Strengthening die-cast Al-Mg and Al-Mg-Mn alloys with Fe as a beneficial element

Jaguar Range Rover (JLR

Microstructure and Creep Behavior of Mg-Al Alloys Containing Alkaline and Rare Earth Additions

In the past few decades governmental regulation and consumer demands have lead the automotive companies towards vehicle lightweighting. Powertrain components offer significant potential for vehicle weight reductions. Recently, magnesium alloys have shown promise for use in powertrain applications where creep has been a limiting factor. These systems are Mg-Al based, with alkaline earth or rare earth additions. The solidification, microstructure, and creep behavior of a series of Mg- 4 Al- 4 X:(Ca, Ce, La, and Sr) alloys and a commercially developed AXJ530 (Mg – 5 Al – 3 Ca – 0.15 Sr) alloy (by wt%) have been investigated. The order of decreasing freezing range of the five alloys was: AX44, AXJ530, AJ44, ALa44 and ACe44. All alloys exhibited a solid solution primary α-Mg phase surrounded by an interdendritic region of Mg and intermetallic(s). The primary phase was composed of grains approximately an order of magnitude larger than the cellular structure. All alloys were permanent mold cast directly to creep specimens and AXJ530 specimens were provided in die-cast form. The tensile creep behavior was investigated at 175 °C for stresses ranging from 40 to 100 MPa. The order of decreasing creep resistance was: die-cast AXJ530 and permanent mold cast AXJ530, AX44, AJ44, ALa44 and ACe44. Grain size, solute concentration, and matrix precipitates were the most significant microstructural features that influenced the creep resistance. Decreases in grain size or increases in solute concentration, both Al and the ternary addition, lowered the minimum creep rate. In the Mg-Al-Ca alloys, finely distributed Al2Ca precipitates in the matrix also improved the creep resistance by a factor of ten over the same alloy with coarse precipitates. The morphology of the eutectic region was distinct between alloys but did not contribute to difference in creep behavior. Creep strain distribution for the Mg-Al-Ca alloys developed heterogeneously on the scale of the α-Mg grains. As additional bulk strain accumulated, strain localized along grain boundaries between grains with significantly different Schmid factors. At these locations cavities and cracks formed that led to the eventual creep failure. Grain size influenced the plastic strain distribution during creep.Ph.D.Materials Science and EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/58486/1/nsaddock_1.pd

THREE-DIMENSIONAL MICROSTRUCTURAL EFFECTS ON MULTI-SITE FATIGUE CRACK NUCLEATION BEHAVIORS OF HIGH STRENGTH ALUMINUM ALLOYS

An experimental method was further developed to quantify the anisotropy of multi-site fatigue crack initiation behaviors in high strength Al alloys by four-point bend fatigue testing under stress control. In this method, fatigue crack initiation sites (fatigue weak-links, FWLs) were measured on the sample surface at different cyclic stress levels. The FWL density in an alloy could be best described using a three-parameter Weibull function of stress, though other types of sigmoidal functions might also be used to quantify the relationship between FWL density and stress. The strength distribution of the FWLs was derived from the Weibull function determined by fitting the FWLs vs. stress curve experimentally obtained. As materials properties, the FWL density and strength distribution could be used to evaluate the fatigue crack nucleation behaviors of engineering alloys quantitatively and the alloy quality in terms of FWL density and strength distribution. In this work, the effects of environment, types of microstructural heterogeneities and loading direction on FWLs were all studied in detail in AA7075-T651, AA2026-T3511, and A713 Al alloys, etc. It was also found that FWLs should be quantified as a Weibull-type function of strain instead of stress, when the applied maximum cyclic stress exceeded the yield strength of the tested alloys. In this work, four-point bend fatigue tests were conducted on the L-T (Rolling-Transverse), L-S (Rolling-Short transverse) and T-S planes of an AA7075-T651 alloy plate, respectively, at room temperature, 20 Hz, R=0.1, in air. The FWL populations, measured on these surfaces, were a Weibull-type function of the applied maximum cyclic stress, from which FWL density and strength distribution could be determined. The alloy showed a significant anisotropy of FWLs with the weak-link density being 11 mm-2, 15 mm-2 and 4 mm-2 on the L-T, L-S and T-S planes, respectively. Fatigue cracks were predominantly initiated at Fe-containing particles on the L-T and L-S planes, but only at Si-bearing particles on the T-S plane, profoundly demonstrating that the pre-fractured Fe-containing particles were responsible for crack initiation on the L-T and L-S planes, since the pre-fracture of these particles due to extensive deformation in the L direction during the prior rolling operation could only promote crack initiation when the sample was cyclically stressed in the L direction on both the L-T and L-S planes. The fatigue strengths of the L-T, L-S and T-S planes of the AA7075 alloy were measured to be 243.6, 273.0 and 280.6 MPa, respectively. The differences in grain and particle structures between these planes were responsible for the anisotropy of fatigue strength and FWLs on these planes. Three types of fatigue cracks from particles, type-I: the micro-cracks in the particles could not propagate into the matrix, i.e., type-II: the micro-cracks were fully arrested soon after they propagated into the matrix, and type-III: the micro-cracks became long cracks, were observed in the AA7075-T651 alloy after fatigue testing. By cross-sectioning these three-types of particles using Focused Ion Beam (FIB), it was found that the thickness of the particles was the dominant factor controlling fatigue crack initiation at the particles, namely, the thicker a pre-fractured Fe-containing particle, the easier it became a type-III crack on the L-T and L-S planes. On the T-S plane, there were only types-I and III Si-bearing particles at which crack were initiated. The type-I particles were less than 6.5 μm in thickness and type-III particles were thicker than 8.3 μm. Cross-sectioning of these particles using FIB revealed that these particles all contained gas pores which promoted crack initiation at the particles because of higher stress concentration at the pores in connection with the particles. It was also found that fatigue cracks did not always follow the any specific crystallographic planes within each grain, based on the Electron Backscatter Diffraction (EBSD) measurement. Also, the grain orientation did not show a strong influence on crack initiation at particles which were located within the grains. The topography measurements with an Atomic Force Microscope (AFM) revealed that Fe-containing particles were protruded on the mechanically polished surface, while the Si-bearing particles were intruded on the surface, which was consistent with hardness measurements showing that Si-bearing particles were softer (4.030.92 GPa) than Fe-containing ones (8.9 0.87 GPa) in the alloy. To verify the 3-D effects of the pre-fractured particles on fatigue crack initiation in high strength Al alloys, rectangular micro-notches of three different types of dimensions were fabricated using FIB in the selected grains on the T-S planes of both AA2024-T351 and AA7075-T651 Al alloys, to mimic the three types of pre-fractured particles found in these alloys. Fatigue testing on these samples with the micro-notches verified that the wider and deeper the micro-notches, the easier fatigue cracks could be initiated at the notches. In the AA2024-T351 samples, cracks preferred to propagate along the {111} slip plane with the smallest twist angle and relatively large Schmid factor. These experimental data obtained in this work could pave a way to building a 3-D quantitative model for quantification of fatigue crack initiation behaviors by taking into account the driving force and resistance to short crack growth at the particles in the surface of these alloys

Casting and Solidification of Light Alloys

Investigation of the effect of casting and crystallization on the structure and properties of the resulting light alloys and, in particular, research connected with detailed analysis of the microstructure of light alloys obtained using various external influences of ultrasonic, vibration, magnetic, and mechanical processing on the casting and crystallization are discussed. Research on the study of introduction of additives (modifiers, reinforcers, including nanosized ones, etc.) into the melt during the crystallization process, the technological properties of casting (fluidity, segregation, shrinkage, etc.), the structure and physicomechanical properties of light alloys are also included

A holistic review on fatigue properties of additively manufactured metals

Additive manufacturing (AM) technology is undergoing rapid development and emerging as an advanced technique that can fabricate complex near-net shaped and light-weight metallic parts with acceptable strength and fatigue performance. A number of studies have indicated that the strength or other mechanical properties of AM metals are comparable or even superior to that of conventionally manufactured metals, but the fatigue performance is still a thorny problem that may hinder the replacement of currently used metallic components by AM counterparts when the cyclic loading and thus fatigue failure dominates. This article reviews the state-of-art published data on the fatigue properties of AM metals, principally including $S$--$N$ data and fatigue crack growth data. The AM techniques utilized to generate samples in this review include powder bed fusion (e.g., EBM, SLM, DMLS) and directed energy deposition (e.g., LENS, WAAM). Further, the fatigue properties of AM metallic materials that involve titanium alloys, aluminum alloys, stainless steel, nickel-based alloys, magnesium alloys, and high entropy alloys, are systematically overviewed. In addition, summary figures or tables for the published data on fatigue properties are presented for the above metals, the AM techniques, and the influencing factors (manufacturing parameters, e.g., built orientation, processing parameter, and post-processing). The effects of build direction, particle, geometry, manufacturing parameters, post-processing, and heat-treatment on fatigue properties, when available, are provided and discussed. The fatigue performance and main factors affecting the fatigue behavior of AM metals are finally compared and critically analyzed, thus potentially providing valuable guidance for improving the fatigue performance of AM metals.Comment: 201 pages, 154 figure

Ultrafine-Grained Metals

Ultrafine-grained metallic materials produced by severe plastic deformation methods are at the cutting edge of modern materials science. UFG-metals exhibit outstanding properties which make them very interesting for structural or functional engineering applications. Fifteen articles in this special issue address a broad variety of topics: New developments in severe plastic deformation techniques, advances in modeling and simulation of the severe plastic deformation processes, mechanical properties under monotonic and cyclic loading of homogenous and graded UFG structures, dominating deformation mechanisms in UFG materials, advances and strategies for high conductivity UFG-materials, correlation between severe plastic deformation parameters and resulting materials properties and peculiarities in the corrosion behavior of UFG materials. The book covers latest results on ultrafine-grained titanium, aluminum and copper alloys and on UFG iron and steels and thus provides a deep insight to current research activities in the field of ultrafine-grained metals

Recent Advancements in Metallic Glasses

The Special Issue “Recent Advancements in Metallic Glasses” presents ten original papers, considering both scientific and application issues related to metallic glasses. The papers are devoted to general consideration of the formation and defects of the glassy structure, defect evolution due to heat treatment, deformation behavior upon compression and high-pressure torsion, amorphous-crystalline transformation, hydrogenation behavior, and biomedical applications

Light Weight Alloys: Processing, Properties and Their Applications

There is growing interest in light metallic alloys for a wide number of applications owing to their processing efficiency, processability, long service life, and environmental sustainability. Aluminum, magnesium, and titanium alloys are addressed in this Special Issue, however, the predominant role played by aluminum. The collection of papers published here covers a wide range of topics that generally characterize the performance of the alloys after manufacturing by conventional and innovative processing routes

Development of Ti-Fe-based powders for laser additive manufacturing of ultrafine lamellar eutectics

Years of academic research has gone into developing Ti-Fe-based ultrafine eutectic and near-eutectic alloys with remarkable mechanical properties. Cast ingots (few mm in dimensions) have demonstrated high compressive strengths (> 2 GPa) similar to bulk metallic glasses (BMGs), while retaining more than 15 % plasticity at room temperature [1–3]. However, conventional casting methods are incapable of providing uniform and high cooling rates necessary for growing such ultrafine microstructures over large dimensions without introducing significant heterogeneities. On the other hand, laser-based Additive Manufacturing (AM) techniques with inherently very high cooling rates like Selective Laser Melting (SLM) (ranging 106 K/s) or Laser Metal Deposition (LMD) (ranging 104 – 105 K/s) are appropriate for such microstructural growth and their track and layer-wise building approach maintains an almost constant cooling rate throughout bulk. This strongly motivates the development of high-quality powders for SLM and LMD trials. In this work, pre-alloyed powder of Fe-rich near-eutectic composition Fe82.4Ti17.6 (at %) was developed for LMD, while powders of two Ti-rich compositions: near-eutectic Ti66Fe27Nb3Sn4 (at %) and off-eutectic Ti73.5Fe23Nb1.5Sn2 (at %) were explored for SLM trials. Three gas atomisation methods, namely Crucible-based Gas atomisation (CGA), Crucible-Free atomisation (CFA) and Arc-melting Atomisation (AMA) were investigated for optimising powder production. In addition to conventional techniques, a novel methodology was proposed for one-step screening of powders’ key features based on advanced image analysis of X-Ray Computed Tomography (XCT) data. The methodology generated volume-weighted particle size distributions (which were validated against conventional laser diffraction), provided accurate estimations of internal porosity and quantitatively evaluated the 3D morphology of powders. In order to create a solidification knowledge dataset and further optimise the processing of powders under high cooling rates, in-depth microstructural studies were performed on these powders sieved into different particle size ranges (experiencing different solidification rates during atomisation). Results revealed that powder particle size is clearly related to, and can possibly predict, the solidification pathway followed during gas atomisation as well as its degree of completion. The ultrafine interlamellar spacing λ (< 190 μm) of lamellar eutectics observed in powders of near-eutectic compostitions increased almost linearly with particle size and revealed solidification rates similar to those encountered during SLM/LMD processing of the same or similar compositions. Therefore, this work highlights the potential of gas atomisation as a method to study rapid solidification and Laser-AM processing. Finally, two alloys were consolidated by AM using pre-alloyed powders and characterised mechanically, i.e. LMD-built Fe82.4Ti17.6 with lamellar eutectic microstructure and SLM-built Ti73.5Fe23Nb1.5Sn2 (off-eutectic) showing a unique “composite” microstructure of α-Ti and β-Ti grains strengthened by FeTi dispersoids that partially arranged themeselves as fine lamellas. Both alloys showed high compressive yield strengths (≈ 1.8 GPa and ≈ 1.9 GPa) at room temperature, with Ti73.5Fe23Nb1.5Sn2 showing high plasticity up to 20 %. The alloy showed higher tensile yield strength and elongation at intermediate temperatures (450 °C to 600 °C) than popular (α+β) aerospace alloys, like Ti-6Al-4V built by laser-AM [4–6]. LMD-built Fe82.4Ti17.6 largely remained brittle below 500 °C, but out-performed similar induction cast [7] and sintered alloys in compressive yield strength, thus proving an impressive candidate for compression-based applications (like tools) in the intermediate temperature range.Programa de Doctorado en Ciencia e Ingeniería de Materiales por la Universidad Carlos III de MadridPresidenta: Mónica Campos Gómez.- Secretaria: Carmen Cepeda Jiménez.- Vocal: María San Sebastián Ormazába

Magnesium Alloys Structure and Properties

Magnesium Alloys Structure and Properties is a comprehensive overview of the latest knowledge in the field of magnesium alloys engineering. Modern magnesium alloys are promising for a variety of applications in many branches of the industry due to their excellent mechanical properties, high vibration, damping capacity, and high dimensional stability. This book discusses the production, processing, and application of magnesium alloys. It includes detailed information on the impact of alloying additives and selected casting technologies, as well as modern manufacturing technologies based on powder metallurgy, the production of composites and nano-composites with metal matrixes, and methods for improving alloy properties