The Effect of Thermal History on Microstructural Evolution, Cold-Work Refinement and {\alpha}/\b{eta} Growth in Ti-6Al-4V Wire + Arc AM

Wire + arc additive manufacture (WAAM) is an attractive method for manufacturing large-scale aerospace components, however the microstructural changes that occur and the effect of interpass rolling are poorly understood. Therefore two fundamental studies were conducted: the first involved temperature measurement of a wrought dummy wall so that the microstructural changes in the heat affected zone (HAZ) could be related to the thermal cycle. This demonstrated that the white band in the microstructure corresponded to 825 C well below the beta-transus temperature and above this boundary the bi-modal substrate material was converted to lamellar. The second involved peening WAAM material along the side of a deposited wall before applying a typical WAAM thermal heat treatment. This showed that refinement occurred up to the first layer band in the microstructure and the smallest grains were observed just above this boundary at higher temperatures significant grain growth occurred. This study has provided the foundational understanding of microstructural changes that will facilitate future process developments.Comment: 15 Figures, 25 pages, Journal publicatio

Numerical study of rolling process on the plastic strain distribution in wire + arc additive manufactured Ti-6Al-4V

Wire+arc additive manufacturing (WAAM) is an additive manufacturing (AM) process that employs wire as the feedstock and an arc as energy source, to construct near net-shape components at high build rates. Ti-6Al-4V deposits typically form large columnar prior β grains that can grow through the entire component height, leading to anisotropy and lower mechanical properties, compared to the equivalent wrought alloy. Cold-working techniques such as rolling can be used to promote grain refinement in Ti-6Al-4V WAAM parts, thus increasing strength and eliminating anisotropy concomitantly. Additionally, rolling can be beneficial in terms of reduction of residual stress and distortion. The aim of this study is to illustrate the effect of rolling process parameters on the plastic deformation characteristics in Ti-6Al-4V WAAM structures. To produce a certain refinement of the microstructure, a certain amount of strain is typically required; thus suitable design guidelines for practical applications are needed. The effect of different rolling process parameters, in particular, rolling load and roller profile radius on the plastic strain distribution is investigated based on the finite element method. From a numerical point of view, the effect of the stiffness of the roller is investigated, e.g. deformable vs. rigid roller. Results indicate that for an identical rolling load, the deformable roller produces lower equivalent plastic strains due to its own elastic deformation. Additionally, a lower friction coefficient produces higher equivalent plastic strains near the top surface but, it has an insignificant effect on the plastic deformation further away from the top surface. However, numerically the computation time significantly increased for a higher friction coefficient. Larger roller profile radii lead to lower plastic strain near the top surface, but simultaneously had nearly no noticeable effect on plastic strains at deeper depth. In addition, the effect of interspace between rollers on the uniformity of the plastic strain during multi-pass rolling was investigated for a selected example. The results show that a higher uniform plastic strain distribution is obtained when the interspace between two rollers is equal to the residual width of the groove produced by a single rolling pas

The legitimacy-conferring capacity of constitutional courts: Evidence from a comparative survey experiment

Can constitutional court decisions shape public opinion on a governmental policy? Previous studies have focused on the US Supreme Court, which enjoys a high degree of public support as the major resource of power for courts. In this study, we examine the extent to which courts can influence public opinion regarding a government bill at European courts. First, we argue that the public support for courts also allows them to move public opinion on policies into the direction of their decisions. This works in both directions: they can confer legitimacy to a policy that they support, but they can also de-legitimize a policy that they oppose. Second, we argue that this mechanism strongly depends on the amount of support that a court receives. It only has an effect for courts that possess a higher institutional legitimacy and among the group of citizens trusting a court. We test our arguments by combining a most different systems design for France and Germany with a survey priming experiment on a school security bill. France and Germany are selected for a most different systems design as they exhibit different institutional designs as well as different levels of support for the court at the aggregate level. The survey experiment is implemented within large national election surveys, the German Internet Panel and the French National Election Study. Both experiments contain more than 2,600 respondents each. Our survey experiment primes for decision outcomes and different institutions to understand whether there are differences between an institution supporting and opposing a policy and between a court and alternative institutions. Our findings confirm that with higher public support, courts can move the opinion of citizens to both legitimize and de-legitimize a policy. This effect can be found at the aggregate level for a court enjoying higher public support, but also at the individual level for respondents with higher trust in the court. Interestingly, courts can even move the opinion of citizens with strong prior attitudes in the opposite direction, if these citizens highly trust the court. These findings have implications beyond the study itself. First, they confirm that the legitimacy-conferring effect can also be observed for European courts, not only for the US Supreme Court. Second, they show that the relevance of a mechanism identified for a single case, like the US Supreme Court, might only hold for specific conditions. As public support for courts strongly varies across countries in Europe, we also expect the impact of any mechanism relying on public support to strongly vary, as we can observe in our own analysis

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

A guilt-free strategy increases self-reported non-compliance with COVID-19 preventive measures:Experimental evidence from 12 countries

Studies of citizens’ compliance with COVID-19 preventive measures routinely rely on survey data. While such data are essential, public health restrictions provide clear signals of what is socially desirable in this context, creating a potential source of response bias in self-reported measures of compliance. In this research, we examine whether the results of a guilt-free strategy recently proposed to lessen this constraint are generalizable across twelve countries, and whether the treatment effect varies across subgroups. Our findings show that the guilt-free strategy is a useful tool in every country included, increasing respondents’ proclivity to report non-compliance by 9 to 16 percentage points. This effect holds for different subgroups based on gender, age and education. We conclude that the inclusion of this strategy should be the new standard for survey research that aims to provide crucial data on the current pandemic

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

Interpass rolling of Ti-6Al-4V wire + arc additively manufactured features for microstructural refinement

In-process deformation methods such as rolling can be used to refine the large columnar grains that form when wire + arc additively manufacturing (WAAM) titanium alloys. Due to the laterally restrained geometry, application to thick walls and intersecting features required the development of a new ‘inverted profile’ roller. A larger radii roller increased the extent of the recrystallised area, providing a more uniform grain size, and higher loads increased the amount of refinement. Electron backscatter diffraction showed that the majority of the strain is generated toward the edges of the rolled groove, up to 3 mm below the rolled surface. These results will help facilitate future optimisation of the rolling process and industrialisation of WAAM for large-scale titanium components