Metallurgical Joint Research Articles

Compliant Interconnects Based on Single Micrometer-sized Metal-Coated Polymer Spheres.

The rapid evolution of multifunctional electronics necessitates interconnection technologies appropriate for large dies with high-density and/or ultrafine pitch input/output pins. Existing technologies face numerous challenges, including demands for bonding equipment that can deliver extremely high force as well as thermo-mechanical stresses induced in the assembled packages due to mismatched thermal expansion of materials involved. This study proposes an approach to compliant interconnects comprising single micrometer-sized metal-coated polymer spheres, being joined to mating electrodes by sintering of Ag nano ink at low temperature (140 °C) and low pressure (∼15 mN/particle). Such an interconnection technology is expected to enhance the thermo-mechanical robustness of the assembled packages as well as be capable of high-density, ultrafine pitch interconnects. Our approach demonstrates control over conductive particles during assembly, achieving a 98% success rate in individual interconnects with a single captured particle. The use of sintered Ag not only secures free-standing particles on electrical pads (with an adhesion force above 2 μN) but also results in a 15% reduction in interconnect resistance, with measured resistance as low as 0.5 Ω, compared to interconnects without Ag ink. This method presents an alternative to metallurgical joints, particularly suited for high-density, ultrafine pitch applications, offering low bonding pressure and temperature, along with improved interconnect compliance to enhance the thermo-mechanical robustness of the packages.

Understanding the mechanisms behind ultrasonic vibration in resistance spot welding of aluminum and steel

An ultrasonic-assisted resistance spot welding process, which can improve the quality of aluminum/steel spot-welded joints, has been developed. However, the mechanism of the thermal-electrical-mechanical behavior caused by the ultrasonic longitudinal vibration to welding process has not been clarified. In this study, the role and impact mechanisms of ultrasonic longitudinal vibration during the welding process to form joints were confirmed by combining the microstructural characterization of the formed joints with the signal analysis of the process, as well as the analysis of the independent finite element numerical calculation results. Additionally, the interface metallurgical reaction behavior, welding defects, fracture behavior, and joint strength characteristics of ultrasonic-assisted aluminum/steel resistance spot welding joints were comprehensively analyzed, and the results indicated that the sizes of the aluminum/steel nuggets decreased. It could be attributed to the competition among multiple ultrasonic effects. Specifically, ultrasonic vibration decreased the contact resistance between the aluminum/steel sheets, whereas the acoustic cavitation and acoustic streaming effects increased the electrical conductivity of molten steel and thermal conductivity of molten aluminum, respectively. Moreover, ultrasonic vibration promoted the radial creep of molten and near-molten aluminum, which expanded in the effective bonding area between the two workpieces. Furthermore, ultrasonic vibration effectively reduced the thickness of the intermetallic compound layer (<3.0 µm), concurrently inhibited the formation of interfacial welding defects. Finally, the peak tensile-shear load of aluminum/steel joints is significantly increased under multiple ultrasonic optimization mechanisms.

An investigation on metallurgical features and joint characteristics of ultrasonic spot welded Al-Ni sheets

Several potential benefits in joining thin, dissimilar and highly conductive metals in a solid-state environment make ultrasonic welding (USW) a popular method for building Li-ion battery packs. The USW method is highly complex and relies on multiple factors. In this study, aluminum (AA1100) and nickel (UNS N02200) dissimilar sheets were ultrasonically welded using various process parameters. Plastic deformation, microstructural development, and the corresponding mechanical characteristics of the joints were evaluated qualitatively and quantitatively. The joint strength and critical intensity factor values seemed to rise until the welding duration of 0.3 s, following which there was a subsequent drop in their values. Based on the mechanical analysis, it is observed that the optimal condition records an amplitude of 68 µm, a weld time of 0.3 s, and a clamping pressure of 0.24 MPa. It showed that the joint performance was significantly impacted by the plastic deformation and microstructural changes at the weld region. On the Al side, substantial plastic deformation was observed, and it was primarily attributed to the sonotrode knurls’ penetration. The Al sheet’s thickness was reduced by more than 40%. The microhardness readings at the welding region experienced diminution due to the acoustic softening effect. The cross-section and fracture surfaces of the Al/Ni weld exhibited several distinct microstructural characteristics, such as atomic diffusion, crimped interface, dimple pattern, and swirl pattern flow. Additionally, the diffusion zone is also investigated using X-ray diffraction analysis. These observations offer valuable insights for enhancing weld quality and controlling the overall welding process.

Laser Welding of Titanium/Steel Bimetallic Sheets with In Situ Formation of Fex(CoCrNiMn)Tiy High-Entropy Alloys in Weld Metal.

Due to the notable disparities in the physical and chemical characteristics between titanium and steel, the direct fusion of titanium/steel bimetallic sheets results in a considerable formation of fragile intermetallic compounds, making it difficult to achieve excellent metallurgical welded joints. In this study, a multi-principal powder of CoCrNiMn was designed and utilized as a filler material in the welding of the TA1/Q345 bimetallic sheet. It was expected that the in situ formation of Fex(CoCrNiMn)Tiy high-entropy alloys would be achieved using the filler powders, combined with the Ti and Fe elements from the melting of the TA1 and Q345 so as to restrain the generation of Fe-Ti IMCs and obtain the promising welded joints of the TA1/Q345 bimetallic sheet. An interesting finding is that high-entropy alloys were successfully obtained in the weld metal. The Fe-Ti intermetallic compounds at the welding interface were significantly reduced. The tensile strength was ~293 MPa, accounting for 60% of the strength of the base metal. Dimples were observed at the fracture of the welded joint.

Process parameter assessment on the dissimilar deposition of AA2024-T351 on AA7475-T761 by friction surfacing

Friction surfacing as a solid-state deposition process allows the joining of materials with different chemical and physical properties at temperatures below their respective melting points. This experimental work focuses on generating sound, defect-free metallurgical joints between single friction surfacing deposits and substrate surfaces from dissimilar Al alloys of the 2xxx and 7xxx series. In this context, the influence of axial force and deposition speed on surface morphology and deposit geometry of the two heat-treatable Al alloys AA7475 as substrate and AA2024 as deposit are investigated. Process parameter variation shows that an increase in axial force from 8 to 12 kN, in conjunction with a deposition speed of 8 mm/s, leads to smooth surface morphology and consistent deposit width along its length. The AA2024 deposits consist of fine-grained microstructure with higher hardness at the top and lower hardness at the deposit-substrate interface. The joining mechanism is by interdiffusion, with a 7.5 μm thick diffusion zone across the dissimilar interface. Three-point bending tests reveal excellent bonding in the lateral surface of the advancing side due to the absence of delamination for all conditions tested. Minor delamination appears predominantly on the retreating side region for process parameter sets with low axial force. Tensile test results reveal that the AA2024 deposit on the AA7475 substrate presents an ultimate tensile strength equivalent to the AA7475-T651 base material and an increase of 37 % in ductility.

Additive manufacturing multi-material components of SAF 2507 duplex steel and 15-5 PH martensitic stainless steel

The study reports the successful deposition of 15–5 precipitation-hardened (PH) martensitic steel on SAF 2507 duplex stainless steel (and vice versa) to form functionally graded materials (FGMs) using powder-based, directed-energy-deposition (DED) technology. The evolution of the microstructure and the mechanical properties of the 15–5 PH/SAF 2507 functionally graded material in the as-built state were investigated systematically. The results proved that the microstructural transition zone (MTZ) is formed above the fusion line due to the dilution effect of both steels. The phase composition of the MTZ consists of ferrite and, in comparison to the base materials, an increased amount of austenite. The interface has the lowest hardness owing to the formation of a significant proportion of the austenitic phase. However, the tensile mechanical properties were not affected by the interface as failure occurred in both SAF 2507 and at the interface regions. The research presents a promising application of FGMs in a horizontal configuration to form a high - quality, metallurgical joint between heterogeneous materials. This study introduces a novel approach by additive manufacturing (AM) of heterogeneous multi-materials, merging the favorable properties of duplex SAF 2507 and martensitic 15-5 PH stainless steels through superior metallurgical bonding. The combination of SAF 2507 and 15-5 PH in a functionally graded material has not been previously explored, as the existing studies deal with the duplex stainless steels SAF 2507 and either martensitic stainless-steel 15-5 PH deposited separately by laser powder-bed-fusion (L-PBF) or DED methods. This research pioneers the investigation of these materials in tandem, paving the way for the development of novel FGMs with optimized properties for specific applications. The resulting FGM exhibits exceptional corrosion resistance and high strength, making it highly versatile for applications in diverse industries such as offshore oil and gas production, aerospace, aviation, and chemical processing equipment.

In-situ isothermal aging TEM analysis of a micro Cu/ENIG/Sn solder joint for flexible interconnects

Sn/ENIG has recently been used in flexible interconnects to form a more stable micron-sized metallurgical joint, due to high power capability which causes solder joints to heat up to 200 °C. However, Cu6Sn5 which is critical for a microelectronic interconnection, will go through a phase transition at temperatures between 186 and 189 °C. This research conducted an in-situ TEM study of a micro Cu/ENIG/Sn solder joint under isothermal aging test and proposed a model to illustrate the mechanism of the microstructural evolution. The results showed that part of the Sn solder reacted with Cu diffused from the electrode to form η´-Cu6Sn5 during the ultrasonic bonding process, while the rest of Sn was left and enriched in a region in the solder joint. But the enriched Sn quickly diffused to both sides when the temperature reached 100 °C, reacting with the ENIG coating and Cu to form (NixCu1-x)3Sn4, AuSn4, and Cu6Sn5 IMCs. After entering the heat preservation process, the diffusion of Cu from the electrode to the joint became more intense, resulting in the formation of Cu3Sn. The scallop-type Cu6Sn5 and the seahorse-type Cu3Sn constituted a typical two-layered structure in the solder joint. Most importantly, the transition between η and η’ was captured near the phase transition temperature for Cu6Sn5 during both the heating and cooling process, which was accompanied by a volume shifting, and the transition process was further studied. This research is expected to serve as a reference for the service of micro Cu/ENIG/Sn solder joints in the electronic industry.

Dynamic recrystallization in friction stir welded AA2014 aluminium alloy joints to replace riveted joints

Abstract The demand for lightweight materials (aluminium and magnesium alloy) in structural applications is increasing due to strength ratio, corrosion resistance, formability, and recyclability. Fusion welding of aluminium and its alloy is difficult due to the formation of hot cracking, alloy segregation, porosity, etc. Henceforth, fusion welding is not an ideal process for joining aluminium and its alloy. Steel rivets are being used to join similar and dissimilar alloys in different joint configurations. Since the use of steel rivets, aircraft weight has increased drastically. Although, dissimilar metal corrosion has been encountered. These two are the main problems in structural fabrication industries. The solid-state welding friction stir welding process eliminates the issues mentioned earlier. This process can weld materials well below the melting point. Moreover, the formation of weld in the weld line could be achieved by severe plastic deformation and recrystallized grains. This metallurgical joint may replace the rivets.

An X-ray Microscopy Study of the Microstructural Effects on Thermal Conductivity in Cast Aluminum-Copper Compounds

A metallurgical joint between aluminum and copper established by compound casting provides for high thermal conductivity, which is required for lightweight cooling solutions in applications such as high-power light-emitting diodes or computer processors. If casting is employed in a silane-doped inert gas atmosphere whose oxygen partial pressure is adequate to extreme high vacuum, reoxidation of the active surfaces of aluminum and copper is prevented, and thus a metallurgical bond can be created directly between aluminum and copper. With this approach, thermal conductivities as high as 88.3 W/m·K were realized. In addition, X-ray microscopy was used to shed light on the microstructure–thermal property relationship. It is demonstrated that both porosity and non-bonded areas have a substantial impact on the thermophysical properties of the compound zone. Based on the data obtained, casting parameters can be developed that provide for defect-free bonding zones and optimal heat transfer between the joining partners.

Effects of substrate surface treatments on hybrid manufacturing of AlSi7Mg using die casting and selective laser melting

To balance the manufacturing cost and customizability of automotive parts, a hybrid manufacturing process combining die-casting and selective laser melting (SLM) is proposed: starting with a conventional cast substrate, SLM is utilized to add additional geometric elements on top of it. For this hybrid process, the first priority is to prepare a substrate surface suitable for the subsequent SLM addition of the top-on elements. In this study, the original cast surface of AlSi7Mg was processed by sandblasting, wire electro-discharge machining, and laser remelting, respectively. Then, additional AlSi7Mg components were built on both the original cast and treated surfaces through SLM. After hybrid builds, these surfaces and resultant interfaces were examined by optical and scanning electron microscopes. Results indicate that the defect-free metallurgical joint between the cast and additively added parts can be formed on all surfaces except for the one processed by electro-discharge machining. The observed epitaxial grain growth crossing the interface implies a strong connection between the cast and the SLMed component. Despite these benefits, also mismatches in microstructure, residual stress level and element distribution between the two parts are identified. After a comprehensive assessment, laser remelting with no additional machining is recommended as the optimal surface treatment preceding SLM fabrication, because of its user-friendly operation, low cost, and high industrial feasibility.

A fundamental study of improving thermal interface connection by using silane modified, in-situ formed silver-graphene fillers in epoxy polymers for future semiconductor device packaging

Over the past half-century, the increase of on-chip power and on-chip integration density has created new thermal management challenges for 2.5D/3D semiconductor packaging. High-performance thermal management materials in electronic encapsulation are very important to ensure the performance and reliability of the electronic devices. Graphene-based polymer composites have attracted much attention due to the ultrahigh thermal conductivity and large surface area of graphene. However, graphene nanosheets easily aggregate and lack functional groups on their surfaces, leading to phonon scattering within the interfaces. In this work, silver nanoparticles are in-situ formed on the graphene surface by a facile method, then the graphene-silver nanofillers are modified by (3-Mercaptopropyl)trimethoxysilane (MPTS). MPTS reacts with silver nanoparticles to connect them with epoxy. Also, silver nanoparticles on the surface of graphene can fuse together to form metallurgical joints that connect graphene nanosheets.With the fundamental understanding of sintering mechanisms and reducing two types of thermal interfacial resistances (filler-filler and filler-epoxy interface resistances) simultaneously, the resultant epoxy nanocomposites achieve a high through-plane thermal conductivity of 0.99 W/mK at 8 wt% loading. This corresponds to a 465.7% increase in thermal conductivity as compared to that of neat epoxy. Also, the nanocomposites present a low CTE and high thermal stability. They show strong cooling capability and heat dissipation as thermal interface materials (TIMs) through both experimentation and simulation, providing a promising new insight into thermal management materials to meet the demands of next generation high-power and high-density semiconductor packaging.

Microstructure Characterization and Mechanical Property of the GH4065A Superalloy Inertia Friction Welded Joints

Structural characteristics and design requirements for the integration of the integral rotor and disc shaft of the engine, the welding quality, and mechanical properties of superalloy weldments have received more and more attention in recent years. Inertia friction welding (IFW) was carried out with the typical fiber structure of the solid solution GH4065A alloy as the research object, the microstructure evolution rules of the plastic deformation zone (PDMZ), the thermally affected zone (TMAZ), and the welding zone (WZ) were studied, and the formation mechanism of metallurgical joints was explored. The size difference of the γ′ phase at the grain boundary and in the fiber structure was revealed. The reason is that the γ′ phase located at the grain boundary has lower diffusion activation energy and higher diffusion rate. The microhardness and tensile properties of the IFW joints were explored, the study found that the microhardness of the TAMZ is the highest, followed by the PDMZ and the WZ. The tensile test results show that with the increase in temperature, the fracture position shifts from the BM to the WZ, the microstructure at the fracture changed significantly, and the yield strength decreased from 1372 to 1085 MPa.

Effect of powder size in microwave joining of oxygen-free copper to SS304

This paper investigates the prospect of microwave hybrid heating to join oxygen-free copper with stainless steel (SS304). The effect of size of copper (Cu) powder interlayer in the microstructure and mechanical properties of the joint formed is investigated, along with the incites on the mechanism of joint formation. Cu powder of average sizes 4 and 45 μm was studied. The microwave heating rate was observed to be high for 4 μm Cu in the absence of voids or porosities in the joint region. Voids were observed in the interlayer when 45 μm Cu powder was used. The joint was formed due to volumetric diffusion of the major elements of the substrates, e.g. Fe, Cu, Cr that was verified by energy dispersive spectroscopy. Oxides were present for 45 μm Cu which was absent for 4 μm Cu. X-ray diffraction and energy-dispersive X-ray spectroscopy analysis concluded that no intermetallic compound was formed along the interlayer. The micro-hardness of ∼128 HV was observed to be uniform across the interlayer depicting a good metallurgical joint. The maximum tensile shear strength of 83.12 MPa was recorded for 4 μm copper powder and microwave time of 14 min, exhibiting ductile failure of the joint.

Fundamental study of multi-track friction surfacing deposits for dissimilar aluminum alloys with application to additive manufacturing

Friction surfacing is an emerging solid-state coating technology based on frictional heat induced plastic deformation at the tip of a consumable metallic stud that allows to deposit layers with a fine-grained recrystallized microstructure at temperatures below the melting point. The generation of sound, defect-free metallurgical joints between multiple adjacent overlapping friction surfacing deposits, also referred to as multi-track friction surfacing, from dissimilar aluminum alloys is the focus of this experimental work. An extensive volumetric defect analysis is carried out for various overlap configurations, including post-processing strategies in order to assess the inter-track bonding integrity using microscopic characterization techniques and micro-computed tomography. The effect of layer arrangement and overlap distance on the volumetric defect formation in both inter-track and layer-to-substrate interface is quantified and discussed. Post-processing via hybrid friction diffusion bonding process demonstrates a significant reduction in defect volume ratio, proving higher material efficiency. The gained knowledge was used to successfully build a multi-track multi-layer friction surfacing stack, demonstrating the suitability of this process for large-scale additive manufacturing components. The subsequent mechanical analysis reveals excellent homogeneous isotropic tensile properties of the additive structure in the range of the base material tensile strength.

ПРЕДПОСЫЛКИ ФОРМИРОВАНИЯ СОВРЕМЕННОЙ СИСТЕМЫ ЭКОНОМИЧЕСКОГО ВЗАИМОДЕЙСТВИЯ РУМЫНИИ И РОССИИ

More than three decades have passed since the end of the confrontation and the Cold War between the two superpowers. Romanian-Russian political relations remain tense. This factor actively influences on the development of bilateral economic relations, investment dynamics, infrastructure, energy, logistics, agro-industrial, metallurgical joint projects. In many economic initiatives of the Romanian side there is lack of consistency and ambivalence, failure to hold the positions taken, despite external factors of influence from outside, change of political party elites in power. The author of the article tried to analyze the premises for the formation and development of Romanian-Russian economic relations at the present stage.

Optimization of FSW process parameters for welding dissimilar 6061 and 7075 Al alloys using Taguchi design approach

Friction stir welding (FSW) produce a strong metallurgical joint with the application of severe deformations and frictional heating in the metals and alloys by using a non-consumable rotating tool consisting of pin and tool shoulder. During welding, the plastic deformations of base metals vary for the varying mechanical properties such as tensile strength (TS), impact strength (IS), and hardness (HV) which in turn varies the welding conditions and parameters. Therefore, selection of optimal weld parameters plays an important role in enhancing the quality of weld joint. In this article, friction stir butt welds made of 6061 and 7075 Al alloys are performed with various welding parameters such as rotational speed of tool, angle of tilt, and axial force using tool has taper pin profile. Experiments are carried out on twenty-seven joints that are made on 6061 and 7075 Al Alloy plates of 6.50 mm thick of same nature and tested for its tensile, impact and HV properties.

The study of hot deformation on laser cladding remanufactured 316L stainless steel

Laser cladding deposition (LCD) is widely used to remanufacture/repair workpieces because of its high design freedom to rebuild areas of damage. However, the process often introduces a columnar grain structure in the cladding layer, resulting in a large variation of microstructure and hardness across the cladding layer, welding interface, and base metal. Under fatigue and tensile loading, fractures can initiate in the lower hardness cladding layer. This study explores the feasibility of a new hybrid remanufacturing method integrating the LCD with a subsequent hot deformation process to refine grain structures, reduce hardness variations, and enhance mechanical properties. The effects of deformation temperatures and imposed plastic strains were studied by examining the microstructural and stress–strain behaviour of laser cladded 316L stainless steel. After LCD, compressive deformation was imposed at temperatures of 900 and 1100 °C, with engineering strain levels of 0.1 and 0.5. A high-quality metallurgical joint was achieved, with the optimal ultimate tensile strength and yield strength under process conditions of an engineering strain level of 0.5 imposed at 900 °C (35% improvement compared to the directly laser cladding remanufacturing process). Dynamic recrystallization process was observed by the electron back scatter diffraction technique to reveal the underlying mechanism.

REW Application Possibilities for the Production of Combined Metal - Plastic Joints

In this paper, innovative resistance element welding (REW) technology for joining galvanized steel sheets to thermoplastics (PMMA) is introduced. The essence of the innovation is in the use of a special bimetallic joining element consisting of the core made of a Sn60Pb solder, and the sleeve made of a Cu tube. During resistance heating, the solder melts, thus allowing the formation of a metallurgical joint with galvanized steel sheet. Since Sn60Pb solder melting occurs at temperatures (from 183 to 190 °C) be-low the thermal decomposition temperature of most thermoplastics (for PMMA above 300 °C), there is no thermal destruction of the PMMA material around the joint. The mechanical fixation of the thermo-plastic material at the overlap joint is provided by the sleeve made of Cu tube which has a substantially higher strength than a Sn60Pb solder.

Interfacial Reactions between AlSi10 Foam Core and AISI 316L Steel Sheets Manufactured by In-Situ Bonding Process

Aluminum foam sandwiches (AFS) with AlSi10 foam cores and AISI 316L steel skins are manufactured by an in-situ bonding process. The precursor of the core foam was made with the powder compacted method. The precursor and skins, coupled together, were then heated up to the melting point of the Al alloy. The gas released by the blowing agent formed hydrogen bubbles in the melt. producing the foam. Such a porous structure was kept frozen at room temperature via cooling in cold water. To optimize the process conditions, some foaming experiments have been conducted with different holding times and temperatures. Such manufactured AFS were cut, chemically etched and studied with an optical microscope associated with image analysis software to get information about pores morphology in terms of circularity and equivalent diameter. The interface AlSi10-AISI316L has been characterized by SEM and EDX to investigate the bonding conditions between cores and skins. Finally, the AFS have been polished and etched to analyze the microstructure. Quasi-static compressive tests have been performed on the AFS. Obtained results showed that the interface formed during the foaming can be characterized by the inter-diffusion of alloying elements, as confirmed by the good quality of metallurgical joints.

Influence of laser pulse on solidification of molten pool during laser welding of dissimilar Ti-based amorphous alloys

Dissimilar Ti-based amorphous alloy sheets were butt welded by a pulsed laser and metallurgical joints were obtained. The β-Ti crystal grains in nanometre scale unevenly distribute on the amorphous background in the fusion zone. With the increase of laser pulse energy and pulse duration, the crystalline β-Ti grains grow up from spherical to dendritical profile, but the crystalline size reaches its maximum of about 1 µm, which is much smaller than the grains in base metal. The refinement of the grains in fusion zone results from the fast cooling speed during welding. The microhardness in the welded joints ranges from 470 to 525 HV. Average tensile strength of the welded joint is about 1510 MPa (86% of pure Ti-based amorphous alloy BM and 91% of β-Ti dendrite reinforced Ti-based amorphous alloy BM).