Microstructure, mechanical properties and cracking behaviour in a gamma'-precipitation strengthened nickel-base superalloy fabricated by electron beam melting

The influences of EBM processing and post-processing on microstructure, mechanical properties and cracking behaviour in a γ′ precipitation-strengthened nickel-base superalloy DZ125 are critically assessed. Results show that a solution treating and ageing (STA) is required to obtain γ′ precipitates with a cuboidal shape. The columnar grain width was found to gradually increase from the bottom to the top of the as-EBM sample having a total build height of 80 mm, whereas there was little gradient in γ′ size. The presence of EBM induced intergranular cracks can be closed by hot isostatic pressing (HIP), however reappearance of intergranular cracks with a much wider crack opening width was observed after STA treatment. EBM induced cracks are classified as liquid-state cracking, as the classic dendritic morphology were found on the cracked surfaces. The post-processing induced cracks can be attributed to the effect of γ′ dissolution and re-precipitation of fine γ′ during the cooling stage of solution treatment. The results suggest that HIP is not an effective approach in healing liquid-state cracks in EBM fabricated γ′ precipitation-strengthened superalloys

Tribological performance of an H-DLC coating prepared by PECVD

Carbon-based coatings are of wide interest due to their application in machine elements subjected to continuous contact where fluid lubricant films are not permitted. This paper describes the tribological performance under dry conditions of duplex layered H-DLC coating sequentially deposited by microwave excited plasma enhanced chemical vapour deposition on AISI 52100 steel. The architecture of the coating comprised Cr, WC, and DLC (a-C:H) with a total thickness of 2.8 μm and compressive residual stress very close to 1 GPa. Surface hardness was approximately 22 GPa and its reduced elastic modulus around 180 GPa. Scratch tests indicated a well adhered coating achieving a critical load of 80 N. The effect of normal load on the friction and wear behaviours were investigated with steel pins sliding against the actual coating under dry conditions at room temperature (20 ± 2°C) and 35-50% RH. The results show that coefficient of friction of the coating decreased from 0.21 to 0.13 values with the increase in the applied loads (10-50 N). Specific wear rates of the surface coating also decrease with the increase in the same range of applied loads. Maximum and minimum values were 14 × 10-8 and 5.5 × 10-8 mm-3/N m, respectively. Through Raman spectroscopy and electron microscopy it was confirmed the carbon-carbon contact, due to the tribolayer formation on the wear scars of the coating and pin. In order to further corroborate the experimental observations regarding the graphitisation behaviour, the existing mathematical relationships to determine the graphitisation temperature of the coating/steel contact as well as the flash temperature were used

Characterisation and correlation of areal surface texture with processing parameters and porosity of High Speed Sintered parts

High Speed Sintering is an advanced powder bed fusion polymer Additive Manufacturing technique aimed at economical production of end-use parts in series manufacture. Surface finish is thus of high importance to end users. This study investigates the surface topography of High Speed Sintered parts produced using a range of different energy-related process parameters including sinter speed, lamp power and ink grey level. Areal surface texture was measured using Focus Variation microscopy and the sample porosity was systematically examined by the X-ray Computed Tomography technique. Surface topography was further characterised by Scanning Electron Microscopy, following which the samples were subject to tensile testing. Results showed that areal surface texture is strongly correlated with porosity, which can be further linked with mechanical properties. Certain texture parameters i.e. arithmetic mean height Sa, root-mean-square Sq and maximum valley depth Sv were identified as good indicators that can be used to compare porosity and/or mechanical properties between different samples, as well as distinguish up-, down-skins and side surfaces. Sa, Sq and Sv for up- and down-skins were found to correlate with the above energy-related process parameters. It was also revealed that skewness Ssk and kurtosis Sku are related to sphere-like protrusions, sub-surface porosity and re-entrant features. Energy input is the fundamental reason that causes varying porosity levels and consequently different surface topographies and mechanical properties, with a 10.07 μm and a 30.21 % difference in Sa and porosity, respectively, between the ‘low’ and ‘high’ energy input

Characterisation of Nickel-Based Superalloys Manufactued by Electron Beam Melting

Nickel-based superalloys have been manufactured by electron beam melting (EBM). EBM is an additive manufacturing process for direct production of metal parts. The part is build up by successive melting of metal powder layers in a vacuum chamber using electron beam. In this study, the microstructure, texture and the physical properties of nickel-based superalloys manufactured by EBM were thoroughly characterised by different methods, e.g. XRD, DTA, SEM, EDS, EBSD, TEM and APFIM. The investigated alloys were Inconel alloy 600, Udimet alloy 720 and Inconel alloy 718; they are strengthened by different hardening mechanisms. The EBM materials were highly textured and oriented with the directions in the building direction and in the scanning directions of the electron beam. Furthermore, the grains are elongated in the building direction and are up to several millimetres in length. The mechanical properties of heat treated Inconel alloy 718 manufactured by EBM are to large extent comparable with conventional material. Additionally, the same alloys were manufactured by casting and directional solidification, in order to compare the solidification and precipitation behaviour with the EBM samples. The solidification rate was fastest in the EBM process and slowest in directional solidification. Macrosegregation did not occur in the EBM samples, but in the cast and directionally solidified samples.Enhanced understanding about rapid solidification and local melting has been gained during this study. This should be of value also for similar processes, e.g. additive manufacturing and welding. EBM is a promising technology for direct manufacturing of highly textured nickel-based superalloy parts. A suitable application would be parts currently manufactured by directional solidification. Using EBM, both cost and time can be reduced as no mould is required. Furthermore, it would be possible to tailor both the geometry and the texture of each single part in the EBM process

Characterisation of Nickel-Based Superalloys Manufactued by Electron Beam Melting

Nickel-based superalloys have been manufactured by electron beam melting (EBM). EBM is an additive manufacturing process for direct production of metal parts. The part is build up by successive melting of metal powder layers in a vacuum chamber using electron beam. In this study, the microstructure, texture and the physical properties of nickel-based superalloys manufactured by EBM were thoroughly characterised by different methods, e.g. XRD, DTA, SEM, EDS, EBSD, TEM and APFIM. The investigated alloys were Inconel alloy 600, Udimet alloy 720 and Inconel alloy 718; they are strengthened by different hardening mechanisms. The EBM materials were highly textured and oriented with the directions in the building direction and in the scanning directions of the electron beam. Furthermore, the grains are elongated in the building direction and are up to several millimetres in length. The mechanical properties of heat treated Inconel alloy 718 manufactured by EBM are to large extent comparable with conventional material. Additionally, the same alloys were manufactured by casting and directional solidification, in order to compare the solidification and precipitation behaviour with the EBM samples. The solidification rate was fastest in the EBM process and slowest in directional solidification. Macrosegregation did not occur in the EBM samples, but in the cast and directionally solidified samples.Enhanced understanding about rapid solidification and local melting has been gained during this study. This should be of value also for similar processes, e.g. additive manufacturing and welding. EBM is a promising technology for direct manufacturing of highly textured nickel-based superalloy parts. A suitable application would be parts currently manufactured by directional solidification. Using EBM, both cost and time can be reduced as no mould is required. Furthermore, it would be possible to tailor both the geometry and the texture of each single part in the EBM process