion (LPBF). However, powders for LPBF must present appropriate rheological properties. Indeed, if the initial batch of powders is not homogeneous, both spreadability and laser - powder interaction suffer, affecting the final part quality. This work thus focuses on the preparation of mixed AISI S2 tool steel and silicon carbide (SiC) powders for use in LPBF. To promote the complete dissolution of SiC in the melt pool, spray dried granules of SiC nanoparticles were selected. A combination of sieving, ball milling and thermal treatment was finally selected as it resulted in good rheological properties of the powders mixture and in a good quality of the final part

Unveiling the complex wear sequence of a directed energy deposited 316L+WC hierarchical composite against alumina

peer reviewedIn this work, the addition of 20 vol. % of tungsten carbide (WC) to 316L powder processed by direct energy deposition (DED) leads to a hierarchical composite. The modified austenitic microstructure is reinforced by partially dissolved WC, and by a network of solidification carbides. Wear tests for DEDed composite sliding against alumina were carried out, highlighting a cyclical wear regime different from the steady state one achieved on the reference DEDed 316L under similar conditions. The wear behaviour of the DEDed composite sliding against alumina was investigated when considering significant variations of both the friction coefficient (CoF) and the penetration depth (PDe). A novel approach based on interrupted wear tests was applied to better understand the role of the complex hierarchical microstructure of the DEDed composite on its wear behaviour. The wear tracks were analysed through scanning electron microscopy and profilometer at the different stages. This approach allows to observe the evolution of the wear rate with the progress of the test and to identify the corresponding mechanisms that involve adhesive, abrasive and oxidative wears. The influence of WC reinforcements, of the composite matrix or of the counterbody considered solely or in association under dry sliding conditions are discussed thus helping to unveil the complex wear sequence of the DEDed composite. The interrupted wear tests approach also helps to highlight the contribution of the short run-in period and that of the subsequent cyclical regime to the wear rate while considering repeated compaction, breakdown and reformation of a protective tribolayer.IAWATH