Al-rich Intermetallic Compounds Research Articles

Microstructure and formation mechanism of the transition layer at the interface of Al–Cu EMPW joint

In the dissimilar metal welding field, electromagnetic pulse welding (EMPW) is a kind of potential technology. To further reveal the bonding mechanism of EMPW, this work carried out a Cu–Al sheet EMPW experiment with a discharge voltage of 15 kV and a gap of 1.5 mm. It analyzed the microstructure and formation mechanism of the Cu–Al EMPW joint bonding interface transition layer in detail by SEM, EDS, TEM, and FIB. The transition layer was found at the vortex zone, and there was an isolated long strip Cu particle separated from the main body of the Cu sheet. The tested results showed that the transition layer contained Al-rich intermetallic compound (IMC) Al2Cu, and some bulk particles in the transition layer were Cu-rich IMCs Al4Cu9 and AlCu4. There were Cu-rich IMCs Al4Cu9, AlCu4, and AlCu between the long strip Cu particle and the IMC transition layer. In the violent impact process between the Cu sheet and Al sheet, the Cu rapidly diffused in the Al matrix to form the Al-rich IMC (Al2Cu) as the primary transition layer, and the interface was high temperature and high pressure, forming a plastic flow and vortexes swirling. Some Cu particles separated from the Cu sheet and became the vortex piece during the plastic flow process, and continued to interact with the Al2Cu primary transition layer to form the secondary transition layer. This work could provide a basis for revealing the structure composition and formation mechanism of the Cu–Al EMPW joint interface transition layer.

Investigation of CuAl IMCs Corrosion in Chloride Environment and its Prevention Strategy

Abstract In most current wire-bonding applications, electrical interconnection is accomplished by fine Cu wires bonded to Al bonding pads microfabricated on an IC chip and external contact pins of the PCB board. Corrosion-related failure defects between the Cu wire and Al bond pad have been an ongoing un-trackable reliability issue plaguing the IC packaging industry for the past ten years, despite approaching ppb levels. Most prior studies hypothesized that intermetallic compounds (IMCs) like Cu9Al4, and CuAl2 were responsible for the observed acute wire-bond lift-off corrosion defects. Further studies sought to quantify the rates of corrosion of these IMCs and explore the effects of mitigation efforts of adding Pd, relevant to Pd-coated Cu wire-bonding. However, utilizing a novel real-time corrosion screening approach, we previously established that peripheral bimetallic contact between Cu ball-bonds and Al bond pads also plays a substantial role in the aggressive Al pad corrosion, induced by chloride ion penetration, which often leads to device failure. In this work, we further explored the role of IMCs corrosion in wire-bond lift-off failure utilizing fundamental electrochemical studies to quantify rates of galvanic-induced corrosion of IMCs within the broader context of an interconnected stack of Al bond pad, Cu-Al IMCs and Cu bonding wire. We also explored the use of a corrosion inhibitor to suppress the galvanic corrosion currents of Al bond pad, and Al-rich IMCs when electrically connected to Cu wire or Cu-rich IMCs.

An investigation of mechanical and electrical properties of friction stir welded Al and Cu busbar for battery pack applications

The aluminum and copper (Al–Cu) busbar is widely used as a core component in Lithium-Ion (Li-ion) batteries. The Al–Cu busbar is challenging to fabricate with the traditional welding processes because of its high thermal conductivity. The Al–Cu busbar is fabricated using the friction stir welding method in the present study. The effect of temperature and vibration generated during the welding process on intermetallic compounds (IMCs) is studied using the effective formation model and found that Al2Cu is the first to form at the interface. The IMC formation at the joint interface had detrimental (Al-rich IMC) and beneficial (Cu-rich IMC) effects. The presence of detrimental IMCs affects the joint strength of about 36% as compared to the sample with the highest tensile load. The surface electrical conductivity is measured by using a Gaussian profile method and found in the range of 0.94–5.37 μ Ω·mm. The welded samples with the presence of Al2Cu3 and Al4Cu9 IMC at the interface are found to have higher electrical conductivity. Interestingly, the sample with a higher tensile load had observed higher electrical conductivity due to the formation of Cu-rich IMC, i.e., Al4Cu9.

Friction stir alloying of AZ61 and mild steel with Al-CNT additive

The friction stir alloying (FSA) procedure successfully produced dissimilar joints between AZ61 magnesium alloy and mild steel with Al-CNT additions. Before welding, Al-CNT additives were placed in a gap between the AZ61 and steel specimens. The experimentation was performed using different welding speeds while tool rotational speed was kept constant for varying weight percentages of carbon nanotubes (CNT) in Al powder. The influence of additives and welding speed on tensile strength and characterisation across the welding interface was studied. Tensile strength of joint with 3% CNT improved up to 222 MPa. Al-rich intermetallic compounds were found at the welding contact, while the formation of Al2MgC2 carbide was detected, which contributed to CNT strengthening effects all to improved welding strength.

Intrinsic Diffusivities Ratio Analysis in Double Multiphase Systems

Our investigations show that electrochemical corrosion of copper is faster than electrochemical corrosion of aluminium at temperatures below 100°C. Literature data analysis shows that the Al atoms diffuse faster than the Cu atoms at temperatures higher than 475°C, Al rich intermetallic compounds (IMCs) are formed faster in the Cu-Al system, and the Kirkendall plane shifts toward the Al side. Electrochemical corrosion occurs due to electric current and due to diffusion. An electronic devise working time, for example, depends on initial copper cover thickness on aluminium wire, connected to the electronic devise, temperature, and volume and dislocation pipe diffusion coefficients, so copper, iron, and aluminium electrochemical corrosion rates are investigated experimentally at room temperature and at temperature 100°C. Intrinsic diffusivities ratios of copper and aluminium at different temperatures and diffusion activation energies in the Cu-Al system are calculated by proposed here methods using literature experimental data. Dislocation pipe and volume diffusion activation energies of pure iron are calculated separately by earlier proposed method using literature experimental data. Aluminium dissolved into NaCl solution as the Al3+ ions at room temperature and at temperature 100°C, iron dissolved into NaCl solution as the Fe2+ (not Fe3+) ions at room temperature and at temperature 100°C, copper dissolved into NaCl solution as the Cu+ ions at room temperature and as the Cu+ and the Cu2+ ions at temperature 100°C. It is found experimentally that copper corrosion is higher than aluminium corrosion, and ratio of electrochemical corrosion rates, kCu/kAl>1, decreases with temperature increasing, although iron electrochemical corrosion rate does not depend on temperature below 100°C. It is obvious, because the melting point of iron is more higher than the melting point of copper or aluminium. It is calculated that the copper electrochemical corrosion rate is approximately equal to aluminium electrochemical corrosion at temperature about 300°C, so copper can dissolve into NaCl solution mostly as the Cu2+ ions at temperature about 300°C. The ratio of intrinsic diffusivities, DCu/DAl<1, increases with temperature increasing, and the intrinsic diffusivity of aluminium could be approximately equal to the intrinsic diffusivity of copper at temperature about 460oC. Intrinsic diffusivities ratios in the Cu-Zn system at temperature 400°C and in the Cu-Sn system at temperatures from 190°C to 250°C are analyzed theoretically using literature experimental data. Diffusion activation energies and pre-exponential coefficients for the Cu-Sn system are calculated combining literature experimental results.

Investigation of Weld Zone and Fracture Surface of Friction Stir Lap Welded 5052 Aluminum Alloy and 304 Stainless Steel Joints

Dissimilar material joining of aluminum and steel in the present scenario is an important criterion in the manufacturing industry, especially because of their low weight and technical performance. In the present investigation, AA5052 and SS304 are friction stir welded in lap configuration with different tilt angles, welding speed, pin depth, and tool rotational speed, with aluminum as the top plate. A maximum of 3.16 kN shear strength was achieved at 2.5° tilt angle when the penetration depth was 4.3 mm. The shear strength samples were studied for fracture analysis and it was found that fracture of the samples mainly occurred on the aluminum side and the fracture demonstrated both brittle and ductile failure, consisting of quasi-cleavage, trans-granular, and intergranular fracture areas. Field emission scanning electron microscope images at the interfacial region of the weld show that different intermetallic compounds were formed at various zones of the joint with respect to the change in process parameters. It was observed from energy dispersive spectroscopy that Al-rich intermetallic compounds were formed at the interfacial region of the welded samples. Amongst the process parameters, change in the tilt angle affected the weld zone significantly. The thickness of the intermetallic compound (IMC) layer formed with 800 and 1000 rpm at 2.5° tilt angle was between 2.5 and 3 μm, which resulted in achieving better joint strength. AlFe, AlFe3, Al13Fe4, and Al5Fe2 were the different intermetallic compounds detected using X-ray diffraction with different process parameters. The hardness of the samples ranged between (300 and 630) HV, which further supports the formation of AlFe and AlFe3 intermetallic compounds.

Microstructure and wear resistance of the laser-cladded Al0.8CrFeCoNiCu0.5Bx high-entropy alloy coating on aluminum

Coatings with excellent mechanical properties are difficult to prepare on the aluminum alloy surfaces by laser surface engineering because of the low melting point and active chemical properties of aluminum (Al). Based on the unique entropy effect of a high-entropy alloy, we suppress the cracks in the cladding layer caused by the dilution ratio of the substrate, obtaining coatings exhibiting excellent wear resistance. The Al0.8CrFeCoNiCu0.5Bx coating exhibited a BCC1 + BCC2 + FCC phase when the x ≤ 0.1. When x ≥ 0.2, boride was formed in the Al0.8CrFeCoNiCu0.5Bx coatings, and it became the main phase of the Al0.8CrFeCoNiCu0.5B0.3 and Al0.8CrFeCoNiCuB0.4 coatings; however, no Al-rich intermetallic compound with a complex phase structure was formed. When 0 ≤ x ≤ 0.3, the bonding strengths of the Al0.8CrFeCoNiCu0.5Bx coatings were 70.6–176.2 MPa. An increase in the boron content increased the hardness of all the coatings, except the Al0.8CrFeCoNiCu0.5B0.3 coating. The hardness was 479–705 HV (6–9 times that of the substrate). The wear rate of the coating ranged from 1.50 × 10−7 to 1.19 × 10−5 mm3/Nm, which was only 0.01%–5.43% that of the substrate. The Al0.8CrFeCoNiCu0.5B0.2 coating exhibited the highest resistance to wearing, and its wear mechanism was identified as abrasive wear.

Effect of pouring temperature on microstructure and mechanical properties in Al-16Si-2%Al2O3 hypereutectic alloys

The Al-Si hypereutectic alloys are taking vital role in day to day life and industrial applications for its excellent microstructural and mechanical properties. The grain size, volume fraction and distribution of both primary as well as eutectic silicon enhances significantly on these properties among the materials. The modification of primary and secondary Si particles in hypereutectic Al-Si alloys is quite difficult in conventional processes. This paper investigates the microstructural and mechanical properties changes as changing in pouring temperature. During solidification, the effect of pouring temperature on primary and secondary Si particles is showing the vital role in hypereutectic Al-16Si-2%Al₂O3 alloy. In order to achieve better microstructure and mechanical properties, the interfacial bonding between the dispersed phase and liquid matrix was good in the synthesized composites produced by casting route over other conventional processes. The cast product of hypereutectic Al-16Si alloy shows significant variation over wear behaviour and hardness at different pouring temperatures. However in XRD analysis of sample indicating the formation of harder phases like SiO2, Al2O3 and Al-rich intermetallic compounds.

Brittle-to-ductile transition of Au2Al and AuAl2 intermetallic compounds in wire bonding

Au–Al intermetallic compounds (IMCs) were conventionally regarded as damage sources within wire bonding interfaces. Among the Au–Al IMCs system, Au2Al and AuAl2 IMCs were commonly observed after long-term service. Here, the intrinsic mechanical properties of Au2Al and AuAl2 IMCs were investigated by in-situ bending tests. The results clearly revealed that the transition from brittle to ductile facture behavior was determined by the atomic ratio of Au and Al elements, while Al-rich Au–Al IMC exhibits higher bending strength than that of Au-rich Au–Al IMC. The Al-rich Au–Al IMC such as AuAl2 indeed showed brittle fracture behavior, in which cracks propagated through the grain boundaries. In contrast, the Au-rich Au–Al IMC such as Au2Al appeared to be quite ductile and crack resistant, and its plastic deformation was undertaken by slip bands. This work suggested that not all Au–Al IMCs cause brittleness and lead to damages, only Al-rich IMCs should be avoided at wire bonding interfaces.

A Correlation between the Ultimate Shear Stress and the Thickness Affected by Intermetallic Compounds in Friction Stir Welding of Dissimilar Aluminum Alloy–Stainless Steel Joints

In this work, Friction Stir Welding (FSW) was applied to join a stainless steel 316L and an aluminum alloy 5083. Ranges of rotation and translation speeds of the tool were used to obtain welding samples with different heat input coefficients. Depending on the process parameters, the heat generated by FSW creates thin layers of Al-rich InterMetallic Compound (IMC) mainly composed of FeAl3, identified by energy dispersive spectrometry. Traces of Fe2Al5 were also depicted in some samples by X-ray diffraction analysis and transmission electron microscopy. Monotonous tensile tests performed on the weld joint show the existence of a maximum mechanical resistance for a judicious choice of rotation and translation speeds. It can be linked to an affected zone of average thickness of 15 µm which encompass the presence of IMC and the chaotic mixing caused by plastic deformation in this area. A thickness of less than 15 µm is not sufficient to ensure a good mechanical resistance of the joint. For a thickness higher than 15 µm, IMC layers become more brittle and less adhesive due to high residual stresses which induces numerous cracks after cooling. This leads to a progressive decrease of the ultimate shear stress supported by the bond.

Effect of alumina on grain refinement of Al-Si hypereutectic alloys

The size, volume fraction and distribution of primary as well as eutectic silicon affect the mechanical properties of the Al-Si hypereutectic alloys. It is very difficult for the simultaneous refinement and modification of primary and secondary Si particles in hypereutectic Al-Si alloys through traditional processes. This paper explores the role of γ-Al2O3 nanoparticles on Si particles in the course of solidification in hypereutectic Al-Si alloys at particular pouring temperature. The present study involves incorporation of varying contents dispersed γ-Al2O3 nanoparticles into a molten base metal during stir casting and followed by solidification. It has been reported that the synthesized composites having good interfacial bonding (wetting) between the dispersed phase and the liquid matrix was achieved in order to provide improved mechanical properties of the composite. The cast product of hypereutectic Al-16Si alloy with the reinforcement of nanoparticles, illustrated a significant improvement in both wear behaviour and hardness. The dry sliding wear test has been performed on a group of specimens with varying parameters (different loads and sliding velocities) in a pin on disc wear testing machine. Moreover, the wear rate and specific wear rate also affected in different load and different sliding velocities. However in XRD analysis of the samples, the enhancement of wear resistance as well as hardness was due to the formation of brittle phases like SiO2, Al2O3 and Al-rich intermetallic compounds. The hardness value of the materials increases nearly 6% in addition to increase in the density of only 0.8%. As per literature, the large plate eutectic Si particles were modified in to the fine core particles and it acquires enough potential for various applications.

Electrochemical studies of Pd-doped Cu and Pd-doped Cu-Al intermetallics for understanding corrosion behavior in wire-bonding packages

This study investigated the electrochemical characterizations in the field of wire metallurgy (Pd concentration) and molding compound chemistry (chloride concentration) to find ways to reduce metallic entities' susceptibility to corrosion at ball-pad interfaces. The open circuit potentials and potentiodynamic polarization curves of various metallic entities found in a Cu(Pd)-Al bonding interface were obtained in near-neutral electrolytes of 100ppm, 20ppm, and 1ppm of NaCl in high-purity water. From X-ray diffraction spectra, it was found that Pd could be homogeneously incorporated into Cu9Al4, the Cu-rich intermetallic compound (IMC) also referred to as γ but not into CuAl2, the Al-rich IMC also referred to as θ for arc-melted specimens. For Cu-Pd alloys, at a given chloride concentration, increasing Pd concentration causes the value of open-circuit potential (Eoc) to increase and corrosion current density (icorr) to decrease. Likewise, for a given amount of Pd in Cu-Pd alloy, decrease in the NaCl concentration causes the value of Eoc to increase and icorr to decrease. Interestingly, for high concentration of Pd as in the case of Cu-9Pd, Eoc and icorr became less sensitive to the NaCl concentrations investigated. This can be attributed to the Pd enrichment on the corroding surface that reduces the anodic dissolution rate of Cu. For Pd-doped γ intermetallics, increasing Pd concentration causes a systemic increase in the value of Eoc, but at a lower concentration of Pd, the value of icorr was increased. The addition of Pd to γ causes an increase in the cathodic current density due to the high cathodic activity of Pd, while the passivation of Al in γ reduces the extent of the anodic current density reduction due to the addition of Pd, which leads to a higher value of icorr at a low Pd concentration. This is true even when the NaCl concentration is as low as 1ppm. On the other hand, the influence of NaCl concentration on the Eoc and icorr of γ IMC was always observed, even with Pd addition.

Effect of Cr and Nb on the phase transformation and pore formation of Ti-Al base alloys

Abstract The phase transformation and its relation to the porosity of Ti-Al base alloys are investigated in order to understand the effect of composition on the pore formation during sintering at low and high temperatures. Ti-Al base alloys with nominal compositions of Ti52Al48, Ti50Al48Cr2, Ti50Al48Nb2 and Ti48Al48Cr2Nb2 are prepared by the conventional elemental powder metallurgy route. The sintering of mixed powders was carried out at 600° C (low-temperature sintering below the Al melting point) and 1200° C (high-temperature sintering). Al-rich intermetallic compound TiAl3 was primarily formed during low-temperature sintering while Ti-rich intermetallic compound Ti3Al was formed during high-temperature sintering. The phase transformations increase the overall porosity and pore size of alloys to a similar extent regardless of the alloy composition during the low-temperature sintering. However, large differences in the pore size and pore volume distribution occur depending on the composition of Ti-Al alloys during high-temperature sintering. Chromium and niobium do not affect the formation of Kirkendall voids and thus show a similar void formation behavior at 600° C, but affect the void agglomeration behavior to a different extent due to the different diffusion activities at 1200° C.

Crystallographic orientations of intermetallic compounds of a multi-pass friction stir processed Al/Mg composite materials

The crystallographic orientation relationships between intermetallic compounds and matrices were investigated in the interface of an overlapped multi-pass friction stir processed Al/Mg composite materials. During the friction stir processing, an Mg rich intermetallic compound (γ-Al12Mg17) was formed close to the bottom Mg alloy and an Al rich intermetallic compound (β-Al3Mg2) was formed close to the top Al alloy. Two new orientation relationships between the Mg matrix and the γ-Al12Mg17 were observed and analyzed. One was identified as 011¯2Mg//1¯01γ, 01¯11Mg//(101)γ, and the other one was identified as 12¯10Mg//[531]γ, (0001)Mg 1.7° form 1¯21¯γ. The stereographic projections of two new orientation relationships were calculated and discussed. Moreover, high resolution transmission electron microscopy studies of the interface between the Mg matrix and the γ-Al12Mg17 indicated that the γ-Al12Mg17 had a coherent interface with the Mg matrix. It was also observed that the β-Al3Mg2 had a cubic-to-cubic orientation relationship with the Al matrix in the Al/Mg composite material interface.

In pursuit of light intermetallic hydrides

The search for a “magic” intermetallic compound, absorbing reversibly and easily copious quantities of hydrogen, has been pursued since the discovery of the first intermetallic hydrides, forty–fifty years ago. Obviously, such intermetallic compounds should contain light metal elements like Li, Mg, Al, Ca, etc. We summarize in this work the experimental evidence that sheds light on the hydrogen absorption abstinence of Al-rich intermetallic compounds. The elastic moduli and/or bonding properties of Al-containing pseudobinary compounds initially decrease and then increase as a function of the Al composition. The shear and the bulk moduli behave differently in the Al-rich portion, where they present significantly higher and lower values, respectively, than the corresponding moduli in the Al-poor region. We claim that the extensive shear stiffening inhibits the hydrogen absorption in the Al-rich compounds. An explanation for the choice of the LaSn0.22Ni0.78 compound as a sorption cryocooler in the Planck mission is suggested in view of its shear softening with respect to LaNi5. Recent attempts to find pseudobinary intermetallic hydrides, containing Li and Ca, are also presented. These attempts are conducted by considering some of the factors which regulate hydrogen absorption in intermetallic compounds.

Explaining Nondestructive Bond Stress Data From High-Temperature Testing of Au-Al Wire Bonds

The application of an alternative method of bond monitoring during high-temperature aging is reported using a custom made test chip with piezoresistive integrated CMOS microsensors located around test bond pads. The sensor detects radial stresses originating from the bond pad and can resolve changes because of intermetallic compound (IMC) formation, voiding, or crack formation at the bond interface. Optimized Au ball bonds are aged for over 2000 h at 175 °C. It is found that stress sensors next to the bonds are capable of showing the stages of IMC growth, consumption of pad Al layers, and monitoring the formation of low-density and Al-rich IMC (AuAl <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> ) which shows an advanced stage of aging. In particular, a first stress signal increase corresponds to the conversion of all Al above the diffusion barrier into IMCs. The second increase in stress signal after a period of stability corresponds to conversion of all Al below the barrier into IMCs. The IMC formation in these periods causes shear strength increase. After complete bond Al consumption, the bond, however, reaches maximum strength. As bond degradation starts, e.g., by lateral IMC formation, voiding, and oxide formation, as well as because of lateral pad Al transformation to IMC, the signal exhibits a strong decrease. The findings are corroborated by results obtained from classical methods such as interruptive or destructive testing including visual inspection, shear testing, cross sectioning, and by bond resistance monitoring.

Phase equilibria in the aluminium-rich side of the Al–Zr system

The Al-rich phase equilibria in the Al–Zr binary system were investigated experimentally. The phase diagram for compositions up to 40 at.% Zr was determined experimentally by differential thermal analysis and metallography. Three stable intermetallic compounds exist in this region of the diagram: Al3Zr2, Al2Zr, and Al3Zr. The peritectic melting of Al3Zr2 and the congruent melting of Al2Zr were confirmed. Al3Zr, the most Al-rich intermetallic compound, melts peritectically, which contradicts information available in the literature. In addition, the reaction between Al3Zr and the (Al) solid solution seems to be of eutectic nature, in contradiction with previous results found in the literature. Based on these new experimental evidence, a revised phase diagram is drawn.

Oxidation Behavior of AZ91D Magnesium Alloy Containing Nd or Gd

The influence of Nd and Gd additions on the evolution of microstructure, surface hardness and oxidation behavior of the extruded AZ91D magnesium alloy exposed to air was investigated in the temperature range from 290 to 410 °C. The kinetics of the oxidation process was obtained from gravimetric measurements performed during 28 days. Nd and Gd formed Al-rich intermetallic compounds (Al2Gd, Al2Nd, Al–Mn–Gd and Al–Mn–Nd) and reduced the volume fraction of β-Mg17Al12 phase. Surface degradation of the AZ91D alloy, with and without Gd or Nd, was negligible for temperatures below 370 °C. At 410 °C, the grain size and oxidation rate significantly increased, although lanthanide-containing alloys revealed lower degradation compared with the unmodified AZ91D alloy.

Effect of cold rolling and subsequent annealing on hot pressed Ni/Al laminates

Nickel/Aluminum laminates have been prepared by hot pressing of stacked alternate layers of very thin foils of nickel and aluminium. It is found that ductile nickel layers exist in hot pressed Ni/Al laminates which are annealed up to 600 °C. However, at these temperatures Al-rich intermetallic compounds are also formed with a very thin irregular layer of pure aluminium and few voids. As the annealing temperature is increased, nickel layer is consumed and Ni-rich intermetallic compounds are formed. Cold rolling up to 8% reduction in area resulted in the cracking of the regions having intermetallic compounds with cracks perpendicular to the rolling direction. The tensile strength values are high for the laminates which contain ductile Ni layers and reach a value of 320 MPa for the specimens which are hot pressed at 420 °C. However, the laminates which are annealed at 800 °C show brittle fracture and lower tensile strength of 174 MPa. The fractographic examination indicated that Ni/Al laminates which are hot pressed at 420 °C undergo plastic deformation before breaking. On the other hand, Ni/Al laminates which are annealed at 800 °C with Ni present in the form of intermetallic compounds only, exhibit brittle intergranular fracture.