Investigation of Tensile Properties of Bulk and SLM Fabricated 304L Stainless Steel Using Various Gage Length Specimens

The complex solidification dynamics and thermal cycling during Selective Laser Melting process is expected to result in non-equilibrium material characteristics. There is an essential need for characterization techniques which are critical towards the estimation of anisotropies. The current investigation is targeted towards establishing tensile testing methodologies and their relation to differing gage lengths. Dog-bone shaped specimen designs with gage lengths of 1”, 0.3” and 0.12” were employed in this research. The characterization was performed on hot rolled-annealed 304 stainless and SLM fabricated 304L stainless. It was theorized that smaller gage length specimens would be instrumental in mapping material property anisotropy at a better spatial resolution. The ultimate tensile and yield strength data were used to identify the material property distribution and assess the anisotropy. The material property distributions were used to successfully assess the testing methodologies and material characteristics.Mechanical Engineerin

Calibration of the Johnson-Cook Material Model for Additively Manufactured 304L SS Parts: Modeling and Experiments

Selective laser melting (SLM) is a type of additive manufacturing technique which uses a powder bed to form complex metal parts in a layer-by-layer process. This study aims to understand the material flow of parts manufactured by SLM process using 304L stainless steel powder, which is widely used in numerous applications. The tensile specimens were manufactured using 304LSS powder through SLM process. Low strain-rates, high temperature tensile tests were carried out to calibrate the parameters of the constitutive Johnson-Cook strength model. To conduct the tensile tests, different temperatures (25 oC, 150 oC and 250 oC) and strain-rates (0.1 s-1, 0.01 s-1 and 0.001 s-1) were used. The material model developed was used in numerical simulation of the tensile tests and compared with the experimental results.Mechanical Engineerin

JOHNSON-COOK FAILURE MODEL FOR ADDITIVELY MANUFACTURED 304L STAINLESS STEEL PARTS

Laser powder bed fusion (LPBF) process is a type of additive manufacturing technique which uses a powder bed to form complex metal parts in a layer-by-layer process. This study aims to understand the damage initiation in the parts manufactured by LPBF process using 304L stainless steel powder, which is widely used in numerous applications. The tensile specimens were manufactured using 304LSS powder through LPBF. Tensile specimens with varying notches were tested to calibrate the parameters of the constitutive Johnson-Cook failure model. To obtain the strength parameters, the tensile tests were performed at different temperatures and strain-rates. The material model developed was used in numerical simulation of the tensile tests and compared with the experimental results.Mechanical Engineerin

INFLUENCE OF STEEL ALLOY COMPOSITION ON THE PROCESS ROBUSTNESS OF AS-BUILT HARDNESS IN LASER-DIRECTED ENERGY DEPOSITION

To ensure consistent quality of additively manufactured parts, it is advantageous to identify alloys which can meet performance criteria while being robust to process variations. Toward such an end, this work studied the effect of steel alloy composition on the process robustness of as-built hardness in laser-directed energy deposition (L-DED). In-situ blending of ultra-high-strength lowalloy steel (UHSLA) and pure iron powders produced 10 alloys containing 10-100% UHSLA by mass. Thin-wall samples were deposited, and the hardness sensitivity of each alloy was evaluated with respect to laser power and interlayer delay time. The sensitivity peaked at 40-50% UHSLA, corresponding to phase fluctuations between lath martensite and upper bainite depending on the cooling rate. Lower (10-20%) or higher (70-100%) alloy contents transformed primarily to ferrite or martensite, respectively, with auto-tempering of martensite at lower cooling rates. By avoiding martensite/bainite fluctuations, the robustness was improved.Mechanical Engineerin

A Model for the Stray Light Contamination of the UVCS Instrument on SOHO

We present a detailed model of stray-light suppression in the spectrometer channels of the Ultraviolet Coronagraph Spectrometer (UVCS) on the SOHO spacecraft. The control of diffracted and scattered stray light from the bright solar disk is one of the most important tasks of a coronagraph. We compute the fractions of light that diffract past the UVCS external occulter and non-specularly pass into the spectrometer slit. The diffracted component of the stray light depends on the finite aperture of the primary mirror and on its figure. The amount of non-specular scattering depends mainly on the micro-roughness of the mirror. For reasonable choices of these quantities, the modeled stray-light fraction agrees well with measurements of stray light made both in the laboratory and during the UVCS mission. The models were constructed for the bright H I Lyman alpha emission line, but they are applicable to other spectral lines as well.Comment: 19 pages, 5 figures, Solar Physics, in pres

Near-Limb Zeeman and Hanle Diagnostics

"Weak" magnetic-field diagnostics in faint objects near the bright solar disk are discussed in terms of the level of non-object signatures, in particular, of the stray light in telescopes. Calculated dependencies of the stray light caused by diffraction at the 0.5-, 1.6-, and 4-meter entrance aperture are presented. The requirements for micro-roughness of refractive and reflective primary optics are compared. Several methods for reducing the stray light (the Lyot coronagraphic technique, multiple stages of apodizing in the focal and exit pupil planes, apodizing in the entrance aperture plane with a special mask), and reducing the random and systematic errors are noted. An acceptable level of stray light in telescopes is estimated for the V-profile recording with a signal-to-noise ratio greater than three. Prospects for the limb chromosphere magnetic measurements are indicated.Comment: 11 pages, 3 figure

Coronal Shock Waves, EUV Waves, and Their Relation to CMEs. III. Shock-Associated CME/EUV Wave in an Event with a Two-Component EUV Transient

On 17 January 2010, STEREO-B observed in extreme ultraviolet (EUV) and white light a large-scale dome-shaped expanding coronal transient with perfectly connected off-limb and on-disk signatures. Veronig et al. (2010, ApJL 716, 57) concluded that the dome was formed by a weak shock wave. We have revealed two EUV components, one of which corresponded to this transient. All of its properties found from EUV, white light, and a metric type II burst match expectations for a freely expanding coronal shock wave including correspondence to the fast-mode speed distribution, while the transient sweeping over the solar surface had a speed typical of EUV waves. The shock wave was presumably excited by an abrupt filament eruption. Both a weak shock approximation and a power-law fit match kinematics of the transient near the Sun. Moreover, the power-law fit matches expansion of the CME leading edge up to 24 solar radii. The second, quasi-stationary EUV component near the dimming was presumably associated with a stretched CME structure; no indications of opening magnetic fields have been detected far from the eruption region.Comment: 18 pages, 10 figures. Solar Physics, published online. The final publication is available at http://www.springerlink.co

Interchange Slip-Running Reconnection and Sweeping SEP Beams

We present a new model to explain how particles (solar energetic particles; SEPs), accelerated at a reconnection site that is not magnetically connected to the Earth, could eventually propagate along the well-connected open flux tube. Our model is based on the results of a low-beta resistive magnetohydrodynamics simulation of a three-dimensional line-tied and initially current-free bipole, that is embedded in a non-uniform open potential field. The topology of this configuration is that of an asymmetric coronal null-point, with a closed fan surface and an open outer spine. When driven by slow photospheric shearing motions, field lines, initially fully anchored below the fan dome, reconnect at the null point, and jump to the open magnetic domain. This is the standard interchange mode as sketched and calculated in 2D. The key result in 3D is that, reconnected open field lines located in the vicinity of the outer spine, keep reconnecting continuously, across an open quasi-separatrix layer, as previously identified for non-open-null-point reconnection. The apparent slipping motion of these field lines leads to form an extended narrow magnetic flux tube at high altitude. Because of the slip-running reconnection, we conjecture that if energetic particles would be traveling through, or be accelerated inside, the diffusion region, they would be successively injected along continuously reconnecting field lines that are connected farther and farther from the spine. At the scale of the full Sun, owing to the super-radial expansion of field lines below 3 solar radii, such energetic particles could easily be injected in field lines slipping over significant distances, and could eventually reach the distant flux tube that is well-connected to the Earth

Recent Advances in Understanding Particle Acceleration Processes in Solar Flares

We review basic theoretical concepts in particle acceleration, with particular emphasis on processes likely to occur in regions of magnetic reconnection. Several new developments are discussed, including detailed studies of reconnection in three-dimensional magnetic field configurations (e.g., current sheets, collapsing traps, separatrix regions) and stochastic acceleration in a turbulent environment. Fluid, test-particle, and particle-in-cell approaches are used and results compared. While these studies show considerable promise in accounting for the various observational manifestations of solar flares, they are limited by a number of factors, mostly relating to available computational power. Not the least of these issues is the need to explicitly incorporate the electrodynamic feedback of the accelerated particles themselves on the environment in which they are accelerated. A brief prognosis for future advancement is offered.Comment: This is a chapter in a monograph on the physics of solar flares, inspired by RHESSI observations. The individual articles are to appear in Space Science Reviews (2011