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At least 667 records · Page 37
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DOE OSTI2019

Porosity Prediction in A356 Wheel Casting

A FLUENT™-based thermal-fluid-compositional model has been developed and applied to a low-pressure, die-cast, A356 aluminum alloy wheel to explore the effects of macro-segregation of hydrogen and late-stage liquid encapsulation on pore formation during solidification. Based on the evolution of temperature, pressure, and hydrogen concentration in the liquid output from FLUENT, the porosity size distribution has been predicted at a number of locations in a commercially produced wheel using an in-house micro-porosity model. The porosity model incorporates pore nucleation kinetics based on hydrogen supersaturation, and two modes of pore growth: the first, occurring at low solid fractions is based on hydrogen diffusion; and the second, occurring at high solid fractions is based on conservation of volume. Samples from a commercially cast wheel have been analyzed using X-ray micro-tomography to provide basic validation of the micro-porosity model. The predicted results showed that the evolution in pressure has the dominant effect on pore growth, but only under conditions where pores have nucleated prior to late-stage liquid encapsulation. Otherwise, the cooling rate appears to have the dominant effect. The accuracy of the model is discussed in terms of the predicted pore size distribution and number density of pores. Areas for continued development are presented.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Numerical Simulation and Experimental Validation of Nondendritic Structure Formation in Magnesium Alloy Under Oscillation and Ultrasonic Vibration

In this study, the formation of nondendritic structures in the primary phase of magnesium alloy solidified under oscillation and ultrasonic vibration was investigated by numerical simulation and experimentally. The growth and motion of a dendrite during solidification was simulated by a combination of the lattice Boltzmann method and the phase-field method. The simulation and experimental results indicated that higher oscillation amplitudes and acoustic streaming made the microstructures change from dendritic to nondendritic in the α-Mg primary phases. A sufficient shear stress and an appropriate flow time in the barrel should be satisfied when a given inclined angle is selected. The effects of the flow and the thermal field on the nucleation behavior, the constitutional undercooling, and the growth morphology are also discussed. It was found that a high shear rate and high turbulence can help homogenize the temperature and the concentration fields and collide and rotate the α-Mg primary phases.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Formation Mechanisms and Microstructure Characterization of Al/Al{sub 3}Ni In-situ Composite by Compound Casting

The Compound casting method was used to produce an Al/Ni bimetal composite. The mechanisms of formation of the reinforcing intermetallic phases were studied and the produced bimetal composite’s microstructure was then characterized. The results showed that the Al/Ni interface consisted of two intermetallic layers including Al{sub 3}Ni{sub 2} and Al{sub 3}Ni. It is suggested that the Al{sub 3}Ni reinforcing particles of the composite originated from two sources. (1) Some Al{sub 3}Ni particles detached from the Al{sub 3}Ni layer formed at the interface between the Ni core and the Al melt and dispersed in the liquid Al and (2) Al-Al{sub 3}Ni eutectic phase, which formed during the solidification process. The increase in the temperature led to the formation of more reinforcing particles and extended the depth of dispersion of the particles in the Al matrix.

36 MATERIALS SCIENCE↗
DOE OSTI2013

The Role of Elastic and Plastic Anisotropy of Sn in Recrystallization and Damage Evolution During Thermal Cycling in SAC305 Solder Joints

Because failures in lead-free solder joints occur at locations other than the most highly shear-strained regions, reliability prediction is challenging. To gain physical understanding of this phenomenon, physically based understanding of how elastic and plastic deformation anisotropy affect microstructural evolution during thermomechanical cycling is necessary. Upon solidification, SAC305 (Sn-3.0Ag-0.5Cu) solder joints are usually single or tricrystals. The evolution of microstructures and properties is characterized statistically using optical and orientation imaging microscopy. In situ synchrotron x-ray measurements during thermal cycling are used to examine how crystal orientation and thermal cycling history change strain history. Extensive characterization of a low-stress plastic ball grid array (PBGA) package design at different stages of cycling history is compared with preliminary experiments using higher-stress package designs. With time and thermal history, microstructural evolution occurs mostly from continuous recrystallization and particle coarsening that is unique to each joint, because of the specific interaction between local thermal and displacement boundary conditions and the strong anisotropic elastic, plastic, expansion, and diffusional properties of Sn crystals. The rate of development of recrystallized microstructures is a strong function of strain and aging. Cracks form at recrystallized (random) boundaries, and then percolate through recrystallized regions. Complications arising from electromigration and corrosion are also considered.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Homogenizing Advanced Alloys: Thermodynamic and Kinetic Simulations Followed by Experimental Results

Segregation of solute elements occurs in nearly all metal alloys during solidification. The resultant elemental partitioning can severely degrade as-cast material properties and lead to difficulties during post-processing (e.g., hot shorts and incipient melting). Many cast articles are subjected to a homogenization heat treatment in order to minimize segregation and improve their performance. Traditionally, homogenization heat treatments are based upon past practice or time-consuming trial and error experiments. Through the use of thermodynamic and kinetic modeling software, NETL has designed a systematic method to optimize homogenization heat treatments. Use of the method allows engineers and researchers to homogenize casting chemistries to levels appropriate for a given application. The method also allows for the adjustment of heat treatment schedules to fit limitations on in-house equipment (capability, reliability, etc.) while maintaining clear numeric targets for segregation reduction. In this approach, the Scheil module within Thermo-Calc is used to predict the as-cast segregation present within an alloy, and then diffusion controlled transformations is used to model homogenization kinetics as a function of time and temperature. Examples of computationally designed heat treatments and verification of their effects on segregation and properties of real castings are presented.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Process Evaluation of AISI 4340 Steel Manufactured by Laser Powder Bed Fusion

Laser powder bed fusion (L-PBF) involves the consolidation of metal powder, layer by layer, through laser melting and solidification. In this study, process parameters are optimized for AISI 4340 steel to produce dense and homogeneous structures. The optimized process parameters produce mechanical properties at the center of the build plate that are comparable to wrought in the vertical and horizontal orientations after heat treatment and machining. Four subsequent builds are filled with specimens to evaluate the mechanical behavior as a function of location and orientation. Variations in the mechanical properties are likely due to recoater blade interactions with the powder and uneven gas flow. The results obtained in this study are analyzed to assess the reliability and reproducibility of the process. A different build evaluates the performance of near-net-shaped tensile specimens angled 35°-90° from the build plate surface (horizontal). Ductility measurements and surface roughness vary significantly as a function of the build angle. In the stress-relieved and as-built conditions, the mechanical behavior of vertically oriented specimens exhibits somewhat lower and more variable ductility than horizontally oriented specimens. Therefore, several process variables affect the mechanical properties of parts produced by the L-PBF process.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Abnormal Transient Liquid Phase Bondability of High-Volume Fraction SiC Particle-Reinforced A356 Composite for Cu Interlayer and the Interlayer Improvement Routes

Features of transient liquid phase (TLP) bondability of high-volume fraction (70 vol.%) SiC particle-reinforced A356 composite for popular Cu interlayer were investigated. Although Cu foil was melted, molten Cu was completely expulsed, indicating that the numerous SiC particles significantly deteriorated wettability not only at particle/braze metal (P/M) interface, but also at the matrix/braze metal (M/M) interface. For the reason of poor wettability at P/M interface, a model showing there is a “window period” for actual wetting reaction (allowing liquid phase to be in contact with surficial SiC particles) much shorter than nominal isothermal solidification time during TLP bonding of the composite containing sintered ceramic preform was proposed. For the poor wettability at M/M interface, both size effect of too small matrix unit (2 ~ 20 μm) and restraint effect of the rigid porous ceramic preform on suppressing oxide film crack on the small matrix unit surface were proposed. Thus, chemical route to disrupt oxide film on the small matrix unit surface should be preferred in braze composition design. For example, Al-27Cu-5Si-2Mg-1Ti braze was developed by co-addition of Mg and Ti to improve wettability and to in situ reinforce bond seam, respectively; Ti also could remedy Mg loss for improving wettability.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Tribological Performance of Ni{sub 3}Al Matrix Self-Lubricating Composites Containing Multilayer Graphene Prepared by Additive Manufacturing

In order to improve tribological performance of Ni{sub 3}Al-based alloy, Ni{sub 3}Al matrix composites containing 1.5 wt.% multilayer graphene (MLG) are prepared through additive manufacturing (AM) and spark plasma sintering (SPS), which are denoted as NMAM and NMSPS, respectively. Tribological behaviors of NMAM and NMSPS against Si{sub 3}N{sub 4} balls are researched under constant speed (0.2 m/s) and varied loads (from 4 to 16 N) for evaluating the tribological properties of NMAM and NMSPS. The results present that NMAM exhibits the excellent tribological properties [low friction coefficients (0.26-0.40) and considerable wear resistance (2.8-4.6 × 10{sup −5} mm{sup 3} N{sup −1} m{sup −1})] as compared to NMSPS, which attributes to the uniform enrichment of MLG with properties of high tensile strength and being easily sheared off on the worn surfaces. Owing to the use of spherical prealloyed powder containing multilayer graphene and the characteristics of layer by layer depositing in the AM process, NMAM has a more compact and uniform substrate, which persistently provides a source of the formation of continuous and stable frictional layer. Due to the characteristics of AM rapid solidification, NMAM has the small grain size and well-compacted microstructure, which can effectively reduce the probability of spalling wear and lead to the increase in wear resistance of materials. The research can offer the reference for self-lubricating materials prepared by AM technology.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Ca Addition Effects on the Microstructure, Tensile and Corrosion Properties of Mg Matrix Alloy Containing 8 wt.% Mg{sub 2}Si

The microstructure, tensile properties and corrosion behavior of the Mg-8 wt.% Mg{sub 2}Si-x%Ca alloy have been studied by the use of optical microscopy, scanning electron microscopy equipped with energy-dispersive spectroscopy, x-ray diffraction, standard tensile testing, polarization test and electrochemical impedance spectroscopy (EIS) measurements. Microstructural studies indicated that Ca modifies both primary and eutectic Mg{sub 2}Si phase. It was found that the average size of primary Mg{sub 2}Si particles is about 60 μm, which is dropped by about 82% in the alloy containing 0.05 wt.% Ca. By the addition of different Ca contents, Ca-rich intermetallics (i.e., CaSi{sub 2} and CaMgSi) were formed. The modification mechanism of adding Ca during solidification was found to be due to the strong effect of CaMgSi phase as a heterogonous nucleation site, apart from CaSi{sub 2} which was reported before, for Mg{sub 2}Si intermetallics. Tensile testing results ascertained that Ca addition enhances both ultimate tensile strength (UTS) and elongation values. The optimum amount of Ca was found to be 0.1 wt.%, which improved UTS and elongation values from about 130 MPa and 2% to 165 MPa and 5.5%, whereas more Ca addition (i.e., 3 wt.%) reduced the tensile properties of the alloy to about 105 MPa and 1.8%, which can be due to the formation of CaMgSi intermetallics with deteriorating needle-like morphology. Polarization and EIS tests also showed that the Mg-3%Si-0.5%Ca alloy pronounces as the best anti-corrosion alloy. Nevertheless, further added Ca (up to 3 wt.%) deteriorated the corrosion resistance due to predominance of worse galvanic coupling effect stemmed from the presence of stronger CaMgSi cathode in comparison with Mg{sub 2}Si. With higher Ca additions, an adverse effect was seen on corrosion resistance of the Mg-3%Si alloy, as a result of forming a weak film on the alloy specimen surface.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Nucleation and Grain Refinement of 7A04 Aluminum Alloy Under a Low-Power Electromagnetic Pulse

The effects of a low-power electromagnetic pulse on the grain size and cooling curve of high-strength aluminum alloy 7A04 were investigated for various pulse duty cycles. This electromagnetic pulse treatment was found to effectively produce fine grains with globular crystals and a uniform microstructure for pulse duty cycles between 20 and 40%. The key factors that affected grain refinement under the electromagnetic pulse included the electromagnetic energy and the conversion frequency between B and E. The nucleation rate increased as the nucleation period was extended. A new kinetic condition of magnetic nucleation was explored by decreasing the critical Gibbs free energy in the electromagnetic pulse, which was more sensitive under low undercooling. In addition, the crystal orientation was controlled in such a solidification environment.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Experimental Investigation of Microstructure and Phase Transitions in Ag-Cu-Zn Brazing Alloys

Microstructure and phase transitions of selected brazing alloys from the Ag-Cu-Zn ternary system were investigated. Four ternary alloys with silver content in the compositional range from 25 to 60 wt.% were studied using x-ray diffraction (XRD) and scanning electron microscopy coupled with the energy-dispersive spectroscopy (SEM–EDS). Phase transitions of the investigated alloys were measured using differential scanning calorimetry (DSC). Experimentally obtained results were compared with the results of a thermodynamic calculation of the phase equilibria according to the CALPHAD method. The experiments confirmed the optimized thermodynamic parameters for the calculations from the thermodynamic assessment in literature. Phase compositions, liquidus and solidus temperatures were confirmed by the EDS and DTA methods. Additionally, the calculated solidification paths and predicted phase transformations were in agreement with the SEM images.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Effect of Al on Grain Refinement and Mechanical Properties of Mg-3Nd Casting Alloy

The effect of Al on the grain refinement and mechanical properties of as-cast Mg-3Nd alloy was investigated systematically by a series of microstructural analysis, solidification analysis and tensile tests. The results show that Al has an obvious refining effect on the as-cast Mg-3Nd alloy. With increasing Al content, the grain size of the as-cast Mg-3Nd alloy decreases firstly, then increases slightly after the Al content reaching 3 wt.%, and the minimum grain size of the Mg-3Nd alloy is 48 ± 4.0 μm. The refining mechanism can be attributed to the formation of Al{sub 2}Nd particles, which play an important role in the heterogeneous nucleation. The strength and elongation of the Mg-3Nd alloy refined by Al also increase with increasing Al content and slightly decrease when the Al content is more than 3 wt.%, and the strengthening mechanism is attributed to the grain refinement as well as dispersed intermetallic particles. Furthermore, the microstructural thermal stability of the Mg-3Nd-3Al alloy is higher than that of the Mg-3Nd-0.5Zr alloy. Overall, the Mg-3Nd alloy with Al addition is a novel alloy with wide and potential application prospects.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Characterization of the Interface of an Alloy 625 Overlay on Steels Using Nanoindentation

Industry standards require postweld heat treatment (PWHT) to reduce the heat-affected zone hardness of steels such as F22 (2.25Cr-1Mo) and AISI 8630 overlaid (clad) with Alloy 625 weld metal. PWHT results in carbon diffusion and accumulation at the interface between the steel and overlay. The accumulation of carbon in a planar solidification growth zone adjacent to the fusion boundary results in high hardness and the potential for hydrogen-assisted cracking. The planar growth zone (PGZ) is so narrow that normal Vickers hardness testing cannot fully reveal the hardness distribution in this zone. This study focused on the application of nanoindentation to characterize the hardness in the narrow microstructural regions adjacent to the fusion boundary. The development of nanohardness maps revealed that the PGZ is not necessarily the region that exhibits peak hardness after PWHT. The highest hardness values were associated with clusters of M{sub 7}C{sub 3} carbides in specific subregions in the PGZ and also in the partially-mixed zone adjacent to the fusion boundary or in steel “swirl” structures. It was also confirmed in this study that nanohardness has a linear correlation with Vickers hardness values. The results presented here provide new insight into the role of carbon diffusion during PWHT and its effect on interface embrittlement associated with Alloy 625 overlays on steel.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Formation and Toughening Mechanisms of Dispersions in Interfacial Intermetallics of Dissimilar Laser Al/Steel Joints

The key to improve the strength of Al/steel dissimilar joint is to toughen the hard and brittle interfacial intermetallics. Introducing soft and tough dispersions into the interfacial intermetallics would be a promising method. In the present study, Zn-Al alloy (Zn-22Al) was used as the filler metal to generate dispersions in the interfacial intermetallics matrix of dissimilar laser Al/steel joint. The results have shown that soft and tough dispersions, i.e., FeZn{sub 10,} Al-rich amorphous phase, and Zn solid solution, were successfully introduced into the hard and brittle Fe{sub 2}Al{sub 5−x}Zn{sub x} matrix. The formation of these dispersions was resulted from diffusion of elements Fe and Al and infiltration of liquid Zn along the grain boundaries of Fe{sub 2}Al{sub 5−x}Zn{sub x} matrix and the subsequent chemical reaction and solidification. With this kind of interfacial microstructure, the joint exhibited a stable peak fracture load of 1200 N even though the interfacial layer thickness increased from 20 to 45 μm. This was because the interfacial layer was toughened by the soft and tough dispersions. Using this example, introduction of soft and tough dispersions into hard and brittle intermetallics matrix to toughen interfacial layer and then strengthening of dissimilar joints will be highlighted.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Processing Parameter Effects on Residual Stress and Mechanical Properties of Selective Laser Melted Ti6Al4V

Selective laser melting (SLM) process is characterized by large temperature gradients resulting in high levels of residual stress within the additively manufactured metallic structure. SLM-processed Ti6Al4V yields a martensitic microstructure due to the rapid solidification and results in a ductility generally lower than a hot working equivalent. Post-process heat treatments can be applied to SLM components to remove in-built residual stress and improve ductility. Residual stress buildup and the mechanical properties of SLM parts can be controlled by varying the SLM process parameters. This investigation studies the effect of layer thickness on residual stress and mechanical properties of SLM Ti6Al4V parts. This is the first-of-its kind study on the effect of varying power and exposure in conjunction with keeping the energy density constant on residual stress and mechanical properties of SLM Ti6Al4V components. It was found that decreasing power and increasing exposure for the same energy density lowered the residual stress and improved the % elongation of SLM Ti6Al4V parts. Increasing layer thickness resulted in lowering the residual stress at the detriment of mechanical properties. The study is based on detailed experimental analysis along with finite element simulation of the process using ABAQUS to understand the underlying physics of the process.

36 MATERIALS SCIENCE↗
DOE OSTI2018

The Influence of Galvanizing Parameters on the Structural Development of Zn-Al-Based Coatings

The evolution of some layers’ characteristics, deposited by galvanization, was monitored according to the technological parameters of the process. Two types of alloys from the Zn-Al and Zn-Al-Ti-B systems were used for deposition. The structure of the deposited layers was analyzed by optical and electron microscopy. The chemical composition, phases and compounds in the structures were identified. The mechanical properties of the structural constituents of the layers were determined by nanoindentation. These techniques highlighted the typical non-uniformities in terms of composition and properties in the layers, mainly due to the diffusion and distribution of iron from the steel substrate in the deposited layers. The evaluation of iron diffusion in the deposited layer was done by differential thermal analysis. The values of the critical temperatures (melting temperature, eutectic temperature and solidification temperature) were correlated with the parameters used during deposition. It was observed that the structure of the coatings is layered, with a specific order from the steel substrate to the edge. In these layers, the constituents are mixtures of soft solid solutions of Al in Zn (η and β) and hard intermetallic compounds. Moreover, increasing the immersion time in the galvanizing bath and the bath temperature leads to an increase in iron diffusion in the coating layer, which contributes to lower coating thickness, thus reducing its durability in operation.

36 MATERIALS SCIENCE↗
DOE OSTI2018

The Application of Centrifugal Atomization Method for Preparation of Rapidly Solidified Nd-Fe-B Flakes Used for Production of Permanent Magnets

Nd-Fe-B-type permanent magnets are used in applications that require a high-energy product/volume ratio in order to reduce weight. Automotive industry, hard drives, or wind turbines are just examples of an application where their use can be found. Conventional casting techniques reveal the formation of a high quantity of α-Fe and large Nd-rich regions. On the other hand, new techniques, like strip casting, melt-spinning and centrifugal atomization, produce homogeneous and fine-scaled microstructures. This paper discusses the application of rapid solidification by means of the centrifugal atomization method for preparation of Nd-Fe-B flakes. The effect of alloy composition and various process parameters of centrifugal atomization on the microstructure of rapidly solidified Nd-Fe-B alloy were investigated. The microstructures and the phase composition were examined by metallographic techniques, namely optical and scanning electron microscopy. Additionally, the influence of the processing parameters on the microstructures of as-cast flakes and subsequent magnetic properties of the prepared magnets will be discussed.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Novel Fabrication of a TiB{sub 2} Grain Refiner and its Effect on Reducing Hot Tearing in AZ91D Magnesium Alloy

Despite the increased use of magnesium (Mg) alloys in diverse engineering applications in recent years, further incorporation of complex Mg alloy castings remains challenging due to the alloy’s relatively high susceptibility to hot tearing. A significant reduction in hot tearing via grain refinement was achieved in aluminum and ferrous alloys; however, an effective, long-lasting and inexpensive grain refiner for aluminum-containing Mg alloys remains elusive. In this work, a TiB{sub 2} grain refiner was fabricated via a novel spark plasma sintering (SPS) powder metallurgy process. Then, its effectiveness in grain refining an AZ91D magnesium alloy was compared to that of a commercially available TiB{sub 2} grain refiner produced by hot extrusion. Due to the homogeneous distribution of the TiAl{sub 3} and TiB{sub 2} particles in the grain refiner, as well as their effect on the alloy’s microstructure evolution and solidification behavior, the SPS-TiB{sub 2} grain refiner was observed to achieve a significant reduction in the grain size and hot tearing of the Al-containing AZ91D magnesium alloy.

36 MATERIALS SCIENCE↗