Copper Side Research Articles

Design and Qualification of Joints for ITER Magnet Busbar System

The joints connecting the ITER magnet busbars and coils utilize the twin-box “shaking hands” concept: inside a helium leak tight box, a bare cable is pressed in an indium-tinned copper base. To form the joint, two boxes are tightly compressed against each other on the copper side with a layer of indium in between. This concept was chosen to address the different requirements of the joints: to provide low electrical resistance without degradation and need of maintenance during ITER lifecycle, to sustain cyclic electromagnetic and pressure loads, to be easily dismountable in case of failure, to be tolerant to manufacturing and assembly misalignments, to provide low coolant flow impedance, and to limit ac losses and at the same time to facilitate current redistribution in the busbars. This paper presents the design; the key results of the electromagnetic, thermal, and stress analysis; and the major manufacturing and qualification steps. The latter includes cryogenic fatigue test of the joint welds and joint resistance measurements.

Effect of beam offset on the characteristics of copper/304stainless steel electron beam welding

Copper (T2) and 304stainless steel were joined by EBW with different beam offset. Full-strength joint as well as the offset joints of sufficient strength can be obtained. The highest tensile strength reaches 250Mpa, which is almost equal to the strength of copper. By analyzing phase structure and distribution of weld zone, bonding interface of fusion zone, width and grain size of heat affected zone, diffusion behavior of Fe/Cr, cooling rate and fracture behavior, the effect of beam offset on the joint strength was detailed studied. The results show that beam offset is flexible and allowed to be in the range of −0.2 mm to 1 mm. Under the condition of small beam offset, the performance of the joint is nice because of narrow HAZ and weld zone of few defects. But for large offset which leads to low joint performance on copper side or steel side, the main reasons respectively are deterioration of HAZ, FZ and low metallurgical bonding of root interface. Toughness fracture was proved by dimples for fusion welding, but brittle fracture occurred for re-melt deposit welding of large offset.

Active Brazing of C/C Composite to Copper by AgCuTi Filler Metal

Brazing between the carbon–fiber–reinforced carbon composite (C/C composite) and copper has gained increasing interest because of its important application in thermal management systems in nuclear fusion reactors and in the aerospace industry. In order to examine the “interfacial shape effect” on the mechanical properties of the joint, straight and conical interfacial configurations were designed and machined on the surface of C/C composites before joining to copper using an Ag-68.8Cu-4.5Ti (wt pct) alloy. The microstructure and interfacial microchemistry of C/C composite/AgCuTi/Cu brazed joints were comprehensively investigated by using high-resolution transmission electron microscopy. The results indicate that the joint region of both straight and conical joints can be described as a bilayer. Reaction products of Cu3Ti3O and γ-TiO were formed near the copper side in a conical interface joint, while no reaction products were found in the straight case. The effect of Ag on the interfacial reaction was discussed, and the formation mechanism of the joints during brazing was proposed. On the basis of the detailed microstructure presented, the mechanical performance of the brazed joints was discussed in terms of reaction and morphology across the joint.

Ballistic impact welding of copper to low carbon steel

A method of ballistic impact welding pure copper to low carbon steel was developed based on the prediction of the critical parameters. The macro profile, microstructure, element distribution across the interface, and the mechanical properties of the welds were studied. It showed that a wavy bond profile was formed at the interface and no intermetallic layer was formed near the interface. The microstructures and the properties exhibited different characteristics in different areas with different distance to the interface for both the copper side and low carbon steel side. Moreover, the additional increase in temperature due to the viscous friction caused by the spinning of projectile between the spinning copper projectile and the low carbon steel results in a recrystallization annealing on the copper side.

Characterization of Clean after Photoresist Removal from Wafers with Copper Pillars with Lead-Free Solder Caps

Opportunities for developing new and enabling packaging schemes are being pursued as part of device improvement strategies for electronic products. Processes such as embedded technologies in wafer level packaging and 3-D chip architecture schemes open up opportunities for realization of a variety of package configurations. As a result, there are many opportunities to impact both device performance and the processes used to create them. In the area of electroplated solder applications, there has been widespread adoption of copper pillar bumps with lead free solder caps. Solder wetting of only the copper pillars after reflow confines the spread of the solder cap in the x,y plane enabling tighter pitch layouts, while the smaller solder cap still improves the robustness of the interconnection. However, as more data is gathered using this process, one area of growing interest is the characterization of polymer photoresist residue on the side walls of plated copper pillars with lead-free solder caps. As has been seen with other processes including for example, redistribution line (RDL) cleaning processes, cleaning challenges have increased in tighter pitch applications. In RDL, cleaning challenges were increased as cleaning processes that were once taken for granted were now being applied to wafers with 10μm line/space and tighter pitches. Analogies can be found in the tight pitch copper pillar cleans. Typical characterization techniques such as optical microscopy have been used traditionally in the industry as a post process inspection method. Advantages such as ease of use, fast throughput, automated tooling, use of filters to highlight certain defect types, and the ability to non-destructively inspect whole wafers have all contributed to its success in the field. With tighter pitch copper pillar applications, inspection using optical microscopy is challenged in an extreme and it is possible to inspect a clean copper pillar wafer and not see photoresist residue on the copper side walls. This paper will describe wet cleaning processes used today for copper pillar cleans, with special focus on characterization of the Cu pillar and solder side walls. Examples using several different photoresists and copper and solder configurations will be examined. Samples with differing copper/solder bump heights and pitches will be discussed. Cleaning and compatibility results will be shown.

Effect of rotation speed on the microstructure and mechanical properties of dissimilar friction stir-welded copper/brass metals

Dissimilar copper and brass metals were friction stir welded at a constant welding speed of 80 mm/min. Groove defects were found on the weld surface with a rotation speed of 1000 rpm, and defect-free welds were successfully obtained with rotation speeds ranging from 1200 to 1600 rpm. The base material of copper consisted of an equiaxed α phase, while that of brass was characterized by a coarse α phase containing certain twins and a β′ phase. Onion rings were developed due to the plastic flow of metal induced by mechanical mixing in the stir zone during friction stir welding. The grain size in the heat-affected zone was refined compared with that in the base material at the brass side of the welded joint, while the situation was completely the opposite for the copper side. With increasing rotation speed, grain size in all zones increased. The weld exhibited higher hardness than the base material. There is little change in tensile properties of all the defect-free welded joints with increasing rotation speed, and all the defect-free welded joints were fractured in the lowest hardness region of the welded joint during tensile tests.

Solid state impact welding of BMG and copper by vaporizing foil actuator welding

The objective of this study was to create impact welds between a Zr-based Bulk Metallic Glass (BMG) and copper at a laboratory scale and subsequently investigate the relationship between interfacial structure and mechanical properties. Vaporizing Foil Actuator (VFA) has recently been demonstrated as a versatile tool for metalworking applications: impact welding of dissimilar materials being one of them. Its implementation for welding is termed as VFA Welding or VFAW. With 8kJ input energy into an aluminum foil actuator, a 0.5mm thick Cu110 alloy sheet was launched toward a BMG target resulting in an impact at a velocity of nearly 600m/s. For this experiment, the welded interface was straight with a few BMG fragments embedded in the copper sheet in some regions. Hardness tests across the interface showed increase in strength on the copper side. Instrumented peel test resulted in failure in the parent copper sheet. A slower impact velocity during a separate experiment resulted in a weld, which had wavy regions along the interface and in peel failure again happened in the parent copper sheet. Some through-thickness cracks were observed in the BMG plate and there was some spall damage in the copper flyers. TEM electron diffraction on a sample, cut out from the wavy weld interface region using a focused ion beam, showed that devitrification of the BMG was completely avoided in this welding process.

Effects of filler wire on residual stress in electron beam welded QCr0.8 copper alloy to 304 stainless steel joints

Abstract The electron beam welding (EBW) of 304 stainless steel to QCr0.8 copper alloy with or without copper filler wire was studied in detail. The temperature fields and magnitude and distribution of stress fields in the joints during the welding process were numerically simulated using finite element method. The temperature cycles and residual stresses were also experimentally measured by thermometric and hole-drilling methods, respectively. The accuracy of the modeling procedure was verified by the good agreement between the calculated results and experimental data. The temperature distribution in the joint was found to be asymmetric along the center of weld. In particular, the temperature in the copper alloy plate is much higher than that in the 304 SS plate owing to the great difference in thermal conductivity between the two materials. The peak three-dimensional residual stresses all appeared at the interface between the copper and steel in the two different joints. Furthermore, the weld was subjected to tensile stress. The longitudinal residual stress, generally the most harmful to the integrity of the structure among the stress components in EBW with filler wire (EBFW), was 53 MPa lower than that of autogenous EBW (AEBW), and the through-thickness residual stress was 12 MPa lower. The transverse residual stress of EBFW was 44 MPa higher than that of AEBW. However, analysis of the von Mises stress showed that the EBFW process effectively reduced the extent of the high residual stress region in the weld location and the magnitude of the residual stresses in the copper side compared with those of the AEBW joint.

Effects of Tilt Angle on the Properties of Dissimilar Friction Stir Welding Copper to Aluminum

In the present investigation, dissimilar materials such as electrolytic tough pitch copper, and aluminum 6061-T651 were welded by friction stir welding technology. Effects of tool tilt angle on the mechanical and metallurgical properties were studied experimentally for dissimilar material systems. In the present study, the tool tilt angle was varied from 0° to 4° with an interval of 1°, while the other parameters such as rotational speed, welding speed, tool pin offset, and workpiece material position were kept constant. Macrostructure analysis, tensile test, macro hardness measurement, scanning electron microscopy, and energy dispersive x-ray spectrographic tests were performed to evaluate the weld properties of dissimilar copper–aluminum joints. The results revealed that a defect free dissimilar copper–aluminum friction stir welding was achieved by tilt angles 2°, 3°, and 4°. The maximum tensile strength was reported to be 117 MPa and the macro hardness was reported to be 181 VH (in the nugget zone) at a tilt angle of 4°. The macro hardness was increased as the tilt angle increases from 0° to 4°. In addition to this, the thermo-mechanically affected zone (at the copper side) was found to be the weakest zone for a dissimilar copper–aluminum friction stir welding system.

The Heat Treatment Effect on Microstructure for Arc Welding of Dissimilar Steel to copper

The purpose of this paper is to be familiarize with the microscopic structure of the welding specimen, before and after heat treatment in which the pure copper ( OFHC ) has been welded to the low carbon steel ( ASTMA36 ) by the method of fusion welding ( Arc Welding ) using covered welding electrode (SMAW) .A 12mm thick plates of the two metals were used.The plate have been prepared (Single Butt Joint ) where the dissimilar plates copper to steel were welded using ( Ecusu8 ) electrodes . The heat treatment has been carried out to the specimen at 600Co,700Co then cooled down in the oven . The microscopic test for the welded specimen has been carried out before and after the heat treatment .Before heat treatment It was clear that there exist a separating limit from the steel side with zigzag indicating that interaction was happened giving a ( Non-epitaxial ) structure with opening windows accompanied by diffusion of masses from the base metal inside the added metal ,while after heat treatment, it is found that more zigzag and opening cracks in the steel side filled with the added metal by the process of ( Brazing ) and excess clearness of diffusion process with diffused metal taking a tree shapes , While from the copper side, it showed ( Epitaxial Growth ) No existence to any cracks was noticed , but there was a granulated growth especially when heated up to 700Co .

Electron beam welding of 304 stainless steel to QCr0.8 copper alloy with copper filler wire

Electron beam welding (EBW) of 304 stainless steel to QCr0.8 copper alloy with copper filler wire was carried out. Orthogonal experiment was performed to investigate the effects of process parameters on the tensile strength of the joints, and the process parameters were optimized. The optimum process parameters are as follows: beam current of 30 mA, welding speed of 100 mm/min, wire feed rate of 1 m/min and beam offset of −0.3 mm. The microstructures of the optimum joint were studied. The results indicate that the weld is mainly composed of dendritic α phase with little globular α phase, and copper inhomogeneity only occurs at the top of the fusion zone. In addition, a melted region without mixing exists near the weld junction of copper side. This region with a coarser grain size is the weakest section of the joints. It is found that the microhardness of the weld decreases with the increase of the copper content in solid solution. The highest tensile strength of the joint is 276 MPa.

Experimental investigations of fracture and fatigue crack growth of copper–steel joints arc welded using nickel-base filler

This paper experimentally investigates fracture toughness and fatigue crack growth behaviour of mode-I cracked bi-metallic compact tension specimens. The specimens were made of weak-copper (UNSC11000) and strong-alloy steel (En31) arc-welded with nickel-filler. Residual stresses induced in constituent materials during welding, due to their different coefficients of thermal expansion (thermal mismatch) were measured at important locations in all the materials near the weld zone. Several notched specimens with positions of crack tip in critical zones of copper, steel and nickel-weld were subjected to fracture tests. The specimens were fatigue precracked prior to fracture testing. Bi-metallic specimens having an equal initial crack length were subjected to constant-amplitude fatigue crack growth rate tests until their fracture occurred. Residual stress intensity factor as a function of crack length was computed theoretically for inclusion in the analysis. Thermal and mechanical mismatches between constituent materials, the latter represented by elastic and plastic property differences, were strongly found to affect the behaviour of crack tip in the vicinity of the weld under both monotonic and fatigue load. The combined effect of “elastic–plastic mismatch” across the material interfaces over the crack tip was described by crack driving force, Jinterface. Under monotonic load (i) fracture toughness of bi-metallic specimens with crack in copper side facing the weld backed by steel was found to be higher than that of monolithic copper with crack arrest noticed in all the specimens when the crack tip was near the weld. Furthermore, Jinterface was found to be positive in this case which implied that the crack driving force, Jtip, was less than the remotely applied crack-driving force, Japplied. (ii) Fracture toughness of bi-metallic specimens with crack in steel side facing the weld backed by copper was found to be lesser than that of monolithic steel with the specimens failing quickly in the form of short brittle fracture within steel and the part of the weld. In addition, Jinterface was negative in this case with Jtip being more than Japplied. Under constant amplitude fatigue load (i) for crack growth from copper to steel side, crack growth rates in copper side retarded in the vicinity of the weld due to shielding caused by the stronger weld and back up steel. The crack deflected away from mode-I plane before penetrating into weld and back up steel. (ii) For crack growth from steel to copper side, crack growth rates in steel accelerated in the vicinity of the weld due to the amplification caused by the “elastic–plastic” property gradient from higher to lower. Furthermore, the crack penetrated through the weld and back up copper with no deflection. The results were also compared with those of monolithic metals subjected to similar fracture and fatigue crack growth rate tests. The shielding and amplification trends in the fatigue crack growth tests were not found to be altered by residual stresses. This implied that the combined effect of “elastic–plastic” mismatch was dominant over that of residual stresses in both directions of crack-transition.

Brazing of copper to stainless steel with a low-silver-content brazing filler metal

The brazing of copper to stainless steel (SUS304 JIS) was performed using a low- silver-content brazing filler metal, Ag-50Cu, under an Ar gas atmosphere with a conventional furnace, owing to the potential economic benefits of using low-silver-content filler metals. The brazeability of the low-silver-content brazing filler metal to copper and SUS304 was investigated. A good joint was obtained, and a drastic dissolution reaction occurred at the copper side. Molten BAg8 penetrated along the crystal grain boundary of the copper base metal when BAg8 was used as the filler metal. This was caused by the dissolution of Ni from the stainless steel into the molten filler metal. Ag-50Cu, which was investigated in this work, can be used instead of BAg8 filler metal.

Selective atomic layer deposition of zirconia on copper patterned silicon substrates using ethanol as oxygen source as well as copper reductant

The authors report a new chemical approach for the selective atomic layer deposition of ultrathin layers of zirconium oxide (ZrO2) on copper patterned silicon surfaces. Instead of using common atomic layer deposition (ALD) oxygen sources such as water, oxygen, or ozone, the authors use ethanol, which serves as oxygen source for the ALD on the silicon side and as effective reducing agent on the copper side, thereby selectively depositing ZrO2 film on the silicon surface of the substrate without any deposition on copper up to at least 70 ALD cycles. The resulting ZrO2 nanofilm is found to be an effective copper diffusion barrier at temperatures at least up to 700 °C.

Microstructure and shear strength of sintered Cu–Al2O3 composite joined to Cu using Ag–Cu and Cu–Zn filler alloys

The microstructure and shear strength of joints between Cu–Al2O3 (50vol.% Al2O3) composite and Cu metal brazed with 72Ag–28Cu and 70Cu–30Zn filler metals were characterized. The composite fabricated by the pulsed electric current sintering of Cu-coated alumina particles showed less than 2% residual porosity and interesting thermophysical properties for electronic packaging applications; thus, the adhesion of the composite to copper metal was evaluated. The composite–Cu assemblies were brazed in an inert atmosphere at working temperatures of 10, 60 and 110°C above the liquidus of the alloys. In the case of Ag–Cu, there was significant diffusion of silver to the composite side, forming a (Ag, Cu) solution. Zinc diffused to the copper side when using the Cu–Zn filler metal, and the formation of fine oxygen-containing Zn precipitates was observed; furthermore, the overheating of the Cu–Zn material originated in interfacial cavities due to evaporation and the zinc deficiency of the fused metal, which reduces its fluidity. The shear strength followed a similar trend with respect to temperature for both joining systems; maximum strengths of 49.9 and 37.1MPa were obtained for the 72Ag–28Cu and 70Cu–30Zn fillers, respectively, at the intermediate temperatures used. Fracture examination revealed that the brazed arrays failed in all cases on the composite side. The microstructure and strength values indicate a strong adhesion between the parts, which constitutes an important requirement for thermal management in electronic assemblies.

Effect of friction stir welding parameters on the mechanical properties and microstructure of the dissimilar Al5083–copper butt joint

In this paper, butt joining of Al5083 to commercially pure copper by friction stir welding method has been investigated. The effect of welding parameters, rotational speed of the tool and tool offset on joint strength and microstructure have been studied experimentally. By examining different situations, joint strength was optimized in terms of rotational speed and offset. Results show that tool offset to the copper side reduces defects and increases the joint strength. Welded joint that was conducted at the rotation speed of 800 r/min, tool traverse speed of 40 mm min−1 and 1 mm offset to the copper side had the highest tensile strength, about 96% of the weak base metal strength. Microstructure in the stir zone had different morphology from that observed in the base metal. The analyses were performed in intermetallic compounds formed in this area. Al4Cu9 and Al2Cu, were the intermetallic compounds detected in stir zone.

On Copper-Steel Joint Arc Welded with Nickel Based Filler

The paper presents some metallurgical and mechanical examinations of the joint between dissimilar metals viz. copper and alloy steel obtained by Shielded Metal Arc Welding (SMAW) using consumable nickel based superalloy electrode as the weld filler. Material microstructures at all the important zones of the weldment are examined with the help of Optical and Scanning Electron Microscopes (SEM). Examinations reveal corrugated weld interfaces with good fusion between the metals and the filler. The weld joints are subjected to tensile and impact tests in ambient conditions. In both the tests, the weldments fracture on copper side with all the weld interfaces remaining intact confirming a strong joint.

Texture Evolution of Nanostructured Aluminum/Copper Composite Produced by the Accumulative Roll Bonding and Folding Process

In this study, the accumulative roll bonding and folding (ARBF) process was used for manufacturing nanostructured aluminum/copper multilayered composites. Textural evolution during the ARBF process of composites was evaluated using X-ray diffraction. Microstructural observation of some samples was evaluated by scanning electron microscopy and transmission electron microscopy. The ARBF process induced formation of a strong preferred orientation along the β-fiber and also to the pronounced copper texture component. In the aluminum side, occurrence of dynamic recovery reduced the intensity of the β-fiber rolling texture due to change in dislocation structure and decrease in the degree of strain hardening. On the other hand, occurrence of discontinuous dynamic recrystallization at the third and fourth ARBF cycles led to decreasing the intensity of fibers and texture components in the copper side. The average grain sizes of the final sample for the copper and aluminum sides were ~50 and ~200 nm, respectively.

On the initial growth of atomic layer deposited TiO2 films on silicon and copper surfaces

Atomic layer deposition (ALD) of TiO2 using tetrakis(diethylamino)titanium precursor and H2O was studied on silicon and copper surfaces in order to examine differences in nucleation. Both surfaces were patterned on the same substrate to assure identical deposition conditions. Spectral ellipsometry, X-ray photoelectron spectroscopy and surface profilometry were used to probe nucleation phenomena, growth rates, and surface morphology on both surfaces. The TiO2 deposition on copper was found to exhibit a significant induction period of about 20–25 ALD cycles with no observable TiO2 during the first 10–15 cycles on the copper side; in contrast, no such inhibited growth was observed in the TiO2 deposition on silicon. This result opens up potential for selective ALD of TiO2 films on silicon-based substrates patterned with a metal without the use of a mask, a self-assembled monolayer or soft lithography which is impractical for some nanoscale semiconductor fabrication processes. After film nucleation, the TiO2 growth rate on both surfaces was found to be 0.10nm/cycle.

Microstructure and mechanical properties of dissimilar pure copper/1350 aluminum alloy butt joints by friction stir welding

The dissimilar friction stir welding of pure copper/1350 aluminum alloy sheet with a thickness of 3 mm was investigated. Most of the rotating pin was inserted into the aluminum alloy side through a pin-off technique, and sound welds were obtained at a rotation speed of 1000 r/min and a welding speed of 80 mm/min. Complicated microstructure was formed in the nugget, in which vortex-like pattern and lamella structure could be found. No intermetallic compounds were found in the nugget. The hardness distribution indicates that the hardness at the copper side of the nugget is higher than that at the aluminum alloy side, and the hardness at the bottom of the nugget is generally higher than that in other regions. The ultimate tensile strength and elongation of the dissimilar welds are 152 MPa and 6.3%, respectively. The fracture surface observation shows that the dissimilar joints fail with a ductile-brittle mixed fracture mode during tensile test.