A Review on Laser-Assisted Joining of Aluminium Alloys to Other Metals

Modern industry requires different advanced metallic alloys with specific properties since conventional steels cannot cover all requirements. Aluminium alloys are becoming more popular, due to their low weight, high corrosion resistance, and relatively high strength. They possess respectable electrical conductivity, and their application extends to the energy sector. There is a high demand in joining aluminium alloys with other metals, such as steels, copper, and titanium. The joining of two or more metals is challenging, due to formation of the intermetallic compound (IMC) layer with excessive brittleness. High differences in the thermophysical properties cause distortions, cracking, improper dilution, and numerous weld imperfections, having an adverse effect on strength. Laser beam as a high concentration energy source is an alternative welding method for highly conductive metals, with significant improvement in productivity, compared to conventional joining processes. It may provide lower heat input and reduce the thickness of the IMC layer. The laser beam can be combined with arc-forming hybrid processes for wider control over thermal cycle. Apart from the IMC layer thickness, there are many other factors that have a strong effect on the weld integrity; their optimisation and innovation is a key to successfully delivering high-quality joints.publishedVersio

Material Characterization of Intermetallic Compound Formation with Respect to Thermosonic Bonding Duration

Intermetallic Metallic Compounds (IMCs) formation is a common cause for wire bond failures. This research studied the effect of US vibration duration on IMC formation and growth in Copper-Aluminum (Cu-Al) wire bonded samples. Wire bonded samples, using 2.5 mil (63.5 μm) thick Palladium coated Copper wire, is ultrasonically bonded on a 2 cm thick Aluminum (1”x1”) coupon. Segmented bonding technique using 200 gf force and 220 gf force are applied during segment 1 and segment 2 of the bonding respectively. Ultrasonic (US) vibration frequency of 115-117 khz and a bonding temperature of 175°C is used. A pair of 5 samples with bonding duration: 20 milliseconds (ms), 40ms, 60ms, 80ms, 100ms is created. Keeping the temperature constant at 250 °C, a tube furnace is used to annealing one set for 2 hours and the other set for 4 hours. Backscattered Electrons (BSE) images were used to detect IMC growth. Backscattered images revealed formation of IMC at the Cu-Al bond interface, mostly around the center of and bond periphery. Using BSE images to identify location of IMC, Energy Dispersive Spectroscopy (EDS) linescans were then performed. Only EDS analysis was taken into account for final results assuming it was more accurate than visual inspection of BSE images. EDS linescan analysis for 2 hour heat treated samples showed IMC thickness growing from 0.6 μm to 1 μm as bond duration increased from 20ms to 100ms. Linescan results for 4 hour samples had IMC thickness ranging from 0.8μm to 1.5 μm, and hence showed an increase xiv with bond duration from 20ms to 100 ms. Using micro indentations, hardness of both Cu ball and Al was measured. Change in hardness for Cu and Al was compared with bond duration and annealing time. Cu hardness decreased from 20ms to 60ms bond time and then increased in value from 80ms to 100ms bond time. When compared to anneal time, overall hardness in Cu increased with increase in annealing time. Overall hardness in Aluminum increased with increasing bond duration but decreased with increase in anneal time, such behavior is related to the concurrent effect of annealing and IMC growth

Characterization of innovative rotary swaged Cu-Al clad composite wire conductors

Cu/Al composites are perspective for applications in a wide range of industrial and commercial branches, from transportation to elecatrotechnics. This study focuses on Cu/Al clad composite wires with 5 mm diameter featuring unique sequencing produced via the technology of rotary swaging at the processing temperatures of 20 degrees C and 250 degrees C. During the swaging process, we continuously acquired samples for investigations and used our own KOMAFU S600 system for dynamic detection of swaging forces. The composite wires subjected to electrical resistivity measurement were further analysed via electron microscopy, neutron diffraction, and mechanical testing. The results showed that both the total imposed strain (swaging degree) and swaging temperature influenced the investigated parameters non-negligibly. The samples subjected to high reduction ratios (swaging degree > 3) at the temperature of 250 degrees C exhibited formation of intermetallics at the interfaces, which deteriorated the electric conductivity. However, the conductivity was also affected by structural phenomena, such as work hardening, texture development, dislocations density, and recrystallization. All the final 5 mm samples exhibited sufficient bonding of both the components and recrystallized ultra-fine grained structures providing them with the ultimate tensile strength of >200 MPa.Web of Science16083582

A transmission electron microscopy study of defect generation and microstructure development in ultrasonic wire bonding

Ultrasonic wire bonding is widely used in the electronic industry to connect semiconductor chips to packages. Even though the popularity of the technique has increased in recent times, questions remain about the bonding mechanism, and factors affecting bondability and reliability. In this thesis, answers were provided to many of these questions using TEM to examine bonded cross section and plan view specimens. A detailed investigation of the Al wire and substrate showed dynamically annealed well recovered grains while microstructural observations of other substrates revealed wide varieties of response mechanisms. For example, Ni formed a dislocation cell structure, Cu formed a partially recovered structure, while Cu alloys and stainless steel formed planar dislocation arrays. These observed transformations were correlated with basic material properties and literature reported cyclic deformation studies to determine factors contributing to substrate plastic deformation during bonding. It appeared that the plastic deformation of the substrate is not a requirement for good bonding, but since extensive plastic deformation can occur during bonding, it could have important implications in bond strength and reliability. A model developed to estimate microstructural transformations was effective when applied to different metal substrates but somewhat less effective with Cu alloys. The extent and type of intermetallic phases that formed at the wire-substrate interface after thermal aging, thermal cycling and in the as-bonded conditions were characterized for different Au and NiB plated substrates using EDS. Specimens were also examined for the extent of Kirkendall porosity and the conditions of the unreacted portions of the wire and substrate. It was found that the extent of interfacial reactions depended strongly on substrate metallurgy. For example, in the NiP/immersion Au specimen the original Au layer was still present after bonding, and transformed completely to Al-Au intermetallics after only 1.5 h at 90\sp\circC. The same treatment resulted in no intermetallic phase formation for Ni-B specimens with the interface remaining chemically and structurally sharp. Finally, mechanisms of bonding and microstructural development were proposed

The role of rapid solidification processing in the fabrication of fiber reinforced metal matrix composites

Advanced composite processing techniques for fiber reinforced metal matrix composites require the flexibility to meet several widespread objectives. The development of uniquely desired matrix microstructures and uniformly arrayed fiber spacing with sufficient bonding between fiber and matrix to transmit load between them without degradation to the fiber or matrix are the minimum requirements necessary of any fabrication process. For most applications these criteria can be met by fabricating composite monotapes which are then consolidated into composite panels or more complicated components such as fiber reinforced turbine blades. Regardless of the end component, composite monotapes are the building blocks from which near net shape composite structures can be formed. The most common methods for forming composite monotapes are the powder cloth, foil/fiber, plasma spray, and arc spray processes. These practices, however, employ rapid solidification techniques in processing of the composite matrix phase. Consequently, rapid solidification processes play a vital and yet generally overlooked role in composite fabrication. The future potential of rapid solidification processing is discussed

Dissimilar Welding and Joining of Magnesium Alloys: Principles and Application

The growing concerns regarding fuel consumption within the aerospace and transportation industries make the development of fuel-efficient systems a significant engineering challenge. Currently, materials are selected because of their abilities to satisfy engineering demands for good thermal conductivity, strength-to-weight ratio, and tensile strength. These properties make magnesium an excellent option for various industrial or biomedical applications, given that is the lightest structural metal available. The utilization of magnesium alloys, however, requires suitable welding and joining processes that minimizes microstructural changes while maintaining good joint/bond strength. Currently, magnesium are joined using; mechanical fastening, adhesive bonding, brazing, fusion welding processes or diffusion bonding process. Fusion welding is the conventional process used for joining similar metals. However, the application of any welding technique to join dissimilar metals presents additional difficulties, the principal one being; the reaction of the two metals at the joint interface can create intermetallic compounds that may have unfavorable properties and metallurgical disruptions which deteriorates the joint performance. This chapter investigates the welding and joining technologies that are currently used to join magnesium alloys with emphasis on the development of multi-material structures for applications in the biomedical industries. Multi-material structures often provide the most efficient design solution to engineering challenges

Update - Body of Knowledge (BOK) for Copper Wire Bonds

Copper wire bond technology developments continue to be a subject of technical interest to the NASA (National Aeronautics and Space Administration) NEPP (NASA Electronic Parts and Packaging Program) which funded this update. Based on this new research, additional copper bond wire vulnerabilities were found in the literature - Crevice corrosion, intrinsic degradation of palladium coated copper wire, congregation of palladium near ball bond interface leading to failure, residual aluminum pad metallization impact on device lifetimes, stitch cracking phenomena, package delamination's that have resulted in wire bond failures and device failure due to elemental sulfur. A search of the U.S.A. patent web site found 3 noteworthy patents on the following developments: claim of a certain IMC (Intermetallic Compound) thickness as a mitigation solution to chlorine corrosion; claim of using materials with different pHs to neutralize contaminants in a package containing copper wire bonds; and a discussion on ball shear test threshold values for different applications. In addition, an aerospace contractor of military hardware had a presentation on copper bond wires where it was reported that there was a parametric shift and noise susceptibility of devices with copper bond wires which affected legacy design performance. A review of silver bond wire (another emerging technology) technical papers found that an electromigration failure mechanism was evident in device applications that operate under high current conditions. More studies may need to be performed on a comprehensive basis. Research areas for consideration are suggested, however, these research and or qualification/standard test areas are not all inclusive and should not be construed as the element (s) that delivers any potential copper wire bond solution. A false sense of security may occur, whenever there is a reliance on passing any particular qualification, standard, or test protocol

Study on strengthening mechanism of Ti/Cu electron beam welding

Welding-brazing method is widely used for dissimilar metals welding. However, it is becoming increasingly difficult to further improve the connection strength by controlling the formation of the transition layer. In this study, an innovative welding method referred to as adjacent welding was addressed, which greatly improved the tensile strength of Ti/Cu dissimilar joint. The strength of new joint could reach up to 89% that of copper base metal, compared to the use of a traditional welding-brazing method which strength coefficient is within the limit of 70%. In order to determine the strengthening mechanism of adjacent welding, optical microscopy, SEM, EDS and XRD were applied for the analysis of microstructure and phase structure. Furthermore, tensile strength was also tested. The results show that due to the process of remelting and reverse solidification of intermetallic compounds (IMCs) layer, a less complex and thinner IMCs layer was formed and TiCu (553 HV) with high embrittlement existing in the front of titanium substrate was changed into Ti2Cu (442 HV). Performances of joints were optimized by these changes. An interpretation module was presented for the mechanism

Kinetics of transformation in an in-situ aluminum-strontium deformation processed metal-metal composite

Efficient electricity transmission is a key component in all plans to increase the amount of renewable power used in the decades ahead. Prime solar and wind generation sites are usually distant from major population centers, resulting in the need for improved conductor wires that are stronger, lighter, and have better conductivity than conventional conductors. Deformation processed metal-metal composites (DMMCs) have a desirable combination of high strength and good conductivity. One such DMMC, aluminum-strontium, was investigated in this study. The composite wire was created by extrusion, swaging, and wire drawing of bundled Al and Sr wires. Intermetallic compound formation between Al and Sr is of particular interest to produce a strong, conductive wire with good high-temperature strength. Samples of swaged and drawn Al-Sr composite wire were heat treated at 483K, 513K, 543K and 573K to produce samples at varying stages of intermetallic compound formation. Resistivity measurements were taken from samples over a range of heat treatment times and temperatures to calculate the activation energy for Al-Sr intermetallic compound formation. Scanning electron microscopy, differential scanning calorimetry, and x-ray diffraction were used to investigate the changes in the microstructure occurring in the samples as a function of heat treatment. In addition, mechanical properties data were generated for pure Sr metal