Belotti, L. Palmeiravan Dommelen, J. A. W.Geers, M. G. D.Goulas, C....
13页
查看更多>>摘要：Wire arc additive manufacturing (WAAM) is a class of technologies suitable for producing large parts due to its high material deposition and building rates. Among the many possible materials processed by WAAM, austenitic stainless steels, e.g. 316L, are commonly employed. The structure of WAAM 316L thin parts has been studied extensively before. However, multiwalled or thick WAAM 316L parts remain largely unexplored. Hence, in this study, the microstructure of a thick 316LSi WAAM part is characterised in detail. The microstructure of the part consists of large and highly-oriented columnar grains dominated by epitaxial and competitive growth. The overlapping regions between neighbouring fusion zones contain long grains with a dominant 100 texture, which cross several layers and are aligned with the building direction. The grains' internal microstructure consists of an austenite matrix, ferrite with locally varying dendritic morphologies and dispersed oxide inclusions. The texture spatially varies across the part, and this variation is correlated to the local thermal gradient induced by the building strategy and processing conditions used during the manufacturing of the thick-walled part.
查看更多>>摘要：The melting process during laser powder bed fusion (LPBF) is accompanied by complex physical phenomena, which can be rarely scaled with acceptable effort to utilize the melting process in component design to obtain tailored mechanical properties with adequate machine productivity. A constitutive law relating melt pool geometry to the process parameters thereby plays an essential role in practically fostering machine productivity and component quality in LPBF. In this work, we derive an improved dimensionless scaling law based on dimensional analysis to characterize the melt pool width, which is also capable of scaling the beam-size-dependent laser attenuation. Measurements from LPBF-produced specimens and 3D phenomenological LPBF finite element simulations are also performed in batch to validate the derived law. The regression analysis presents the correlation similar to 97-98% for the simulated melt pool widths and similar to 70-90% for experimental measured ones, with fabricated minimum strut thickness of 113 mu m. After scaling the effective laser power with the calibrated laser attenuation, the proposed scaling law can be extended to be beam-size independent, as demonstrated by the regression of overall experimental measurements and simulation results, performed under different laser beam diameter, to the unified law with correlation >= 97%. It demonstrates the feasibility of proposed scaling law in melt pool controlled LPBF manufacturing for designing and producing thin-walled components with strut thicknesses in the micrometer range.
查看更多>>摘要：In specialised solidification processing techniques such as High Pressure Die Casting, Twin-Roll Casting and others, an additional external deformation load is applied to achieve the required shape, leading to the formation of microstructural features such as shear bands. The mechanism for forming these features is believed to be dependent on dynamically evolving strain fields, which are dependent on the local solid fraction, applied strain rates and casting geometry. To investigate this, a semisolid (similar to 50 % solid fraction) Al-10 wt.% Cu alloy is isothermally injected into a bespoke die using a custom-designed thermo-mechanical rig. The semisolid deformation, formation of Cu-rich dilatant bands and subsequent pore nucleation and growth are captured using fast synchrotron X-ray radiography. The local normal and shear strains acting on the mush are quantified using digital image correlation to identify the dilatant shear bands and the dominant local strain component. Correlating the radiographs with strain maps reveals that gas pores within the dilated interstices grow, while those in compressed regions are squeezed out. A linear correlation between accumulated volumetric strain and porosity volume fraction demonstrates that higher dilations give rise to a local increase in both gas and shrinkage porosity.
查看更多>>摘要：In this review the authors provide a comprehensive insight into additive manufacturing process mechanics which explore how thermodynamics give rise to characteristic microstructure and property. Discussion is then given over to how solidification microstructure is modified using processing conditions and utilisation of traditional casting methods which give rise to grain refinement and some cases 'texture by design'. The review then makes observation and provides analysis on the typical material types reported in powder bed fusion and directed energy deposition. These are organised to efficiently highlight the key observations in terms of alloy creation and adaption with a specific focus on the relevant additive manufacturing technique. The emergent alloy families are also explored alongside the rapidly expanding topic of functionally graded alloys produced by the breadth of additive manufacturing techniques. The review concludes with a detailed critique of the state-of-the-art and an examination of where opportunities exist to advance the interdependency between process and emergent alloys exhibiting superior properties. The authors propose several areas worthy of consideration and attempt to provide inspiration for our community in the pursuit of new alloys and accompanying process designs for additive manufacturing.
查看更多>>摘要：Bonding of tungsten (W) with oxide dispersion strengthened (ODS) steel is an urgent challenge during the practical manufacturing process in many applications, including advanced nuclear reactor systems, military equipment, etc. To eliminate the generation of brittle intermetallic compounds Fe-W and relieve the residual thermal stresses, a novel composite interlayer Zr/Cu, for bonding W with ODS steel using the transient liquid phase (TLP) diffusion technology, was designed in this work. The bonding materials were prepared as a sandwiched structure of W/Zr-Cu/ODS steel in a vacuum diffusion bonding device. Interfacial structure and tensile strength of the bonded joints were investigated, and the processing parameters were optimized. After the bonding process, a reliable Cu/ODS steel interface could be built at all applied temperatures or holding times. Nevertheless, only as the bonding temperature was 1000 degrees C or even higher, the inserted Zr foil could be completely consumed and a stable W/Cu interface free from intermetallic compounds and micro-cracks was built. Additionally, the bonding ratio was gradually improved with increasing the bonding temperature or holding time. As a result, a high-strength W/Cu/ODS steel joint of -430.4 MPa could be fabricated at 1000 degrees C for a holding time of 60 min, and the bonding ratio was as high as 98 %, which provided a significant guide for the assembly of plasma facing components (PFCs).
查看更多>>摘要：In this study, ultrasonic surface rolling process (USRP) was used to fabricate a gradient nanostructured commercial pure titanium. The site-specific microstructure, refining mechanisms and mechanical properties were investigated by high-resolution transmission electron microscopy, electron backscatter diffraction and tensile test. The results show that the surface layer exhibited multi-gradient features along the depth, including grain size gradient, deformation twin gradient, strain gradient, orientation gradient and hardness gradient. The gradient structure consisting of equiaxed nanograin layer, elongated lamellar layer, deformation twinning profuse layer, and coarse-grained layer along the depth direction. The formation mechanisms of gradient nano-structure are thoroughly elucidated, and the synergistic effect of twin and dislocation plays an important role in grain refinement. In addition, amorphous bands (generated by localized dislocations or phase transformation triggered crystalline to amorphous transition) and nano-thick lamellar with face-centered cubic (FCC) structure (<0001 >(HCP)//<001 >(FCC), {1 (1) over bar 00}(HCP)//{2 (2) over bar0}(FCC)( )and {11 (2) over bar0}(HCP)//{220}(FCC)) were formed in the nanograin layer, and the phase transformation mechanisms were elucidated. The HCP to FCC phase transformation and amorphization play a role in refining grain. Tensile test results suggested that the gradient nanostructure improved the strength (yield strength and ultimate tensile strength improved from 342 MPa and 526 MPa to 412 MPa and 598 MPa, respectively) while maintaining enough plasticity. The strength-plasticity synergy was attributed to the coordinated deformation of the gradient nanostructure and the coarse-grained core.
查看更多>>摘要：Wire -arc additive manufacture (WAAM) is suitable for Inconel 718 components due to its high deposition efficiency. However, large columnar dendrites decrease the mechanical properties and can cause severe mechanical anisotropy. Cold rolling and warm rolling through flame heating have been investigated to analyze their effects on microstructure and tensile properties compared to as-deposited WAAM material. Standard solution and double aging (SA), as well as homogenization followed by solution and aging (HSA) heat treatments were compared. The results show that the large columnar dendrites change to finer equiaxed grains 16.4 mu m and 26.2 mu m in size for warm and cold rolled alloy, respectively. This increases to 22.5 mu m and 30.1 mu m after HSA treatment. The microhardness and strength of rolled material increase significantly and the warm rolled material after HSA treatment exceeds that of the wrought alloy. While the as-deposited and cold rolled samples both show significant anisotropy, isotropic tensile properties are obtained for warm rolled plus HSA heat treated samples. Finer equiaxed grains with more dispersive distributions of gamma' and gamma" strengthening precipitation contribute to the superior mechanical properties for warm rolled material. For both the cold and warm rolled material, there was an elongation decrease due to precipitated particles, which also led to a trans-granular ductile fracture mode. The strengthening mechanism of the hybrid rolling process was analyzed and found to be related to work hardening, grain boundary strengthening, precipitated strengthening phases and the 8 phase.
查看更多>>摘要：A 'co-shaft in-situ rolling-imprinting technique' is proposed for the production of a silver micro-nanowire array with ultra-high slenderness ratio on transparent polyethylene terephthalate (PET) film. Two major mechanisms microrolling-tooth array mold cutting and silver micro-nanowire array rolling-imprinting are designed for the rolling-imprinting system. The setup allowed for mold cutting and micro-nanowire rolling-imprinting to be conducted in the same coordinate system with the same concentric accuracy. To provide steady micro-amounts of silver-paste supply, a silver-paste supply mechanism was designed comprising a fine screw thread, paste storage tank and comb-shaped microchannel array. Experiments show by changing surface roughness so that the contact angle of the silver-paste on the roller mold is larger than on the PET film, it was possible to transfer of silver-paste smoothly onto the PET film. Doctoring blade placement was optimized by using optimal letterpresswidth and minimal letterpress-gap to ensure silver-paste was scraped off the roller mold in a manner that minimized wire-width. Silver-paste molecules were subject to optimal congregation effect to ensure minimal wire-width for a 5-mu m letterpress-width, 1-mu m letterpress-gap, and Ra 6-nm surface roughness on the roller mold. The study utilized silver-paste's innate 'internal force balance characteristic' to great effect. The resultant silver micro-nanowires were 5.1 mu m is width, 1 mu m in thickness and of ultra-high slenderness ratio, high-straightness, -consistency and -regularity. The effectiveness of the array was tested driving a LED device. The influence of convex versus concave microrolling-tooth molds, droplet forces, and silver-paste thixotropy were also all discussed in detail.
查看更多>>摘要：In this study, tungsten and steel were successfully bonded utilizing hot isostatic pressing diffusion bonding with a Ni-Si-B interlayer. The interfacial structures and formation mechanisms of joints with interlayers of various thicknesses were systematically investigated. The results indicated that the interfacial structure of a joint with a thin (0.8 mu m) Ni-Si-B interlayer is a W/Fe-W inter-metallic compound (IMC) reaction layer/(Fe,Ni)ss diffusion layer/steel structure, where through cracks appear at the interface of the W/reaction layer, leading to poor shear strength. As the interlayer thickness increases (3.2-22.9 mu m), the brittle Fe-W IMC no longer appears and the interfacial structure is a W/Ni-W IMCs reaction layer/(Ni,Fe)ss diffusion layer/steel structure. When the interlayer thickness is 3.2 mu m, there are many large voids in the diffusion layer. When the interlayer thickness increases to 5.3 mu m, the most compact interfacial structure can be obtained with the maximum bonding strength of 363.5 MPa. However, when the interlayer thickness increases further to 12.6 and 22.9 mu m, many voids appear in the reaction layer, leading to a decrease in bonding strength. The formation mechanism of a joint with a thin (0.8 mu m) interlayer is a purely solid-solid reaction without the generation of a liquid phase. When the interlayer thickness is large (3.2-22.9 mu m), the formation process of joints includes three stages: solid-solid reaction, remaining interlayer liquefaction, and solid-liquid reaction. Electron probe microanalysis results reveal that the melting-point depressant element Si is segregated in the reaction layer or diffusion layer depending on the interlayer thickness, which is precisely controlled by the staged formation mechanism of the joints.
查看更多>>摘要：In this paper, the limitations of the original Hill48 yield model in predicting uniaxial tensile yield stresses and r- values were analyzed in detail, and then the necessary and sufficient conditions for judging the applicability of the original Hill48 yield model were proposed. On this basis, the approximate satisfactions of the steel sheet DC04, DP980 and the aluminum alloy AA5754-O to the necessary and sufficient conditions were investigated. The results show that the necessary and sufficient conditions can be used to qualitatively analyze the predicted error of the original Hill48 model. Based on the inherent relationship of Hill48 yield function, the direction of principal stress axis was introduced into the anisotropic parameters, and a modified Hill48 model suitable for plane stress state was developed. According to the variation of the predicted error of the original Hill48 yield model in biaxial tensile stress, an exponential interpolation method including material parameter was proposed to correct the asymmetric biaxial stress between uniaxial stress state and symmetric biaxial stress state. The theoretical predicted results show that the modified model can accurately capture the directional yield stresses, r- values and yield locus. The main advantage of the modified model is that it still maintains a simple quadratic form in the stress tensor, and can significantly improve the predicted accuracy of anisotropic behavior of aluminum alloy and other materials from the use of associated flow rule.
NSTL
Elsevier