Numerical investigation of the effect of rolling on the localized stress and strain induction for wire + arc additive manufactured structures

Cold rolling can be used in-process or post-process to improve microstructure, mechanical properties and residual stress in directed-energy-deposition techniques, such as the high deposition rate wire + arc additive manufacturing (WAAM) process. Finite element simulations of the rolling process are employed to investigate the effect of rolling parameters, in particular rolling load and roller profile radius on the residual stress field as well as plastic strain distribution for the profiled roller. The results show the response to rolling of commonly used structural metals in WAAM, i.e., AA2319, S335JR steel and Ti-6Al-4V, taking into account the presence of residual stresses. The rolling load leads to changes in the location and the maximum value of the compressive residual stresses, as well as the depth of the compressive residual stresses. However, the roller profile radius only changes the maximum value of these compressive residual stresses. Changing the rolling load influences the equivalent plastic strain close to the top surface of the wall as well as in deeper areas, whereas the influence of the roller profile radius is negligible. The plastic strain distribution is virtually unaffected by the initial residual stresses prior to rolling. Finally, design curves were generated from the simulations for different materials, suggesting ideal rolling load and roller profile combinations for microstructural improvement requiring certain plastic strains at a specific depth of the additive structure

Quantification of strain fields and grain refinement in Ti-6Al-4V inter-pass rolled wire-arc AM by EBSD misorientation analysis

Inter-pass deformation is an effective method for refining the coarse β-grain structure normally produced in high-deposition-rate additive manufacturing processes, like wire-arc additive manufacturing. The effectiveness of applying contoured surface rolling deformation tracks to each added layer has been studied by developing, and applying, a large-area SEM-based strain mapping technique. This technique is based on calibration of the average point-to-point Local Average Misorientation (LAM) of α-phase lamellar variants in EBSD orientation data to the local effective plastic strain. Although limited in the strain range that can be measured, the technique has proven to be very effective for identifying the size and depth of the plastic zone induced by surface rolling, as well as the local strain distribution, up to a saturation limit of ~12%. The strain fields mapped showed a close correlation to the region and level of recrystallization that occurred in the deformation zones during rapid re-heating through the β transus. The β recrystallization identified was consistent with the local strain distribution within the plastic zones measured by the LAM method and previous work on the recrystallization mechanisms operating in WAAM inter-pass deformation processes

Judicial disagreement need not be political: dissent on the Estonian Supreme Court

I investigate the non-unanimous decisions of judges on the Estonian Supreme Court. I argue that since judges on the court enjoy high de jure independence, dissent frequently, and are integrated in the normal judicial hierarchy, the Estonian Supreme Court is a crucial case for the presumption that judicial disagreement reveals policy preferences. I analyse dissenting opinions using an ideal point response model. Examining the characteristics of cases which discriminated with respect to the recovered dimension, I show that this dimension cannot be interpreted as a meaningful policy dimension, but instead reflects disagreement about the proper scope of constitutional redress

The Effectiveness of Grain Refinement by Machine Hammer Peening in High Deposition Rate Wire-Arc AM Ti-6Al-4V

Surface deformation, applied in-process by machine hammer peening (MHP), has the potential to refine the coarse columnar β-grain structures normally found in high deposition rate Wire-Arc Additive Manufacturing (WAAM) processes with Ti alloys like Ti-6Al-4V. Effective refinement, as well as a reduction in texture strength, has been achieved in relatively thick sections and to a depth that is greater than that expected from the surface deformation induced by MHP. By application of MHP to each deposition track, the average β-grain size could be reduced from cm’s to less than 0.5 mm. Systematic experiments have been performed to investigate the origin of this interesting effect, which included ‘stop-action’ trials and separation of the strain and temperature gradients induced by the two process steps. The maximum depth of the plastic deformation from MHP required to generate new β-grain orientations was determined by electron backscatter diffraction local average misorientation analysis to be < 0.5 mm, which was less than the melt pool depth in the WAAM process. Nevertheless, new β-grain orientations were observed to form within the peened layer ahead of the approaching heat source as the peak temperature rose above the β transus, which then grew into the less deformed core of the wall as the temperature rose. This allowed the new grain orientations to penetrate deeper than the melt pool depth and survive to act as substrates for epitaxial growth at the fusion boundary during solidification, resulting in significant grain refinemen

High cycle fatigue and fatigue crack growth rate in additive manufactured titanium alloys

The Wire + Arc Additive Manufacture (WAAM) process can produce large metal parts in the metre scale, at much higher deposition rate and more efficient material usage compared to the powder bed fusion additive manufacturing (AM) processes. WAAM process also offers lead time reduction and much lower buy-to-fly ratio compared to traditional process methods, e.g. forgings. Research is much needed in the areas of fatigue and fracture performance for qualification and certification of additive manufactured aircraft components. In this study, specimens made of WAAM Ti-6Al-4V alloy were tested and analysed focusing on two key areas of structural integrity and durability: (1) High cycle fatigue and effect of defects: crack initiation at porosity defects was investigated via fatigue and interrupted fatigue-tomography testing performed on specimens with porosity defects purposely embedded in the specimen gauge section. Key findings are as follows. Presence of porosity did not affect the tensile strengths, however both ductility and fatigue strength were significantly reduced. Fatigue life could not be correlated by the applied stress, e.g. in terms of the S-N curves, owing to the different pore sizes. Using the fracture mechanics approach and Murakami’s stress intensity factor equation for pores, good correlation was found between the fatigue life and stress intensity factor range of the crack initiating defects. Predictive methods for fatigue strength reduction were developed taking account of the defect size, location, and distribution. (2) Fatigue crack growth rate: effect of heterogeneous microstructure was investigated via two different material deposition methods and testing two crack orientations. Fatigue crack growth rates were measured for damage tolerance design considerations. Unique microstructure features and their effect on the property anisotropy are discussed

Prinzipal Parlament : die Bedeutung europäischer Parlamente als Wahlorgane

Parlamente sind in parlamentarischen Regierungssystemen zentrale Machtverteilungsorgane und als solche Prinzipale der Regierung und weiterer externer Amtsträger. Dieser Beitrag zeigt, dass parlamentarische Wahlbefugnisse im Rahmen eines delegationstheoretischen Modells erstens Delegationsverluste reduzieren und zweitens zu Abweichungen von der idealtypischen parlamentarischen Delegationskette führen können. Ihre Analyse erlaubt Rückschlüsse auf das tatsächliche Ausmaß der in bestehenden Modellen nur angenommenen Beschränkung der Regierung durch externe Amtsträger. Empirisch werden erstmals die Wahlbefugnisse von 25 europäischen Parlamenten bei der Besetzung von sieben Staatsämtern systematisch vergleichend untersucht. Die Analyse zeigt deutliche Unterschiede zwischen den untersuchten Parlamenten und identifiziert Wahlbefugnisse als eine auch empirisch distinkte Dimension parlamentarischer Machtbefugnisse