Showing papers by "Jesper Henri Hattel published in 2019"

Keyhole-induced porosities in Laser-based Powder Bed Fusion (L-PBF) of Ti6Al4V: High-fidelity modelling and experimental validation

Abstract: Metal additive manufacturing, despite of offering unique capabilities e.g. unlimited design freedom, short manufacturing time, etc., suffers from raft of intrinsic defects. Porosity is of the defects which can badly deteriorate a part’s performance. In this respect, enabling one to observe and predict the porosity during this process is of high importance. To this end, in this work a combined numerical and experimental approach has been used to analyze the formation, evolution and disappearance of keyhole and keyhole-induced porosities along with their initiating mechanisms, during single track L-PBF of a Ti6Al4V alloy. In this respect, a high-fidelity numerical model based on the Finite Volume Method (FVM) and accomplished in the commercial software Flow-3D is developed. The model accounts for the major physics taking place during the laser-scanning step of the L-PBF process. To better simulate the actual laser-material interaction, multiple reflection with the ray-tracing method has been implemented along with the Fresnel absorption function. The results show that during the keyhole regime, the heating rises dramatically compared to the shallow-depth melt pool regime due to the large entrapment of laser rays in the keyhole cavities. Also a detailed parametric study is performed to investigate the effect of input power on thermal absorptivity, heat transfer and melt pool anatomy. Furthermore, an X-ray Computed Tomography (X-CT) analysis is carried out to visualize the pores formed during the L-PBF process. It is shown, that the predicted shape, size and depth of the pores are in very good agreement with those found by either X-CT or optical and 3D digital microscopic images.

Impact of micro-scale residual stress on in-situ tensile testing of ductile cast iron: Digital volume correlation vs. model with fully resolved microstructure vs. periodic unit cell

Abstract: The understanding of the mechanisms controlling deformation of ductile iron at the micro-scale and their coupling to the manufacturing conditions is still far from complete. In this respect, recent synchrotron-based studies have demonstrated that the thermal contraction mismatch between the graphite particles and the matrix during solid-state cooling leads to a complex residual stress state in the microstructure. To investigate its impact on the room-temperature tensile deformation, a computational-experimental analysis extendable to other similar composite materials is presented in this paper. First, a miniaturized specimen is loaded and imaged in-situ with X-ray tomography. Then, the microscale displacement is reconstructed using digital volume correlation (DVC) and used to prescribe the boundary conditions in a finite element model of the full microstructure between two cross-sections. The model predictions at both the macroscale – tensile force and lateral contraction – and the microscale – strain field – are compared to the corresponding experimental and DVC-based data for several choices of the initial stress state, particles’ mechanical behavior and strength of the particles-matrix interface. It is proved that the micro-scale residual stress and a low interface strength are the key to explain the early stages of the tensile deformation of ductile iron. Finally, it is shown that a simple unit cell model of the microstructure would lead to significantly different results, thus demonstrating the superior accuracy and robustness of the present approach.

Roughness Investigation of SLM Manufactured Conformal Cooling Channels Using X-ray Computed Tomography

Abstract: Conformal cooling channels are becoming one of the next big steps in the fabrication of moulds and tools. Mass flow rate and heat transfer are affected by the surface roughness in the cooling channels. The freeform shape of conformal cooling channels makes it difficult to evaluate the internal roughness with respect to classic planar techniques. This work presents a fitted-ellipse method to evaluate internal surface features of helical cooling channels. The investigated cooling channel was made from maraging steel 300 and manufactured with the selective laser melting process. X-ray computed tomography and image analysis were utilized in order to generate a freeform nominal surface by fitting ellipses to the reconstructed surface. The nominal surface was compared to the reconstructed surface and resulted in a point cloud of deviation values. The deviation values were used as input for deviation plots, inner area and volume estimations together with estimations of classic area surface parameters, according to ISO 25178-2:2012. Results showed that the internal surface features were highly orientation dependent, with extreme roughness observed on the downward facing surface of the cooling channel. The arithmetical mean height and average maximum height of the total inner surface were estimated at Sa = 13.7 μm and Sz20 = 251 μm, respectively. The mass distribution was positively skewed, the root mean square height was Sq = 21.8 μm and the peaks observed on the surface were characterized as spiked. The obtained results suggested that the proposed method could evaluate the internal features of a helical cooling channel efficiently and qualitatively, while giving realistic quantitative estimations of the surface roughness characteristics.

Simulation of resin-impregnation, heat-transfer and cure in a resin-injection pultrusion process

Abstract: In this paper has a numerical framework for multiphysical simulation of resin-impregnation, heat-transfer and cure in a resin-injection pultrusion process been developed. Using the framework, the material flow through the pultrusion die was studied for the manufacture of a 100 mm thick glass fibre reinforced polyurethane (thermoset) composite profile. The results demonstrated that while curing is initiated near the heated die-walls, a yet stronger reaction is simultaneously obtained at the centre of the profile. The results were qualitatively compared to measurements from an industrial pultrusion line, which confirmed the trends of the material flow.

Experimental investigation and thermo-mechanical modelling for tool life evaluation of photopolymer additively manufactured mould inserts in different injection moulding conditions

Abstract: There is a growing interest for integrating additive manufacturing (AM) technology in different manufacturing processes such as injection moulding (IM) due to the possibility of achieving shorter manufacturing times and increased cost effectiveness. This paper evaluates IM inserts fabricated by the AM vat photopolymerisation method. The inserts are directly manufactured with a photopolymer material, integrated on an injection moulding tool and subsequently used for IM. Therefore, particular attention has to be paid in order to develop the soft tooling process chain and the IM experimental procedure as detailed in this study. Different combinations of IM parameters are investigated in this work in order to determine the influence of the various process settings on the inserts’ performance (lifetime, crack propagation, consistency of the mould surface features). The mould inserts were analysed by three-dimensional optical metrology and evaluated with regard to the different surface features that were affected by the IM process. A three-dimensional thermo-mechanical with phase change model for the analysis of the effects of the IM process on the additive manufactured tools was accomplished in the FE software COMSOL Multiphysics. The potential causes for the insert failure are identified both by means of the IM experiments and the numerical model. The developed model could also predict the thermally induced deformations produced in the mould and identify where this phenomenon would eventually lead to defects in the shape of the parts. The influence of three different temperatures of the insert at 25 °C, 50 °C and 100 °C on the failure of the insert was investigated. Also a detailed discussion about the solidification and temperature changes is given.

Resistor-Capacitor Approach for Modelling of Temperature and Humidity Response Inside Electronic Enclosures

Abstract: Moisture is an important factor to reliability, functionality and durability of electronic devices. Nowadays, modelling tools have become an integral part of electronics design, because they are less expensive for searching the optimal design of moisture control. CFD and FEM are very time consuming due to their computational effort, therefore it is desirable to have a method such as well-known Resistor-Capacitor (RC) approach which is faster and has less spatial resolution. The paper concerns the development of an in-house code using the RC approach for simulating coupled heat and moisture transport inside electronic enclosure under non-isothermal conditions. Thereafter, the simulations results were compared with corresponding experiments in order to validate the developed code. Based on comparison with experiments, a new configuration RC model was developed for a more accurate humidity prediction. In general new RC model had the adsorption and desorption mechanisms included as a lumped capacitance for describing the surface of a wall on its both sides. Such modification improved the agreement with experiments.

Application of a Projection Method for Simulating Flow of a Shear-Thinning Fluid

Abstract: In this paper, a first-order projection method is used to solve the Navier–Stokes equations numerically for a time-dependent incompressible fluid inside a three-dimensional (3-D) lid-driven cavity. The flow structure in a cavity of aspect ratio δ = 1 and Reynolds numbers ( 100 , 400 , 1000 ) is compared with existing results to validate the code. We then apply the developed code to flow of a generalised Newtonian fluid with the well-known Ostwald–de Waele power-law model. Results show that, by decreasing n (further deviation from Newtonian behaviour) from 1 to 0.9, the peak values of the velocity decrease while the centre of the main vortex moves towards the upper right corner of the cavity. However, for n = 0.5 , the behaviour is reversed and the main vortex shifts back towards the centre of the cavity. We moreover demonstrate that, for the deeper cavities, δ = 2 , 4 , as the shear-thinning parameter n decreased the top-main vortex expands towards the bottom surface, and correspondingly the secondary flow becomes less pronounced in the plane perpendicular to the cavity lid.

Additive manufacturing of metal components ─ process-material interaction in different process chains

Abstract: In this paper we describe metal additive manufacturing (AM) processes in general, and laser bed fusion processes in particular. The process characteristics are described, and the various modelling approaches demonstrated. The open AM architecture developed at DTU Mechanical Engineering is introduced, as well as an alternative process chain based on AM and subsequent injection moulding.

A Characterization Study Relating Cross-Sectional Distribution of Fiber Volume Fraction and Permeability

Abstract: In the present study, Scanning Electron Microscopy (SEM) has been used to assess the influence of a varying cross-sectional fiber volume fraction on local permeability values. The investigated sample is a textured glass fiber reinforced polyurethane composite rod with an Ø5mm cross-section. The SEM investigations show a high variation in local fiber volume fraction with a mean fiber volume fraction of 0.41±0.2, which is in agreement with the value 0.41 obtained by burn-off test by the manufacturer. The additional result from the SEM investigations was a mean fiber diameter of 23.2±2.6μm, which was verified by single fiber tests. The characterization results were successfully used to evaluate the local variation in permeability over the cross-section and could most likely be a very essential input for numerical impregnation simulations.

Build orientation effects on the roughness of SLM channels

Abstract: Increasingly advanced shapes and geometries are being manufactured using additive manufacturing and new characterization techniques must emerge in order to fully utilize the new possibilities given by freeform design. Cooling channels produced by the laser powder bed fusion process has been shown to have high roughness at overhanging areas due to powder particles being fused with the internal surface. Classic techniques for characterizing profile roughness are falling short with respect to internal surfaces in freeform geometries. Hence, this work presents a methodology for characterizing internal surface roughness by extracting roughness profiles through the use of image analysis and X-ray CT. In order to fully describe the internal surface roughness, two orientations were defined, namely the global and local orientations α and β. The internal profile roughness was evaluated in accordance with ISO 4287:1997. Seven selective laser melting manufactured straight channels made in 17-4 PH stainless steel were CT scanned and analyzed with the proposed methodology. Results showed that the Ra-values inside the channel were dependent on both α and β. The average Ra-values and their standard deviations were found to be decreasing rapidly with increasing α. The highest average roughness was found for α = 0°, where an average Ra-value of 70.7 μm was found. The lowest average roughness was found at α = 90°, where an average Ra-value of 6.7 μm was found. Furthermore, it was found that the surface texture and roughness changed dependent on the location along the length of the channel produced at α = 0°. These findings suggest the importance of characterizing the internal surface roughness of cooling channels with respect to both the global build orientation of a channel, the local orientation within a channel and the specific location along the length of a channel. SLM, powder bed fusion, additive manufacturing, cooling channels, X-ray CT, roughness analysis

Fiber segmentation from 3D X-ray computed tomography of composites with continuous textured glass fibre yarns

Abstract: In recent years, the prospects of advanced X-ray Computed Tomography (XCT) for industrial applications continue to increase. The insights into materials microstructure, delivered by XCT, are of uttermost importance for the production of high-quality composite materials, e.g. Fiber Reinforced Polymers (FRP). Hence, knowledge of the microstructure plays a vital role in optimization within manufacturing of FRPs. The cross-sectional (2D) spatial distribution of fibers was investigated using Optical Microscopy (OM) in [1,2] and it was shown that a varying fiber distribution has a significant influence on local material behavior during the manufacturing process, i.e. thermo-chemical curing gradients. Destructive methods for 3D microstructure characterization were investigated in [3]. Non-destructive confocal laser scanning microscopy was applied in [4], but this technique is limited to sufficiently transparent samples and limited fields of view. In the study by Emerson et al. [5], unidirectional (UD) glass and carbon FRP composites were characterized using XCT in combination with a segmentation method that is robust to image quality [6]. The method is based on a probabilistic dictionary-based feature-labelling algorithm that does not require manual correction of fiber detections. The precision of the method used in [5] was validated by comparing the obtained fiber diameters and spatial distribution of fibers with similar results from OM and Scanning Electron Microscopy (SEM) in [6]. The current work studies the applicability of the method developed in [5] for fiber tracking of continuous textured fiber yarns in FRPs. Finally, options for extensions of the method will be discussed.

The influence of shoe-floor contact area, load and velocity on dynamic friction in indoor sports footwear: a small-scale tribology study

Abstract: Traction is widely regarded as essential to sporting performance. However, high traction also increases lower limb injury risk (Pasanen, Parkkari, Rossi, & Kannus, 2008). Indoor sports shoes are ge...

Exhaust Valve Spindles for Marine Diesel Engines Manufactured by Hot Isostatic Pressing

Abstract: The exhaust valve spindle is one of the most challenging components in the marine two-stroke diesel engine. It has to withstand high mechanical loads, thermal cycling, surface temperatures beyond 700 °C, and molten salt induced corrosion. Powder metallurgy gives the opportunity of improving the component using materials not applicable by welding or forging. Therefore exhaust valve spindles have been produced by Hot Isostatic Pressing (HIP) with a spindle disc coating of a Ni-Cr-Nb alloy that cannot be manufactured by welding or forging. This paper presents the service experience gathered by MAN Diesel & Turbo in a number of service tests on ships (up to 18000 running hours): corrosion and degradation phenomena in the spindles produced by HIP are presented and compared with the performance of state-of-the-art exhaust valve spindles. The macroscopic geometrical changes experienced by the spindles are studied by means of Finite Element Method (FEM) calculations and strategies for further development of the component are outlined. Introduction The exhaust valve spindle is one of the most challenging components in the marine two-stroke diesel engine: it has to withstand high mechanical and thermal loads without benefitting from the water cooling applied to the cylinder cover and cylinder liner. Fig. 1 shows a cross section view of the exhaust valve spindle in the closed position, i.e. when the seat area of the spindle is in contact with the bottom piece of the exhaust valve, therefore sealing the combustion chamber. The spindle bottom is directly exposed to the combustion chamber and is the hottest part of the component, with temperatures up to 600-700 °C; the seat can go up to 450-500 °C instead. During each engine cycle the spindle is heated up during the combustion step and thereafter cooled by the gases leaving the combustion chamber: therefore the spindle disc undergoes thermal cycling for each combustion cycle. The high temperatures experienced by the spindle bottom reduce service life of the exhaust valve spindles because of “hot corrosion”, i.e. corrosion due to the formation of molten salts, such as Na2So4 and V2O5, which dissolve the protective Cr2O2 oxide layers on the Ni-Cr alloys currently used. On the bottom of the spindle corrosion rates are usually in the range of 0.1-0.4 mm/1000 hours, but can soar up to 1 mm/1000 hours in harsh conditions. Hot Isostatic Pressing – HIP‘17 Materials Research Forum LLC Materials Research Proceedings 10 (2019) 98-106 doi: http://dx.doi.org/10.21741/9781644900031-14 99 Currently two designs for exhaust valve spindles are available: forged Nimonic80A and DuraspindleTM. In the first the material Nimonic80A (20 wt% Cr, 2.5 wt% Ti, 1.5wt% Al, bal. Ni) has excellent hot corrosion resistance and is precipitation hardened to reach sufficient hardness of the seat area. The DuraspindleTM design is a stainless steel forged substrate with welded Inconel 625 on the spindle bottom and Inconel 718 on the spindle seat. The Inconel 718 layer is cold deformed and aged to increase the hardness of the seat. Fig. 1 Cross section of exhaust valve spindle disc (grey) and bottom piece (blue). It was decided to test a hipped spindle because HIP allows a wider choice of materials for the spindle seat and bottom, without being limited by the forging and welding processes [1]. Moreover, it allows the flexibility of testing multiple materials at once by embedding samples in the spindle bottom [2]. This paper presents the service experience gained by MAN Diesel & Turbo with HIP spindles and the challenges encountered. Fig. 2 The capsule design. The grey capsule parts are produced as deep drawn steel sheet being assembled by gas tungsten arc welding (GTAW). The forged disc substrate (stainless steel) is green, the seat material orange, the bottom coating (NiCr49Nb1) red and the bonding zone material (316L) blue [1] Hot Isostatic Pressing – HIP‘17 Materials Research Forum LLC Materials Research Proceedings 10 (2019) 98-106 doi: http://dx.doi.org/10.21741/9781644900031-14 100 Experimental Manufacturing of the spindles. the spindles were manufactured at Sandvik Powder Solutions by Hot Isostatic Pressing, according to a manufacturing procedure used for a previous test [1]. Figure 2 shows an exploded view of the spindle capsule. The substrate is the same forged stainless steel used for Duraspindle. The seat material is an experimental Ni-Cr-Nb alloy. The bottom coating is NiCr49Nb1 alloy (49% wt Cr, 1.5 wt% Nb, Ni bal.). A 316L buffer layer is applied between the NiCr49Nb1 bottom coating and the forged substrate to act as a diffusion barrier to prevent the formation of chromium carbides. After the hipping at 1100 °C, the spindles undergo cold deformation and a precipitation hardening heat treatment to obtain high hardness in the seat area. The spindles are then put in service on ships running on heavy fuel oil, for up to 18000 hours. Finite element method (FEM) simulations. Because of the axial symmetry of the spindle, it is sufficient to have a 2D model of half the spindle disc (Fig. 3). The material used for the valve seat is the same as used for the spindle bottom: this will influence the stress distribution but only locally. The model consists of ~260000 three node axisymmetric linear elements with 130000 nodes. The model is analyzed using the Abaqus explicit solver. The three material layers are connected at the nodes, i.e. the layers share nodes. The model is held by constraining the nodes on the in the axial direction and the nodes on the centerline in the radial direction. Fig. 3 FEM model of half the spindle disc. The three materials involved are described with isotropic hardening and power law creep. The plastic material parameters are based on tensile tests of the materials whereas the creep parameters are based on literature data. An attempt to measure and apply measured creep parameters is ongoing. The in-service mechanisms have been simplified so that the model experiences only a constant temperature field during the combustion, i.e. the entire spindle disc is uniformly heated up to 700 °C, without external or internal forces applied, meaning that the occurring temperature gradient across the radius of the valve spindle is neglected. The contribution of this to the bending is considered insignificant. The temperature cycling due to the combustion cycle is not considered. Hot Isostatic Pressing – HIP‘17 Materials Research Forum LLC Materials Research Proceedings 10 (2019) 98-106 doi: http://dx.doi.org/10.21741/9781644900031-14 101 The FEM analysis covers the HIP stage and up to five cycles of normal operation. The HIP stage involves uniform cool down from 1100°C to 20°C. Each operating cycle describes heating up to 700°C, holding for 260 h and then cooling to 20°C. Results Corrosion rate of the bottom coating. Because of the bending of the spindle taking place during service (which is illustrated in Section “Bending of the spindle) it is not possible to measure directly the absolute value of the corrosion rate for NiCr49Nb1. However Nimonic80A and Inconel 625 samples were embedded in the bottom coating of two of the spindles during production, therefore it is possible to measure the difference in corrosion rates between these materials and NiCr49Nb1. Both materials have higher corrosion rates than NiCr49Nb1: approximately 0.07 mm/1000 hours for Nimonic80A and 0.15 mm/1000 hours for Inconel 625. Mechanical properties of NiCr49Nb1 and microstructure evolution. Prior to service, samples of NiCr49Nb1 have been heat treated at 700 °C for up to 4400 hours, in order to simulate the effect of service conditions on the material. Fig, 4 shows the mechanical properties of NiCr49Nb1 in the as-hipped conditions and after different heat treatment times. Fig. 4 Mechanical properties of NiCr49Nb1 as a function of time at 700 °C. Yield strength and tensile strength decrease significantly within the first hundred hours at 700 °C and then stay constant over time. Ductility instead increases significantly over a thousandsof-hours timescale. The evolution of the material microstructure is investigated by electron microscopy: a significant change in phase distribution is seen after heat treatment at 700 °C: Fig. 5 shows backscattered electron images of as-hipped material and material after heat treatment at 700 °C. The fraction of α-Cr increases and a Nb rich phase (the bright phase in the electron micrographs) appears. The same has been observed in NiCr49Nb1 after service test. Hot Isostatic Pressing – HIP‘17 Materials Research Forum LLC Materials Research Proceedings 10 (2019) 98-106 doi: http://dx.doi.org/10.21741/9781644900031-14 102 Fig. 5 Backscattered electron images of NiCr49Nb1as-hipped and after 1580 hours at 700 °C. Bending of the spindle. During service the spindle disc bends upwards, as sketched in Fig. 6. This causes the seat angle (defined as the angle between the seat surface and the direction perpendicular to the spindle stem axis, see Fig. 6) to decrease during service from the starting value of 30.3° degrees. The change in seat angle is plotted versus the service time in Fig. 7: the seat angle continues to change even after 10000 hours in service. Fig. 6 Sketch of the spindle showing the seat angle and the direction of the bending. Crack formation at the seat. Penetrant testing reveals that all the spindles, after service, have a crack running along the inner circumference of the seat area. Fig. 8 shows a polished cross section of the seat area after etching with Marble solution. The crack is perpendicular to the surface, about 3 mm long and crosses almost entirely the 316L buffer layer. Hot Isostatic Pressing – HIP‘17 Materials Research Forum LLC Materials Research Proceedings 10 (2019) 98-106 doi: http://dx.doi.org/10.21741/9781644900031-14 103 Fig. 7 Change of seat angle over service time for spindles 2, 3 and 4. “9/12 mm” refers to the thickness of the bottom NiCr49Nb1 coating. Fig. 8 Opt