Suitable Process Window Research Articles

Elucidating the Effect of Additive Friction Stir Deposition on the Resulting Microstructure and Mechanical Properties of Magnesium Alloy WE43

In this work, the effect of processing parameters on the resulting microstructure and mechanical properties of magnesium alloy WE43 processed via Additive Friction Stir Deposition (AFSD), a nascent solid-state additive manufacturing (AM) process, is investigated. In particular, a parameterization study was carried out, using multiple four-layer deposits, to identify a suitable process window for a structural 68-layers bulk WE43 deposition. The parametric study identified an acceptable set of parameters with minimal surface defects and excellent consolidation for the fabrication of a bulk WE43 deposition. Microstructural, tensile, and fatigue life characterization was conducted on the bulk WE43 deposition and compared to commercially available wrought material to elucidate the process-structure-property-performance (PSPP) relationship of the AFSD process. This study shows that the bulk WE43 deposit exhibited a refined homogenous microstructure and a texture shift relative to the wrought material. However, a reduction in hardness and tensile behavior was observed in the as-deposited WE43 compared to the wrought control. Additionally, fatigue specimens extracted from the bulk deposition exhibited a decrease in life in the low-cycle regime but performed comparably to the wrought plate in the high-cycle regime. The outcomes of this study illustrate the potential of the AFSD process in additively manufactured structural load-bearing components made with magnesium alloy WE43 in the as-built condition.

Severe plastic deformation as a processing tool for strengthening of additive manufactured alloys

For the first time, the novel combination of multi-pass equal channel angular extrusion/pressing (ECAE/P) and selective laser melting (SLM) was investigated. Herein, four passes of ECAP via route B c at 150 °C were applied as a severe plastic deformation (SPD) technique on the SLM as-built AlSi12 to promote superior mechanical properties. The microstructure and mechanical behavior of AlSi12 fabricated by SLM were studied before and after ECAP, applying several mechanical and microstructural characterization techniques. Results of the tensile experiments revealed that the yield point, the ultimate strength, and the ductility of the as-built sample were improved by 56%, 11%, and 55% after 4 passes of ECAP, respectively. This enhancement is attributed to the effective grain refinement and the persisting silicon phase network after SPD as evidenced by electron backscatter diffraction and elemental mapping results. Moreover, micro-computed tomography analysis disclosed that ECAP considerably reduces the remnant porosity of the post-treated SLM AlSi12 samples eventually further affecting the strength of the ultra-fine grained AlSi12 in a positive way. Findings presented herein indicate that it is viable to utilize ECAP as a post-AM processing tool for mechanical property improvement of laser powder bed fused microstructures with the virtue of enhanced densification. Even if geometrical restrictions exist in ECAP, results obtained herein are transferrable to other SPD techniques with suitable processing windows, which would pave the way to advanced properties of adequately post-treated conditions.

Investigation of nonisothermal fusion bonding for extrusion additive manufacturing of large structural parts

AbstractFusion bonding is understood to be the decisive mechanism for the interlayer strength in extrusion additive manufacturing. This study links the characteristic temperatures of semicrystalline thermoplastics, namely, PA6 with 40% carbon fibers, to the bond formation in respect to real‐world processing conditions. Based on theoretical investigations, a process window is proposed for bonding to occur without polymer degradation. This range from the glass transition to the initial degradation temperature was determined through differential scanning calorimetry and thermogravimetric analysis. A second process window for optimal bonding is proposed from the extrapolated onset crystallization temperature, , to the melt temperature, . The validation of these process windows was conducted by bending tests according to DIN EN 178. was confirmed as the upper limit, with the part's geometric integrity compromised at higher temperatures. had to be refuted as lower limit as no discrete reduction in bond strength was determined in alignment with . Authors suggest the lower limit is defined by the lowest substrate temperature to lead to interface temperatures above the onset of melt temperature. By utilizing thermal analyses, less time and resources are required to detemine a suitable process window for extrusion additive manufacturing.

Electron Beam Powder Bed Fusion of γ-Titanium Aluminide: Effect of Processing Parameters on Part Density, Surface Characteristics, and Aluminum Content

Gamma titanium aluminides are very interesting for their use in high-performance applications such as aircraft engines due to their low density, high stiffness and favorable high-temperature properties. However, the pronounced brittleness of these intermetallic alloys is a major challenge for their processing through conventional fabrication methods. Additive manufacturing by means of electron beam powder bed fusion (EB-PBF) significantly improves the processability of titanium aluminides due to the high preheating temperatures and facilitates complex components. The objective of this study was to determine a suitable processing window for EB-PBF of the TNM-B1 alloy (Ti-43.5Al-4Nb-1Mo-0.1B), using an increased aluminum content in the powder raw material to compensate for evaporation losses during the process. Design of experiments was used to evaluate the effect of beam current, scan speed, focus offset, line offset and layer thickness on porosity. Top surface roughness was assessed through laser scanning confocal microscopy. Scanning electron microscopy, electron backscatter diffraction (EBSD) and energy-dispersive X-ray spectroscopy (EDX) were used for microstructural investigation and to analyze aluminum loss depending on the volumetric energy density used in EB-PBF. An optimized process parameter set for achieving part densities of 99.9% and smooth top surfaces was derived. The results regarding microstructures and aluminum evaporation suggest a solidification via the β-phase.

Comprehensive study on the influence of different pretreatment methods and structural adhesives on the shear strength of hybrid CFRP/aluminum joints

ABSTRACT The aim of this study was to investigate the influence of laser treatments with varying energy density and wavelength on the surface structure, shear strength and fracture behavior of hybrid compounds of thermoset carbon fiber-reinforced polymer (CFRP) and aluminum (Al) bonded with three different structural adhesives. The CFRP was pretreated by laser with near-infrared (NIR) and ultraviolet (UV) wavelengths and applying different energy densities. Aluminum surfaces were structured with NIR. Reference CFRP/Al-specimens were prepared by acetone cleaning, low pressure oxygen/argon plasma or sand blasting. The surfaces were investigated microscopically before bonding and after mechanical testing. In addition, the surfaces were examined with white light interferometer and sessile droplet test methods. The bonded compounds were characterized in standardized single lap shear tests to reveal the influence of the pretreatment strategies on the joint strength. Suitable process windows for laser pretreatments were identified, including NIR-pretreatment methods that can compete with an UV-laser source. For every adhesive, a suitable pretreatment method was found.

Investigation of High-Depostition-Rate Additive Manufacturing of Ti-6Al-4V via Laser Material Deposition

Ti-6Al-4V is the most prominent titanium alloy widely used e.g. for aerospace applications. Conventionally, many Ti-6Al-4V aerospace components are produced by a multi-stage hot forging process followed by subsequent machining which often generates a high amount of scrap. Additive manufacturing (AM), such as powder-based laser material deposition (p-LMD), enables parts to be made with geometric freedom and near-net-shape, but so far lacks high deposition rates. The present study proposes high-deposition-rate laser material deposition manufacturing using a large laser beam diameter and increased scanning speed to achieve deposition rates up to 5 kg/h. As Ti-6Al-4V is prone to oxygen pick-up, the process was performed in an inert atmosphere. We determined suitable process windows for tracks without fusion defects and low porosity and investigated microstructure and hardness.

A study on the microstructure and mechanical properties of the Ti-6Al-2Sn-4Zr-6Mo alloy produced via Laser Powder Bed Fusion

• The LPBF Ti6246 microstructure is mainly composed of α” orthorhombic martensite. • The martensite provides low tensile strength and hardness but rather high ductility. • The microstructural and mechanical properties are related to the process parameters. • The yield strength can be markedly increased via sub-β annealing. Laser Powder Bed Fusion (LPBF) is an additive manufacturing technology which has been the subject of thorough research and successfully adopted in several industrial sectors. Among all the processable classes of materials, titanium alloys are especially interesting due to their favourable combination of mechanical properties and corrosion resistance. Most of the literature focuses on Ti-6Al-4V, although there are other alloys which are widely applied in fields that can benefit from the advantages of LPBF techniques, such as Ti-6Al-2Sn-4Zr-6Mo, thus far not investigated for this technology. This alloy is generally preferred to Ti-6Al-4V for the production of some components in the aerospace industry, mostly due to its superior strength. In this work, the most suitable process window for this alloy was investigated. Samples produced with two different combinations of process parameters, located in the selected process window, were then thoroughly studied in order to assess the effect of building conditions on the microstructure, phases and mechanical properties of the as-built and heat-treated material. To do so, an X-ray diffraction analysis was conducted with the aim of determining the phase composition and lattice parameters. Moreover, microstructural features, such as α” needles and α lath widths, were analysed in order to correlate the thermal history of the process to the final microstructure of the specimens. Furthermore, the hardness and the tensile properties of the alloy processed by LPBF were quantified and compared with the data available in the literature relative to conventional manufacturing technologies.

Single Scans of Ti-6Al-4V by Directed Energy Deposition: A Cost and Time Effective Methodology to Assess the Proper Process Window

Directed energy deposition is an additive manufacturing technology which usually relies on prototype machines or hybrid systems, assembled with parts from different producers. Because of this lack of standardization, the optimization of the process parameters is often a mandatory step in order to develop an efficient building process. Although, this preliminary phase is usually expensive both in terms of time and cost. The single scan approach allows to drastically reduce deposition time and material usage, as in fact only a stripe per parameters combination is deposited. These specimens can then be investigated, for example in terms of geometrical features (e.g. growth, width) and microstructure to assess the most suitable process window. In this work, Ti-6Al-4V single scans, produced by means of directed energy deposition, corresponding to a total of 50 different parameters combinations, were analyzed, focusing on several geometrical features and relative parameters correlations. Moreover, considering the susceptibility of the material to oxygen pick-up, the necessity of an additional shielding gas system was also evaluated, by comparing the specimens obtained with and without using a supplementary argon flow. A process window, which varies according to the user needs, was found together with a relationship between microstructure and process parameters, in both shielding scenarios.Graphic

High accuracy direct hot bending of hollow profiles

The paper presents the potentialities of thermal assisted processing to manufacture bent hollow profiles in multi-phase High Strength Steels. A new process, named Direct Hot Rotary Draw Bending (DHRDB), was developed and validated in the case of processing 22MnB5 profiles, allowing a localized thermal treatment of the hollow profile during the bending process. Interaction between thermal and tools kinematic cycles were investigated to identify the most suitable process windows capable of granting the required accuracy and mechanical properties. The results show that DHRDB allows overcoming the room temperature process limitations granting high accuracy, nearly absent springback and enhanced mechanical properties.

An Empirical Approach for the Development of Process Parameters for Laser Powder Bed Fusion.

For certain additive manufacturing technologies the choice of available materials is currently limited. The development of process parameters is especially elaborate for powder bed technologies. Currently, there is no common approach concerning the procedure and documentation. This work proposes a methodology for the initial development of process parameters for new L-PBF (laser powder bed fusion) alloys. Key elements are the examination of the laser–powder-bed interaction by single laser track experiments and an iterative design of experiment (DoE) approach for the development of volumetric parameters. Two types of single laser track experiments are presented and provide information regarding the laser track width and depth as well as the resulting surface roughness and melt pool classification. Based on the information gained, suitable process windows for a DoE study can be defined by avoiding parameter settings unsuitable for production or measurement. Gradually, input variables are identified and iterative steps reduce the process window in order to optimize the desired target values. Near-surface exposure parameters are developed by a one-dimensional parameter variation and metallographic investigations. The approach is primarily designed for the initial development of process parameters for new L-PBF alloys. However, the information gained can also be used to optimize established parameter sets regarding new target values (productivity, mechanical properties), optimize process parameters for specific components or for a microstructural design.

Response surface optimization for improving the processing behavior of melamine formaldehyde impregnation resins

AbstractMelamine‐formaldehyde resins are widely used for decorative paper impregnation. Resin properties relevant for impregnation are mainly determined already at the stage of resin synthesis by the applied reaction conditions. Thus, understanding the relationship between reaction conditions and technological properties is important. Response surface methodology based on orthogonal parameter level variations is the most suitable tool to identify and quantify factor effects and deduce causal correlation patterns. Here, two major process factors of MF resin synthesis were systematically varied using such a statistical experimental design. To arrive at resins having a broad range of technological properties, initial pH and M:F ratio were varied in a wide range (pH: 7.9–12.1; M:F ratio: 1:1.5–1:4.5). The impregnation behavior of the resins was modeled using viscosity, penetration rate and residual curing capacity as technological responses. Based on the response surface models, nonlinear and synergistic action of process factors was quantified and a suitable process window for preparing resins with favorable impregnation performance was defined. It was found that low M:F ratios (~1:2–1:2.5) and comparatively high starting pHs (~pH 11) yield impregnation resins with rapid impregnation behavior and good residual curing capacity.

Polymer Selection for Hot-Melt Extrusion Coupled to Fused Deposition Modelling in Pharmaceutics.

Three-dimensional (3D) printing offers the greatest potential to revolutionize the future of pharmaceutical manufacturing by overcoming challenges of conventional pharmaceutical operations and focusing design and production of dosage forms on the patient’s needs. Of the many technologies available, fusion deposition modelling (FDM) is considered of the lowest cost and higher reproducibility and accessibility, offering clear advantages in drug delivery. FDM requires in-house production of filaments of drug-containing thermoplastic polymers by hot-melt extrusion (HME), and the prospect of connecting the two technologies has been under investigation. The ability to integrate HME and FDM and predict and tailor the filaments’ properties will extend the range of printable polymers/formulations. Hence, this work revises the properties of the most common pharmaceutical-grade polymers used and their effect on extrudability, printability, and printing outcome, providing suitable processing windows for different raw materials. As a result, formulation selection will be more straightforward (considering the characteristics of drug and desired dosage form or release profile) and the processes setup will be more expedite (avoiding or mitigating typical processing issues), thus guaranteeing the success of both HME and FDM. Relevant techniques used to characterize filaments and 3D-printed dosage forms as an essential component for the evaluation of the quality output are also presented.

Analysis of weld characteristics of micro plasma arc welded thin stainless steel 306 L sheet

Abstract This work demonstrates joining of thin steel sheet using micro plasma arc welding (MPAW) process. A series of welding experiments are carried out by applying full factorial design of experiment. The process parameters considered are welding current, welding speed and stand-off distance. A sufficient number of trial experiments are conducted to find suitable process window for MPAW of 500 μm thick SS-316 L sheets. Seven weld bead geometry characteristics (viz., area of fusion zone, top bead width, bottom bead width, bead height or penetration, reinforcement, width of left side of heat affected zone (HAZ) and width of right side HAZ), three tensile properties (ultimate tensile strength, yield strength and percentage elongation), micro-hardness and grain size of HAZ are considered as weld quality parameters. Various outlier tests are applied first time in welding for detection of extreme data from the tensile data set. The effects of heat input and process parameters on the measured weld quality parameters are studied. Analysis of variance (ANOVA) is also applied to estimate the relative importance of individual parameter and their interaction on the total variability. Finally, regression models are developed for representing the input and output relationship of the MPAW process.

Friction Stir Welding and Friction Spot Stir Welding Processes of Polymers-State of the Art.

In the last decade, the friction stir welding of polymers has been increasingly investigated by the means of more and more sophisticated approaches. Since the early studies, which were aimed at proving the feasibility of the process for polymers and identifying suitable processing windows, great improvements have been achieved. This owes to the increasing care of academic researchers and industrial demands. These improvements have their roots in the promising results from pioneer studies; however, they are also the fruits of the adoption of more comprehensive approaches and the multidisciplinary analyses of results. The introduction of instrumented machines has enabled the online measurement of processing loads and temperature, and critical understanding of the principal aspects affecting the material flow and welds quality. Such improvements are also clearly demonstrated by the increase of the strength of recent joints (up to 99% of joining efficiency) as compared to those reached in early researches (almost 47%). This article provides a comprehensive review of the recent progresses on the process fundamentals, quality assessment and the influence of process parameters on the mechanical behavior. In addition, emphasis is given to new developments and future perspectives.

Effect of process parameters on additive-free electrospinning of regenerated silk fibroin nonwovens

Silk fibroin is a biomaterial with multiple beneficial properties for use in regenerative medicine and tissue engineering. When dissolving and processing the reconstituted silk fibroin solution by electrospinning, the arrangement and size of fibers can be manifold varied and according fiber diameters reduced to the nanometer range. Such nonwovens show high porosity as well as potential biocompatibility. Usually, electrospinning of most biomaterials demands for the application of additives, which enable stable electrospinning by adjusting viscosity, and are intended to evaporate during processing or to be washed out afterwards. However, the use of such additives increases costs and has to be taken into account in terms of biological risks when used for biomedical applications.In this study, we explored the possibilities of additive-free electrospinning of pure fibroin nonwovens and tried to optimize process parameters to enable stable processing. We used natural silk derived from the mulberry silkworm Bombyx mori. After degumming, the silk fibroin was dissolved and the viscosity of the spinning solution was controlled by partial evaporation of the initial solving agent. This way, we were able to completely avoid the use of additives and manufacture nonwovens, which potentially offer higher biocompatibility and reduced immunogenicity. Temperature and relative humidity during electrospinning were systematically varied (25–35 °C, 25–30% RH). In a second step, the nonwovens optionally underwent methanol treatment to initiate beta-sheet formation in order to increase structural integrity and strength. Comprehensive surface analysis on the different nonwovens was performed using scanning electron microscopy and supplemented by additional mechanical testing. Cytotoxicity was evaluated using BrdU-assay, XTT-assay, LDH-assay and live-dead staining. Our findings were, that an increase of temperature and relative humidity led to unequal fiber diameters and defective nonwovens. Resistance to penetration decreased accordingly. The most uniform fiber diameters of 998 ± 63 nm were obtained at 30 °C and 25% relative humidity, also showing the highest value for resistance to penetration (0.20 N). The according pure fibroin nonwoven also showed no signs of cytotoxicity. However, while the biological response showed statistical evidence, the material characteristics showed no statistically significant correlation to changes of the ambient conditions within the investigated ranges. We suggest that further experiments should explore additional ranges for temperature and humidity and further focus on the repeatability of material properties in dependency of suitable process windows.

Laser powder bed fusion of Nd\u2013Fe\u2013B permanent magnets

In this work, we use laser powder bed fusion (LPBF) to produce Nd–Fe–B magnets. A suitable process window is developed, which allows to fabricate isotropic samples with outstanding magnetic performance. The sample quality is mainly defined by the energy input during LPBF and sintering or delamination occurs, if the process parameter are improperly adjusted. Magnetic and structural properties become better as energy input increases, until the material-specific limit for processability has been reached. Magnets with coercivity of 886 kA/m (µ0Hc = 1.1 T) and maximum energy product of 63 kJ/m3 can be produced from Nd-lean commercial powder without any post treatment. Thereby, our samples represent the new benchmark for permanent magnets produced by additive manufacturing. On the example of coercivity, the impact of laser power, scan velocity and hatch spacing is discussed. It is shown that coercivity can be sufficiently well described by a simple phenomenological model.

Enhancing the Lithium Ion Conductivity of an All Solid-State Electrolyte via Dry and Solvent-Free Scalable Series Production Processes

This work is about the possibility to enhance the lithium ion conductivity of a polymeric solid-state electrolyte via scalable production processes which can directly be utilized for series production. The solid electrolyte consists of PEO, LiTFSI and SiO2 and achieved a maximum ionic conductivity of σIon = 2.4 ·10−3 S cm−1 at 90 °C and σIon = 1.26 ·10−3 S cm−1 at 80 °C. For that, we present a scalable completely dry process chain consisting of granulation, plastification and calendering without the need of any solvents. Within these production processes the influence of process parameters like specific energy input, production temperature and filling degree to the properties of the polymeric solid electrolyte components (among others lithium ion conductivity, chain length, density) are investigated. One key finding is that a suitable process window in terms of specific energy input during plastification is very small and can be quantified for the given process geometry. Under- or overshooting these barriers can directly lead to a degradation of the whole system and as a result directly decreased lithium ion conductivities. Besides process parameters, we also investigate material and formulation parameters, like salt concentration, annealing time and measurement temperatures. Finally, a model is presented to describe the maximum achievable lithium ion conductivity as a function of the specific energy input during production.

Formability Analysis of Micro-Alloyed Sheet Metals Reinforced by Additive Manufacturing

Additive manufacturing (AM) offers the possibility of locally reinforcing sheet metal or sheet metal products by adding patches that are metallurgically bonded to the substrate. Due to the high design freedom of AM, patches can be easily adapted to loads in geometry and thickness. However, the heat input and the high cooling rates during AM processes have a strong influence on the microstructure in the patch as well as in the substrate, which will affect forming properties. The aim of this work is to investigate the influence of patches produced by laser material deposition (LMD) on formability of micro-alloyed sheet metals. After determining a suitable process window for metallurgically bonded patches without cracks and pores, investigations were carried out on the microstructure and mechanical properties of reinforced samples. This work includes metallographic examinations using optical microscopy, hardness measurements and tensile tests. The formability of sheets with local reinforcement was investigated by stretching and Nakajima tests. The heat input creates a heat affected zone (HAZ) directly next to the patches with a reduced strength, caused by recrystallization that may lead to failure in the forming process and thus limits the forming capacity of locally reinforced sheet metals. A subsequent laser heat treatment can homogenize the properties in the HAZ.

Flow behavior and formability of hot-rolled Mg-8Gd-3Y alloy under double-pass isothermal compression

Multi-pass hot rolling can help overcome the poor formability of high-strength Mg-Gd-Y alloys at ambient and intermedium temperature. It is essential to explore deformation characteristics of the alloys under such discontinuous thermomechanical processing. In the current work, double-pass isothermal compression tests were carried out to study the flow behavior and formability of a hot-rolled Mg-8Gd-3Y alloy at the temperature range of 350–450 °C with a strain rate range of 0.001–0.1 s−1. Two kinds of hardening behavior characterized by higher second-pass peak stress than the first-pass unloading stress are discovered. One is caused by precipitation that occurs during holding at a temperature range of 350–400 °C under a strain rate of 0.1 s−1. The other is grain-growth-induced hardening and takes place at 450 °C with a strain rate range of 0.01–0.1 s−1. Based on the dynamic material model, three-dimensional power dissipation and instability maps were established, corroborating improved formability under double-pass isothermal compression. The results clarify that employing inter-pass holding during hot working can widen the suitable processing window and reduce deformation bands.

Poly-Si/SiOx/c-Si passivating contact with 738 mV implied open circuit voltage fabricated by hot-wire chemical vapor deposition

Hot-wire chemical vapor deposition (HWCVD) was utilized to develop a fast and high quality a-Si:H thin film fabrication method for poly-Si/SiOx carrier selective passivating contacts targeting at n-type passivated emitter rear totally diffused crystalline silicon solar cells. The microstructure and hydrogen content of the a-Si:H thin films were analyzed by Fourier-transform infrared spectroscopy in order to understand the influence of film properties on passivation and conductivity. Dense layers were found to be beneficial for good passivation. On the other hand, blistering appeared as a-Si:H layers became more and more dense. However, by adjusting the SiH4 flow rate and the substrate heater temperature, blistering of a-Si:H could be avoided. A suitable process window was found and firing-stable implied open circuit voltage (iVoc) of up to 738 mV was achieved. In addition to high iVoc, a low contact resistivity of 0.034 Ω cm2 was also achieved. The deposition rate of the a-Si:H layers was 7 Å/s by using HWCVD, which is one order of magnitude higher than the deposition rate reported using other deposition methods.