Building metallurgical bonding interfaces in an immiscible Mo/Cu system by irradiation damage alloying (IDA)

For the immiscible Mo/Cu system with a positive heat of mixing （△Hm 〉 0）, building metallurgical bonding interfaces directly between immiscible Mo and Cu and preparing Mo/Cu laminar metal matrix composites （LMMCs） are very difficult. To solve the problem, a new alloying method for immiscible systems, which is named as irradiation damage alloying （IDA）, is presented in this paper. The IDA primarily consists of three steps. Firstly, Mo is damaged by irradiation with multi-energy （186, 62 keV） Cu ion beams at a dose of 2× 1017 ions/cm2. Secondly, Cu layers are superimposed on the surfaces of the irradiation-damaged Mo to obtain Mo]Cu laminated specimens. Thirdly, the irradiation damage induces the diffusion alloying between Mo and Cu when the laminated specimens are annealed at 950 ℃ in a protective atmosphere. Through IDA, Mo/Cu LMMCs are prepared in this paper. The tensile tests carried out for the Mo/Cu LMMCs specimens show that the Mo/Cu interfaces constructed via IDA have high normal and shear strengths. Additionally, the microstructure of the Mo/Cu interface is characterized by High Resolution Transmission Electron Microscopy （HRTEM）, X-ray diffraction （XRD） and Energy Dispersive X-ray （EDX） attached in HRTEM. The microscopic characterization results show that the expectant diffusion between Mo and Cu occurs through the irradiation damage during the process of IDA. Thus a Mo/Cu metallurgical bonding interface successfully forms. Moreover, the microscopic test results show that the Mo/Cu metallurgical interface is mainly constituted of crystalline phases with twisted and tangled lattices, and amorphous phase is not observed. Finally, based on the positron annihilation spectroscopy （PAS） and HRTEM results, the diffusion mechanism of IDA is discussed and determined to be vacancy assisted diffusion.

Recent Progress in Metallurgical Bonding Mechanisms at the Liquid/Solid Interface of Dissimilar Metals Investigated via in situ X-ray Imaging Technologies

The liquid/solid(L/S)interface of dissimilar metals is critical to the microstructure,mechanical strength,and structural integrity of interconnects in many important applications such as electronics,automotive,aeronautics,and astronautics,and therefore has drawn increasing research interests.To design preferential microstructure and optimize mechanical properties of the interconnects,it is crucial to understand the formation and growth mechanisms of diversified structures at the L/S interface during interconnecting.In situ synchrotron radiation or tube-generated X-ray radiography and tomography technologies make it possible to observe the evolution of the L/S interface directly and therefore have greatly propelled the research in this field.Here,we review the recent progress in understanding the L/S interface behaviors using advanced in situ X-ray imaging techniques with a particular focus on the following two issues:(1)interface behaviors in the solder joints for microelectronic packaging including the intermetallic compounds(IMCs)during refl ow,Sn dendrites,and IMCs during solidification and refl ow porosities and(2)growth characteristics and morphological transition of IMCs in the interconnect of dissimilar metals at high temperature.Furthermore,the main achievements and future research perspectives in terms of metallurgical bonding mechanisms under complex conditions with improved X-ray sources and detectors are remarked and discussed.

Interfacial Morphology and Bonding Mechanism of Explosive Weld Joints

Interfacial structure greatly affects the mechanical properties of laminated plates.However,the critical material properties that impact the interfacial morphology,appearance,and associated bonding mechanism of explosive welded plates are still unknown.In this paper,the same base plate(AZ31B alloy)and different flyer metals(aluminum alloy,copper,and stainless steel)were used to investigate interfacial morphology and structure.SEM and TEM results showed that typical sine wave,wave-like,and half-wave-like interfaces were found at the bonding interfaces of Al/Mg,Cu/Mg and SS/Mg clad plates,respectively.The different interfacial morphologies were mainly due to the differences in hardness and yield strength between the flyer and base metals.The results of the microstructural distribution at the bonding interface indicated metallurgical bonding,instead of the commonly believed solid-state bonding,in the explosive welded clad plate.In addition,the shear strength of the bonding interface of the explosive welded Al/Mg,Cu/Mg and SS/Mg clad plates can reach up to 201.2 MPa,147.8 MPa,and 128.4 MPa,respectively.The proposed research provides the design basis for laminated composite metal plates fabrication by explosive welding technology.

Interface behavior and microstructure transformation of welded joint of 35CrMo steel by electro-spark deposition

Electro-spark deposition has been carried out on the 35CrMo steel by filling and restoring the simulated slot specimens, and continuous surface coating with extra low porosity was obtained with proper parameters. The microstructure transformation and interface behavior of deposited joint were observed and analyzed. It shows that a transition region with 15μm in depth was obtained between base metal and deposited metal, and metallurgical bonding was achieved between restored base metal and deposition coating.

Diffusion Behaviour of Fe-Cu Interface of Copper Brazed Double-wall Steel Tubes

By means of electron probe(EPMA),scanning electron microscope(SEM),and optical microscope (QM),the diffusion behaviour on the Fe-Cu interface of copper brazed double-wall steel tubes and the microstructure of the diffusion layer have been investigated.There are three kinds of metallurgical bonds between copper plating layer and steel substrate: (1)the Cu diffusing into steel substrate along grain boundary of ferrite;(2)the Cu diffusion into grain bulk of ferrite: (3)the Fe diffusing into Cu layer.The copper brazed double-wall steel tubes are formed by the combination of the diffusions mentioned above and this is the reason for excellent mechanical and technological properties of the copper brazed double-wall steel tubes.

Mechanical Properties and Microstructure Evolution of AA1100 Aluminum Sheet Processed by Accumulative Press Bonding Process

Accumulative press bonding（APB） is a novel variant of severe plastic deformation processes,which is devised to produce materials with ultra-fine grain.In the present work,the mechanical properties and microstructural evolution of AA1100 alloy,which is produced by APB technique,were investigated.The study of the microstructure of AA1100 alloy was performed by optical microscopy.The results revealed that the grain size of the samples decreased to 950 nm after six passes of APB process.The yield strength of AA1100 alloy after six passes of the process increased up to 264 MPa,which is three times higher than that of the as-cast material（89 MPa）.After six passes,microhardness values of AA1100 alloy increased from 38 to 61 HV.Furthermore,the results showed that the behavior of variations in mechanical properties is in accordance with the microstructural changes and it can be justified by using the Hall-Patch equation.Moreover,the rise in the yield strength can be attributed to the reduction in the grain size leading to the strain hardening.

Microstructural formation and mechanical performance of friction stir double-riveting welded Al-Cu joints

A novel friction stir double-riveting welding(FSDRW) technology was proposed in order to realize the high-quality joining of upper aluminum(Al) and lower copper(Cu) plates,and this technology employed a Cu column as a rivet and a specially designed welding tool with a large concave-angle shoulder. The formations, interfacial characteristics, mechanical properties and fracture features of Al/Cu FSDRW joints under different rotational velocities and dwell times were investigated. The results showed that the well-formed FSDRW joint was successfully obtained.The cylindrical Cu column was transformed into a double riveting heads structure with a Cu anchor at the top and an Al anchor at the bottom, thereby providing an excellent mechanical interlocking.The defect-free Cu/Cu interface was formed at the lap interface due to the sufficient metallurgical bonding between the Cu column and the Cu plate, thereby effectively inhibiting the propagation of crack from the intermetallic compound layer at the lap interface between the Al and Cu plates. The tensile shear load of joint was increased first and then decreased when the rotational velocity and dwell time of welding tool increased, and the maximum value was 5.52 k N. The FSDRW joint presented a mixed mode of ductile and brittle fractures.

Direct alloying of immiscible molybdenum-silver system and its thermodynamic mechanism

Direct alloying is difficult to be realized in an immiscible Mo-Ag system with a positive formation heat due to the absence of thermodynamic driving force at equilibrium.In this work,a direct alloying method is developed to realize the direct alloying between Mo and Ag and construct Mo-Ag interface.The direct alloying method was mainly carried out through a direct diffusion bonding for Mo and Ag rods at a temperature close to the melting point of Ag(Tm Ag).Then the microstructure and phase constitution of the as-constructed Mo-Ag interface are characterized.The results show that Mo-Ag metallurgical bonding interface has been constructed successfully,indicating that a direct alloying in the immiscible Mo-Ag system has been realized.Additionally,mechanical tests are carried out for the Mo-Ag joints prepared through the direct alloying method.The test results show that the average maximum tensile strength of the joints is about 107 MPa.The effect of alloying parameters on the tensile strength is also discussed,which shows that there is an effective temperature range for the direct alloying between Mo and Ag.Lastly,an improved thermodynamic model that considers the formation of Mo-Ag crystalline and amorphous phase is presented to reveal the thermodynamic mechanism of the direct alloying.Combining the calculation and differential scanning calorimetry(DSC)tests results,the Gibbs energy diagram for the direct alloying is obtained.It is confirmed that the co-release of storage energy and surface energy can serve as the thermodynamic driving force to overcome the effect of positive formation heat and lead to direct alloying for Mo-Ag systems.

Nature of CoCrFeMnNi/Fe and CoCrFeMnNi/Al Solid/Solid Interface

To shed light into the application potential of high-entropy alloys as"interlayer"materials for Al-steel solid-state joining,we investigated the nature of the CoCrFeMnNi/Fe and CoCrFeMnNi/Al solid/solid interfaces,focusing on the bonding behavior and phase components.Good metallurgical bonding without the formation of hard and brittle IMC can be achieved for CoCrFeMnNi/Fe solid/solid interface.In contrast to the formation of Al5 Fe2 phase at the Fe/Al interface,Al13Fe4-type IMC,in which the Fe site is co-occupied equally by Co,Cr,Fe,Mn and Ni,dominates the CoCrFeMnNi/Al interface.Although the formation of IMC at the CoCrFeMnNi/Al interface is not avoidable,the thickness and hardness of the Al13(CoCrFeMnNi)4 phase formed at the CoCrFeMnNi/Al interface are significantly lower than the Al5 Fe2 phase formed at the Fe/Al interface.The activation energies for the interdiffusion of Fe/Al and CoCrFeMnNi/Al static diffusion couple are 341.6 kJ/mol and 329.5 kJ/mol,respectively.Despite this similarity,under identical static annealing condition,the interdiffusion coe fficient of the CoCrFeMnNi/Al diffusion couple is significantly lower than that of the Fe/Al diffusion couple.This is thus mainly a result of the reduced atomic mobility/diffusivity caused by the compositional complexity in CoCrFeMnNi high-entropy alloy.