Primary Intermetallics Research Articles

Effects of Multiple Reflow on the Formation of Primary Crystals in Sn-3.5Ag and Solder Joint Strength: Experimental and Finite Element Analysis.

The growth and formation of primary intermetallics formed in Sn-3.5Ag soldered on copper organic solderability preservative (Cu-OSP) and electroless nickel immersion gold (ENIG) surface finish after multiple reflows were systematically investigated. Real-time synchrotron imaging was used to investigate the microstructure, focusing on the in situ growth behavior of primary intermetallics during the solid-liquid-solid interactions. The high-speed shear test was conducted to observe the correlation of microstructure formation to the solder joint strength. Subsequently, the experimental results were correlated with the numerical Finite Element (FE) modeling using ANSYS software to investigate the effects of primary intermetallics on the reliability of solder joints. In the Sn-3.5Ag/Cu-OSP solder joint, the well-known Cu6Sn5 interfacial intermetallic compounds (IMCs) layer was observed in each reflow, where the thickness of the IMC layer increases with an increasing number of reflows due to the Cu diffusion from the substrate. Meanwhile, for the Sn-3.5Ag/ENIG solder joints, the Ni3Sn4 interfacial IMC layer was formed first, followed by the (Cu, Ni)6Sn5 IMC layer, where the formation was detected after five cycles of reflow. The results obtained from real-time imaging prove that the Ni layer from the ENIG surface finish possessed an effective barrier to suppress and control the Cu dissolution from the substrates, as there is no sizeable primary phase observed up to four cycles of reflow. Thus, this resulted in a thinner IMC layer and smaller primary intermetallics, producing a stronger solder joint for Sn-3.5Ag/ENIG even after the repeated reflow process relative to the Sn-3.5Ag/Cu-OSP joints.

De-Ironing of Aluminium Alloy Melts by High Shear Melt Conditioning Technology: An Overview

The main problem of recycling aluminium scrap is the gradual accumulation of impurities, especially iron, which tend to form undesired intermetallic compounds that affect the integrity and the mechanical performance of the castings. In this paper, we aim to provide an overview on the topic of iron removal from aluminium melts through primary intermetallic precipitation and the progress made during the LiME Hub project to understand the process and to develop a more efficient procedure. We cover both thermodynamic analysis and experimental validation. We found that high shear melt conditioning technology enhances the typically slow nucleation and growth of the dense primary intermetallics, speeding up their sedimentation and allowing a faster removal of Fe from the melt by simple gravity sedimentation. It also promotes the formation of smaller and more compact Fe-rich intermetallics, allowing an increased volume fraction recovery and mitigating their effect of being present in the final castings. The technology is not limited to batch processing, with a 90% efficiency, but can also be applied to continuous melt treatment of aluminium scrap, with currently 60% efficiency, and could be combined with other solid–liquid separation techniques to increase the purification efficiency even more.

On the transferability of classical pairwise additive atomistic force field to the description of unary and multi-component systems: applications to the solidification of Al-based alloys.

Multi-component and multiphasic materials are continually being developed for electronics, aircraft, automotive, and general applications. Integrated Computational Materials Engineering (ICME) is a multiple-length scale approach that greatly benefits from atomistic scale simulations to explore new alloys. Molecular Dynamics (MD) allows to perform large-scale simulations by using classical interatomic potentials. The main challenge of using such a classical approach is the transferability of the interatomic potentials from one structure to another when one aims to study multi-component systems. In this work, the reliability of Zr, Al-Cu, Al-Cr and Al-Zr-Ti force field potentials is examined. It has been found that current interatomic potentials are not completely transferable due to the structure dependence from their parameterization. Besides that, they provide an appropriate description of unary and binary systems, notably for liquids, isotropic solids, and partially isotropic compounds. For solidification purposes, it has been found that coherent primary solidification of the FCC-phase in pure Al is highly dependent on the formalism to tune interatomic interactions. For Al-Cr alloys, the icosahedral short-range ordering (ISRO) increased by adding Cr to the melts. The different steps of solidification (formation of nuclei, effective germination of the α-Al phase and end of solidification) have been related to the evolution of the ISRO. The addition of Cr in melts prevented undercooling via icosahedral-enhanced nucleation of the α-Al phase. Precipitation of primary intermetallics in hyper-peritectic Al-Cr alloys was also tested. Contrary to classical thermodynamics predictions, α-Al phase was the primary precipitate for these alloys. This implies that Cr supersaturated the α-Al phase rather than forming intermetallic phases due to the high cooling rates.

Microstructure diversity dominated by the interplay between primary intermetallics and eutectics for Al-Ce heat-resistant alloys

Al-Ce alloys emerge as promising heat-resist aluminum alloys with service temperatures above 250 °C. Here we report the solidification behavior and the microstructure diversity of Al-Ce alloys, aiming to bridge the gap between solidification condition and microstructures. Both conventional casting and arc surface rapid melting (ASRM) are employed to generate varied solidification conditions. Owing to the large entropy of fusion, intermetallic Al11Ce3 phases are grown in a faceted mode with a strong growth anisotropy, and Chinese-script Al11Ce3/α-Al eutectics are produced upon conventional casting. The addition of Mg restrains the coupled growth of eutectics and promotes the generation of divorced eutectics in cast Al-Ce alloys. Upon rapid solidification during ASRM, regular lamellar and rod-like eutectics prevail in Al-Ce binary alloys, and fishbone eutectics dominate the Al-Ce-Mg ternary alloys. Also, the transition from a hypereutectic microstructure to a fully eutectic microstructure is triggered by ASRM. The interplay of intermetallic Al11Ce3 and Al11Ce3/α-Al eutectics is discussed and the formation of refined regular eutectics is highlighted.

Hypereutectic Al-Ca-Mn-(Ni) Alloys as Natural Eutectic Composites

In the present paper, Natural Metal-Matrix Composites (NMMC) based on multicomponent hypereutectic Al-Ca-(Mn)-(Ni) alloys were studied in as-cast, annealed and rolled conditions. Thermo-Calc software and microstructural observations were utilised for analysing the equilibrium and actual phase composition of the alloys including correction of the Al-Ca-Mn system liquidus projection and the solid phase distribution in the Al-Ca-Mn-Ni system. A previously unknown Al10CaMn2 was discovered by both electron microprobe analysis and X-ray studies. The Al-6Ca-3Mn, Al-8Ca-2Mn, Al-8Ca-2Mn-1Ni alloys with representative NMMC structure included ultrafine Ca-rich eutectic and various small-sized primary crystals were found to have excellent feasibility of rolling as compared to its hypereutectic Al-Si counterpart. What is more, Al-Ca alloys showed comparable Coefficient of Thermal Expansion values due to enormous volume fraction of Al-based eutectic and primary intermetallics. Analysis of tensile samples’ fracture surfaces revealed that primary intermetallics may act either as stress raisers or malleable particles depending on their stiffness under deformation. It is shown that a compact morphology can be achieved by conventional casting without using any refining agents. Novel hypereutectic Al-Ca NMMC materials solidifying with the formation of Al10Mn2Ca primary compound have the best ductility and strength. We reasonably propose these materials for high-load pistons.

Effects of Surface Finish on Sn-3.0Ag-0.5Cu Solder Joint Microstructure and Strength

The effects of copper organic solderability preservative (Cu-OSP) and electroless nickel immersion gold (ENIG) surface finish reflowed on Sn-3.0Ag-0.5Cu (SAC305) solder have been investigated in detail. Besides conventional cross-sectional microstructure observation, advanced characterization techniques such as synchrotron radiography imaging and synchrotron micro-x-ray fluorescence (µ-XRF) were utilized to elucidate the microstructural evolution in the solder joints during soldering. Additionally, high-speed shear testing was performed to understand the influence of the surface finish on the solder joint strength. The results indicated that the presence of nickel (Ni) from the ENIG surface finish decreased the growth rate but increased the amount of small Cu6Sn5 primary intermetallics, resulting in a slight reduction of the average interfacial intermetallic compound (IMC) thickness in the SAC305/ENIG solder joints. Due to the refined control of the solder joint microstructure, the average high-speed shear strength was higher for as-reflowed SAC305/ENIG versus SAC305/Cu-OSP solder joints. These results indicate a significant influence of the surface finish on SAC305 solder joint microstructure and strength and could provide a basis to improve solder joint strength.

Novel Al-Cu-Mn-Zr-Sc Compositions Exhibiting Increased Mechanical Performance after a High-Temperature Thermal Exposure

The presence of complex, core–shell Al3(Zr, Sc) dispersoids in Al-Cu alloys has recently been found to significantly refine θ′ precipitates resulting in increased mechanical properties. The purpose of this work was to take advantage of these effects to develop alloys with enhanced resistance to high-temperature thermal exposure compared to industrial baseline products. A series of Sc-bearing Al-Cu alloys were designed, manufactured using tailored homogenization practices and tested. It was found that a novel Al-Cu-Mn-Zr-Sc alloy exhibited higher tensile strength than AA2219-T8511 after a thermal exposure of 280 °C—24 h. The presence of Sc-containing primary intermetallics was also observed in V-bearing compositions.

On the governing fragmentation mechanism of primary intermetallics by induced cavitation

One of the main applications of ultrasonic melt treatment is the grain refinement of aluminium alloys. Among several suggested mechanisms, the fragmentation of primary intermetallics by acoustic cavitation is regarded as very efficient. However, the physical process causing this fragmentation has received little attention and is not yet well understood. In this study, we evaluate the mechanical properties of primary Al3Zr intermetallics by nano-indentation experiments and correlate those with in-situ high-speed imaging (of up to 1 Mfps) of their fragmentation process by laser-induced cavitation (single bubble) and by acoustic cavitation (cloud of bubbles) in water. Intermetallic crystals were chemically extracted from an Al-3 wt% Zr alloy matrix. Mechanical properties such as hardness, elastic modulus and fracture toughness of the extracted intermetallics were determined using a geometrically fixed Berkovich nano-diamond and cube corner indenter, under ambient temperature conditions. The studied crystals were then exposed to the two cavitation conditions mentioned. Results demonstrated for the first time that the governing fragmentation mechanism of the studied intermetallics was due to the emitted shock waves from the collapsing bubbles. The fragmentation caused by a single bubble collapse was found to be almost instantaneous. On the other hand, sono-fragmentation studies revealed that the intermetallic crystal initially underwent low cycle fatigue loading, followed by catastrophic brittle failure due to propagating shock waves. The observed fragmentation mechanism was supported by fracture mechanics and pressure measurements using a calibrated fibre optic hydrophone. Results showed that the acoustic pressures produced from shock wave emissions in the case of a single bubble collapse, and responsible for instantaneous fragmentation of the intermetallics, were in the range of 20–40 MPa. Whereas, the shock pressure generated from the acoustic cavitation cloud collapses surged up to 1.6 MPa inducing fatigue stresses within the crystal leading to eventual fragmentation.

Influence of B and Nb Additions and Heat Treatments on the Mechanical Properties of Cu-Ni-Co-Cr-Si Alloy for High Pressure Die Casting Applications

During the high pressure die casting process (HPDC), it is necessary to develop new designs and alloys for the copper plungers. In this research, two alloys Cu-9Ni-1Co-1.6Cr-2Si-1.3Fe-0.25B wt.% (A1) and Cu-9Ni-1Co-1.6Cr-2Si-0.1Fe-0.2Nb wt.% (A2) under different heat treatments (HT) were studied. Optical microscopy technique was applied to reveal the regions of dendritic morphology, also lower Secondary Dendrite Arm Spacing (SDAS); and different grain orientations. The results reveal that the solidification sequence is primary Cu dendrites and secondary intermetallics; heat treatments increase the redistribution of alloying elements in the interdendritic regions. During the heat treatments, some precipitates were found in the grain boundary after aging heat treatments for both alloys, which were determined by X-ray diffraction. Hardness test HRB presented a decrease with the solution heat treatment and an increase with the aging heat treatments proposed for both alloys. Finally, the wear resistances for both alloys were compared with a commercial alloy C17530, with decreased A1 with B additions having the best result in the as-cast condition 4.07 × 10−4 mm3/Nm, while for A2 with Nb additions wear resistance increased, with the best result in the one with aging heat treatment 1.69 × 10−4 mm3/Nm while for the C17530 alloy this was 2.74 × 10−4 mm3/Nm.

Improving the Tribological Properties of Al-7Fe-5Ni Alloys via Friction Stir Processing

AbstractDue to the limited solid-solubility of Fe and Ni in Al, coarse brittle intermetallics rich in Fe and/or Ni are inevitably formed in the as-cast microstructure of Al-Fe-Ni alloys. Upon formation, Fe(Ni)-rich intermetallics significantly deteriorate the properties and restrict the application of alloy in as-cast condition. The purpose of this study was to investigate the effect of friction stir processing (FSP) on microstructure and tribological properties of cast Al-7Fe-5Ni alloy. The dry sliding wear tests were done by using a pin-on-disk type machine under the applied pressures of 1, 2, and 3 MPa, sliding distance of 1000 m, at room temperature. According to the results, FSP (1250 rpm and 8 mm/min) effectively refined the microstructure, especially the coarse primary Al9FeNi intermetallics and eliminated the alloy casting-related defects. FSP also converted the large columnar grains of the cast alloys to the ultrafine equiaxed grains. These microstructural changes enhanced the substrate mechanical properties thereby increased its resistance against sliding-induced plastic flow, leading to the higher tribolayer stability on the surface, and accordingly, higher wear resistance. The results showed that applying 1-pass FSP reduced the wear by 13% and 53% under applied pressures of 1 and 3 MPa, respectively. The friction results also revealed that, compared with the as-cast sample, FSPed samples exhibited lower friction coefficient and friction coefficient fluctuations.

Influence of trace Zr on the microstructure and elevated temperature properties of an Al–Si–Cu–Ni–Mg piston alloy

The effect of trace Zr on the microstructure, precipitates and mechanical properties on an Al–12.6 wt% Si–4 wt% Cu–1.5 wt% Ni–1.5 wt% Mg piston alloy was studied in this paper. The α–Al dendrites are refined, resulting in a more compact distribution of primary intermetallics. The main precipitates are S (Al2CuMg) after T6 heat treatment. It was found that the S precipitates are denser and finer in the Zr–contained alloy whereas L12–ZrAl3 particles also precipitate. The hardness and elevated temperature tensile strength of the piston alloy were both improved, indicating the positive effect of trace Zr.

Enhancement of hardness of bulk solder by doping Cu nanoparticles at the interface of Sn/Cu solder joint

In this study, Cu NPs prepared by chemical reduction method,were doped into flux at weight proportions (0, 0.2, 0.5, 1 and 2 wt.%) and then reflow soldering was performed for the pure Sn solder (initial diameter = 1400 μm) with Cu substrate at 250 °C for 120 s. The presence of Cu nanoparticles (NPs) in soldering flux was observed to enhance the spreading of solder on Cu substrate. Solder with larger base spread area, characterized with faster diffusion of Cu from substrate attained quicker supersaturation during reflow and larger precipitation of primary Cu6Sn5 intermetallics (IMCs) upon air cooling. The solders containing greater area proportion of primary IMCs are characterized with enhanced effective elastic modulus. Experimentally, the solders prepared with Cu nanoparticles composed flux,were characterized with the increase in Vicker's microhardness. Solder processed with flux containing 2 wt.% Cu nanoparticles possessed Vicker's hardness of 18.6 HV, whereas solder prepared with undoped flux only had hardness of 13.8 HV. Numerical computation of the Cu diffusion and the elasticity have been done using finite element method.

Solidification Conditions for High-Strength Aluminum Alloy Ingots with Increased Transition Metal Content

It is shown that during continuous casting of high-strength aluminum alloy ingots of industrial dimensions there is a possibility of increasing the zirconium content to 0.4% if the production methods provide suppression of stable Al3Zr. Melt temperature and time treatment makes it possible to prepare ingots of alloy 1973 without primary particles intermetallic with diameter of 97 mm containing 0.38% Zr and 178 mm in diameter containing 0.2% Zr. It is determined that the use of extra-furnace modification with addition of Al–5% Zr master alloy bar into the furnace tap-hole box and cavitation action of solidification in the molten pool of an ingot makes it possible to prepare ingots of alloy 01959 300 mm in diameter of guaranteed composition with alloying by zirconium for the lower limit of the technical specifications. It is shown that in spite of the presence within ingots of primary intermetallics the mechanical property level achieved complies with technical specifications for granule shape. Some increase (by 5–7% with respect to strength) of the properties of sections of granules is connected with an increase in fineness of all phases compared with a cast version.

Three Dimensional Morphology of Mn Rich Intermetallics in AlSi Alloys Investigated with X-Ray Tomography

Elementary Mn has a great importance as neutralizer of Fe intermetallics like β-Al5FeSi, which have detrimental effect on mechanical characteristics of AlSi alloys. Presence of Mn in AlSi alloys causes the formation of other intermetallic phases. To understand the effect of solidification conditions and fluid flow on the microstructure of AlSi-based alloys and the addition of Mn leading to Mn-based intermetallics, Al-5 wt pct Si 0.2/0.4/1.0 wt pct Mn alloys have been directionally solidified under defined thermal (gradient 3 K/mm, solidification velocity 0.02-0.12 mm/s) and fluid flow (rotating magnetic field 6 mT) conditions. The primary Al-phase and Mn-based intermetallic phases were studied using 3D X-ray tomography. The spatial morphology of primary phase and intermetallics were characterized with respect to different fluid flow and solidification conditions. The tomography has showed the 3D complicated structure of Mn phases developed.

On the mechanism of the formation of primary intermetallics under ultrasonic melt treatment in an Al-Zr-Ti alloy

Ultrasonic melt treatment (UST) is known to induce grain refinement in aluminum alloys, especially when transition metals like Zr and Ti are present. The refinement of primary intermetallics (e.g. Al3Zr, Al3(Zr,Ti)) caused by UST may influence the subsequent solidification process when intermetallics act as nucleation sites. In this paper, an Al-Ti-Zr alloy is used to analyse the effect of different ultrasonic intensities on the formation of these primary intermetallics. The possible nucleation behaviour of Al3Zr particles during UST is also discussed and an edge to edge matching model is used to make a preliminary analysis of lattice mismatch between aluminum oxide and Al3Zr phase. The experimental results show that when UST is applied, the Al3(Zr,Ti) particles might nucleate on aluminum oxides and remain fine during solidification.

Mechanism and Tendency of Die Erosion in Al-Si Casting Alloy for Permanent Mold Casting Process

The mechanism of die erosion in Al-Si alloys was investigated based on the formation behavior of iron-containing intermetallics. And the metallurgical parameters affecting on the erosion tendency were discussed as functions of chemical composition for a guideline of melt control in Al-Si foundry. It was found that the intermetallic layers of α-AlFeSi, β-AlFeSi and θ-AlFe were formed through metallurgical reactions. The erosion tendency of die material was determined by not only the gradient of iron content through the interface between die material and molten alloy but also the relative formation temperature of primary intermetallics.

Influence of ultrasonic melt treatment on the formation of primary intermetallics and related grain refinement in aluminum alloys.

Ultrasonic melt treatment (UST) is known to induce grain refining in aluminum alloys. Previous studies have clearly shown that in Al–Zr–Ti alloys, the primary Al3Zr intermetallics were dramatically refined by cavitation-assisted fragmentation, and a good refinement effect was achieved. In this article, Al–Ti, Al–Ti–Zr alloys, and some commercial aluminum alloys are used to analyze the effect of UST on primary intermetallics and grain refinement. The addition of a small amount of Al–3Ti–B master alloy is also studied in order to compare with the addition of Ti and Zr in commercial aluminum alloys. Experimental results show that the ultrasonic grain refining effect is not only related to the size of particles which are refined and/or dispersed by UST, but also related to an undercooling available for activation of these particles in the solidification process. Athermal heterogeneous nucleation theory is considered to explain the effect of size and distribution of substrate particles on the grain structure with different undercoolings. The distribution of primary particle sizes results in the distribution of required undercoolings. Grain refining occurs when the undercooling is large enough to activate the refined primary intermetallics or dispersed inoculants.

Measurement of deformations in SnAgCu solder interconnects under in situ thermal loading

Optical microscopy was used to discern the different grain orientations and grain boundaries on the polished cross-sections of near-eutectic lead-free board-level SnAgCu (SAC) solder interconnects. Strain distributions with submicron accuracy of the deformations on the cross-sections of the solder interconnects were measured when the package was subjected to thermal loading from room temperature to 100 °C. The results were correlated with the locations of different grains, grain boundaries and larger primary intermetallics. It revealed anisotropic nature of deformations in different grains of the SAC solder, which is similar to the thermomechanical behavior of pure Sn. The strain distribution in a solder interconnect varied significantly in different grain orientations. The primary intermetallics (Ag 3Sn plates) also behaved very differently from the surrounding Sn matrix under the thermal loading. The demonstrated strain localization along the grain boundaries and bigger primary intermetallics provides a clue for the path of fatigue crack growth that leads to a failure because of anisotropic thermomechanical response of SAC solder during thermal cycling.

Pressure filtration tests of liquid Al–Si cast alloys: I. Flow behaviour

The flow behaviour during the Prifil Footprinter ® tests of liquid Al–Si cast alloys has been investigated for the first time using the first and second derivative methods. The first derivative approach is recommended as the preferred method to determine the transitional points. At the initial transient stage flow rate decreases with filtration time. Higher quality liquid metal usually exhibits a shorter, or even no observable transient period. However, there remains some uncertainty of this conclusion, probably due to the variable waiting periods for the attainment of a designated temperature requirement prior to starting the filtration tests. During the steady state flow rate decreases linearly with filtration time and the second derivative values are around zero. At the initial and terminal transient states a periodic flow mode was also observed. The primary intermetallics possibly precipitated and sedimented prior to or during the tests may significantly influence the filtration behaviour.

Effect of precipitation and sedimentation of primary α-Fe phase on liquid metal quality of cast Al–11.1Si–0.4Mg alloy

A novel approach, the heat treatment of liquid metal, was initiated to investigate the possibility of removing oxide films and primary intermetallics from aluminium melts and its effectiveness. The concept was to remove oxide cracks from liquid metal by precipitation of primary intermetallics onto oxide films, causing them to sink under the weight of the particles. The sedimentation of primary α-Fe particles was investigated at 600°C using an Al–11·1Si–0·4Mg alloy. The liquid metal quality was assessed by reduced pressure and pressure filtration tests. After sedimentation processing there is a clear improvement in liquid metal quality due to the sedimentation and separation of primary α-Fe particles, causing the removal of some oxide films from aluminium melts. Thus the heat treatment of liquid metal has the potential to improve liquid metal quality. Primary α-Fe phase seems to precipitate onto the wetted sides (i.e. the outer interfaces) of double oxide films. However, the gap between the two dry sides of a double film constitutes a crack, thus explaining the common observation of cracks in and around such intermetallics. Hydrogen precipitation into the central oxide films in intermetallics may open the oxide films, or even expand and convert the cracks into open porosity. The oxide films may further open during a later stage of freezing because of shrinkage and cooling stress in the solid state.