Eradicating Brittleness of Ti-6Al-4V/Cu Dissimilar Brazed Joint using a Nb Diffusion Barrier

Influence of the Zn interlayer on the mechanical strength, corrosion and microstructural behavior of friction stir-welded 6061-T6 aluminium alloy and AZ61 magnesium alloy dissimilar joints

Relationship between interface heat input and microstructure of titanium/steel composite material was prepared by laser processing

Niobium and titanium diffusion in the high niobium-containing Ti–54Al–10Nb alloy

Joints of TiBw/Ti6Al4V composites- Inconel 718 alloys dissimilar joining using Nb and Cu interlayers

<div class="section abstract"><div class="htmlview paragraph">A lightweight multi-material combination of steel and aluminium alloy (Al) is becoming a novel approach towards environmentally sustainable transport systems. Studies show that 10% reduction of vehicle weight results into 3-7% reduction in specific fuel consumption in IC engines and a 13.7% improvement in electric range for electric vehicles. However, dissimilar welding of Al/steel is a key challenge because of incompatible thermo-physical properties (melting point, thermal conductivity, and coefficient of thermal expansion) and low miscibility between Al and steel. The formation of brittle and hard Al-steel intermetallic compound (IMC) at the joint interface is the major concern for dissimilar welding of Al/steel. In this work, efforts are made to check the feasibility of Ni interlayer to control IMC formation at the interface of Al/steel dissimilar welded joint. Resistance spot welding is used to join low carbon steel CR01 and Al AA6061-T6 with pure Ni interlayer. Microstructure and IMC morphology of welded joints are investigated by optical and scanning electron microscope. The mechanical performance of welded joints is evaluated by tensile shear strength (TSS), failure energy and failure mode. Ni-interlayer does not have major effect on physical aspects of weld such as electrode indentation, Al thinning and steel bulging height. Two separate interfaces namely, Ni/Al and Ni/steel are formed with Ni interlayer. Higher amount of expulsion is taken place at the Ni/Al interface, resulting into lower TSS for dissimilar welding with Ni-interlayer compared to without interlayer.</div></div>

In the flip-chip (FC) solder joint, solder alloys usually connect with Ni and Cu based under bump metallurgies (UBMs) at chip-side and substrate-side, respectively. The material selection for solder alloys and UBM material is a critical issue to affect the microstructure, phase formation, and the reliability of the solder joints. In this study, the interfacial reaction, microstructure variation, and impact reliability of Sn-based Pb-free solder joints with Ni, Cu, and novel Cu-Zn UBMs are discussed and categorized into six topics. (1) Microstructural variation and interfacial reaction in Ni/Sn-Ag-Cu/Ni assemblies with and without Ni doping Ni and Cu elements cross-interacted between the Cu and Ni UBMs during thermal aging and affected the interfacial reactions at both Cu and Ni sides. Dual-phased (Cu,Ni)6Sn5 intermetallic compounds (IMCs), which has high and low Ni concentration, formed at the solder/Ni interface. In contrast, thicker low-Ni (Cu,Ni)6Sn5 and thin (Cu,Ni)3Sn layers formed at the Cu side. The Ni doping into solder varied the microstructure of solder alloys and the formation of interfacial IMCs. The re-distribution of Ni and Cu was correlated to the formation mechanism of interfacial (Cu,Ni)6Sn5. (2) Effect of dual-phased (Cu,Ni)6Sn5 IMCs on the impact reliability of Sn-Ag-Cu/Ni solder joints By employing the high-speed shear impact test, the impact fracture morphology reveals that the interface of high-Ni (H)/low-Ni (L) (Cu,Ni)6Sn5 facilitates the crack nucleation within the IMC. It is believed that the crack propagation depends on the fracture toughness of interfacial IMCs. The indentation data shows that bulk H-(Cu,Ni)6Sn5 exhibits distinctly lower fracture toughness than bulk L-(Cu,Ni)6Sn5. In correlating the impact fracture behavior and mechanical properties of two kinds of (Cu,Ni)6Sn5, cracks tend to propagate through H-(Cu,Ni)6Sn5 due to the relatively low fracture toughness of H-(Cu,Ni)6Sn5. (3) Development of a novel Cu-Zn UBM for Pb-free solder joints To suppress the thicker IMCs and voids at the solder/Cu interface, the Cu-Zn alloy was designed for a novel UBM material. The interfacial reactions of Sn/Cu-xZn (x = 0, 15 and 30 at.%) solder joints were investigated. Interestingly, the growth of Cu-Sn IMCs was significantly reduced and no void was found in the Sn/Cu-Zn solder joints after thermal aging. Transmission electron microscopy (TEM) images and the field emission electron probe microanalyzer (FE-EPMA) analysis show that there are two types of Zn-rich phases, i.e. CuZn and Cu-Zn-Sn phases, to form in Sn/Cu-Zn joints. The formation mechanisms of IMCs were probed and proposed with regard to the thermodynamics and kinetics. (4) Liquid-state reaction of Sn-Ag-Cu solders and the novel Cu-Zn UBM During the reflow process, Cu and Zn atoms would dissolve from the Cu-Zn UBM into the molten solders, leading to the variation of the composition in the solders. Then, the composition variation further altered the microstructure of the solders. In comparison with the Sn-Ag-Cu/Cu, it was found that the coarser eutectic region and smaller Cu6Sn5 IMCs inside the solder matrix of Sn-Ag-Cu/Cu-Zn. In addition, the interfacial reaction was also affected by Zn dissolution. In this study, it was demonstrated that the microstructural variation and the phase evolution in the solder joints were controlled by the reflow time and the Zn concentration in the Cu-Zn UBM. (5) Characterization of the Cu6(Sn,Zn)5 intermetallic compound Cu6Sn5 is a dominant IMC at the Sn-based solder/Cu joint interface. The crystal structure of Cu6Sn5 varies with temperature. After reflow at 250 oC, the interfacial Cu6Sn5 revealed hexagonal structure (η-Cu6Sn5). During aging at 150 oC, hexagonal η-Cu6Sn5 would transform into monoclinic η’-Cu6Sn5. According to literature, the phase transformation between η’ and η would induce crack easily propagating through the Cu6Sn5 at the solder joint interface. In the novel solder/Cu-Zn joints, and Zn would dissolve into Cu6Sn5 to form the Cu6(Sn,Zn)5 IMC at the interface. X-ray diffraction and differential scanning calorimetry analyses show that doping small amounts of Zn into Cu6(Sn,Zn)5 can stabilize the hexagonal structure during the thermal aging process. Thermodynamic calculation also demonstrates that Zn can stabilize the hexagonal Cu6(Sn,Zn)5. (6) Application of Cu-Zn UBM on the Ni/Sn-Ag-Cu/Cu-Zn assemblies The feasibility of novel Cu-Zn UBM applied for the Ni/solder/Cu-Zn assemblies was also evaluated. In comparison with the Ni/Sn-Ag-Cu/Cu solder joint, Ni/Sn-Ag-Cu/Cu-Zn solder joints revealed thinner Cu6Sn5-based IMCs at both Ni/Sn-Ag-Cu and Sn-Ag-Cu/Cu-Zn interfaces after aging. (Cu,Ni)6(Sn,Zn)5/(Cu,Ni)6Sn5 dual-phase formed at the Ni side while (Cu,Ni)6(Sn,Zn)5 single-phase at the Cu-Zn side. The interfacial IMCs grew very slowly, and no void formed in these Zn-contained solder joints during thermal aging. Additionally, the dissolved Zn in the solder alloy reduced the elemental cross-interaction between the Ni and Cu-Zn substrates. The noticeable thermal stability of Ni/Sn-Ag-Cu/Cu-Zn solder joints is attributed to the Zn re-distribution retarding the reaction of Ni, Cu and Sn. Phase formation and IMCs suppression mechanisms in Ni/Sn-Ag-Cu/Cu-Zn solder joints were probed and discussed. In summary, novel Cu-Zn UBM shows lots of advantages for soldering, including: (I) reduction of Cu-Sn IMCs, (II) suppression of voids at the interface, (III) formation of the a hexagonal Cu6(Sn,Zn)5, and (IV) retardation of the elemental cross-interaction in the Ni/solder/Cu-Zn assemblies. The Cu-Zn alloys could be a potential UBM material for the advanced electronic packaging.

Effect of ultrasonic vibration on the intermetallic compound layer formation in Al/Cu friction stir weld joints

TEM analysis and molecular dynamics simulation of graphene coated Al-Cu micro joints

Tailoring interfacial structure and achieving excellent strength-ductility synergy of Al/Mg/Al composite via porthole die co-extrusion and post-annealing

Tungsten-titanium (WTi) thin films are known as potential adhesion promoters and diffusion barriers. The barrier efficiency of WTi thin films against indium (In) diffusion was experimentally studied by x-ray diffraction (XRD) measurements during in situ annealing. Specific multilayered samples were designed to estimate the diffusion barrier properties using the Ni/In system. These diffusion samples were made up of a 100-nm-thick WTi layer prepared by magnetron sputtering from an alloyed target (W:Ti ≈ 70:30 at.%), sandwiched between Ni and Au/In layers. WTi film microstructures were observed to depend on the working pressure. Diffusion barrier breakdown was monitored upon annealing by the formation of intermetallic compounds (IMC) (intermixing between Ni and In). Annealing was performed at temperatures of 573 K, 623 K, and 673 K (homologous temperatures \(T/T_{\rm m}^{\rm In} \simeq 1.34, 1.45\), and 1.57, respectively) and under primary vacuum. The diffusion coefficients of In in WTi were determined. The correlation between WTi film microstructure and diffusion barrier efficiency was established. Better diffusion barrier performance was obtained for WTi films with dense microstructure associated with a compressive residual stress state. Hence, tuning the sputtering conditions allows significant improvement of barrier performance against diffusion through a change of the film microstructure.

Electro- and thermomigration induced Cu3Sn and Cu6Sn5 formation in SnAg3.0Cu0.5 bumps

There is a subject to reduce the weight of car body by replacing steel to aluminum alloys as some structural body parts for improving fuel consumption and suppressing CO2 generation. As one of the solutions, the hybrid body structure concept using aluminum alloys and high strength steels is proposed recently. However, it is well known that fusion welding of steel and aluminum is difficult due to the formation of brittle intermetallic compounds at the joint interface. Therefore, new welding processes by which these dissimilar materials can be joined in high reliability and productivity are demanded. “Laser roll welding” has been developed for joining of dissimilar metals by Kutsuna, Rathod, and Tsuboi in 2002 (Japanese Patent No. 3535152 and No. 3692135). Controlling the formation of the brittle imtermetallic compounds was tried by application of laser roll welding to get a sound joint. In the present work, laser roll welding of zinc coated steel and aluminum alloy was fundamentally investigated, and the process parameters were studied. Furthermore, the influences of process parameters on the weldability, the formation of intermetallic compound layer, and the joint properties have been investigated to get a sound joint. When intermetallic compound layer thickness was less than 10 μm, failure of tensile shear test specimen occurred in the base metal of the zinc coated steel.

Bimetallic structure of Ti6Al4V and Al6.21Cu fabricated by cold metal transfer additive manufacturing via Nb interlayer added by TIG

Effect of the addition of In on the microstructural formation of Sn–Ag–Zn lead-free solder

With the growing demand for vehicle weight reduction and the increased application of multimaterial design, it is imperative to address the challenges of welding dissimilar light alloys. This paper presents a review on laser welding of Al/Mg, Al/Ti, and Mg/Ti alloys, with focus on the techniques used to suppress the formation of brittle intermetallic compounds (IMCs) and improve joining mechanism. For Al/Mg joints, studies have shown that the use of structural adhesives, the use of interlayers such as Ce, Ni, Ti, and Fe foils and Zn-Al filler metals, and hybrid adhesive-interlayers could suppress the formation of brittle IMCs and improve joint strength. The formation of brittle IMCs during laser welding of Al/Ti alloys could be minimized by offsetting the laser beam at an appropriate distance towards either the Al or Ti alloy, the use of split-beam laser with appropriate joint design, the use of high welding speed, improving the laser energy distribution, and the use of V-groove with Al-Si filler metals. For Mg/Ti alloys, offsetting the laser beam at an appropriate distance towards the Mg alloy, the use of AZ series Mg alloy-based filler wires, and coating the Ti alloy with Ni were found to facilitate the joint formation and improve the joint strength.