Cold Welding Process Research Articles

Atomistic insights into cold welding process of high-entropy alloy nanowires

This research applied molecular dynamics (MD) simulations to investigate the influences of contact pressure, temperature, crystal orientation, and chemical composition on the characteristics of the cold welding joints of CoNiCrFeMn high-entropy alloy (HEA) nanowires. The findings indicate that defect-free cold welded joints can be successfully formed within a contact pressure range of 10 to 10,000 Pa. In addition, changing the contact orientation could cause a significant change in the structure transformation level. The dislocation distribution is continuous when the top and lower blocks are oriented in the same direction. Dislocation lines, on the other hand, stop at the interfacial area and have a discontinuous shape when the weld joints are formed from different orientations. Furthermore, creating from the same orientations leads to a higher ultimate tensile strength (UTS) value than the different orientation cases. In the temperature range of 150 K–350 K, the deformation behaviors of the cold weld joint are stable. Besides, there was no noticeable change in the weld joint when the composition changed. Because of the strong crystal structure of the cold welding joints, the UTS values of the cold-welded joints range from 13.2 to 17.4 GPa, comparable to the bulk samples.

Cold-welded joint characteristics of gold nanowires via atomistic simulation

This study examines the effects of contact depth, contact orientation, contact angle, and nanowires temperature on the tensile strength of the gold nanowire generated via the cold welding process. There is an increase in the deformation rate with increasing contact depth. From 5 Å to 20 Å, the lattice matching of the nanowires is good, leading to a good weld joint. The greatest ultimate tensile strength (UTS) value is achieved at 5 Å contact depth compared to other depth cases. The UTS value is sensitive to the change in the contact orientation and the contact angle. Compared to other orientations, the (001) vs (001) orientation has the highest rate of orientation matching with the highest FCC structure ratio. Other orientations obtain a much lower UTS value. When the contact angle rises, the UTS value mainly decreases due to the higher level of lattice mismatch. Compared to the other angle cases, the maximum UTS value is attained at 0º contact angle. Moreover, changing in nanowires temperature within the 250 K450 K range do not result in significant variations in the sample's surface morphology, structural transformation, or atomic strain. At 250 K, the sample has the highest UTS value compared to other temperature cases.

A comparative study on weld-bead geometry of austenitic stainless-steel produced by cold metal transfer welding and pulse metal inert gas welding process

In recent years, weld cladding has become an emerging technique for the surface modification of carbon steel, which has various industrial applications such as chemical, marine, mining, agriculture, and power generation. In this research, cold metal transfer (CMT) welding and pulse metal inert gas (MIG) welding are utilized to study the weld-clad bead of 308L stainless steel over low-carbon steel. To examine the mechanical and wear characteristics of CMT weld-clad samples, welding speed (3, 4, 5, and 6 mm/s) was used as the input process parameter, while current (175 A), nozzle to workpiece distance (10 mm), and shielding gas flow rate (15 l/min) were kept constant. Microstructural analysis of weld bead samples was done through optical microscopy and field emission scanning electron microscopy. With an increase in welding speed, the microhardness and wear properties of the clad surface were improved. The CMT-clad surface had significant improvements in mechanical, wear properties, and lower residual stresses due to low dilution and lesser heat input.

OPTIMIZATION OF A METHOD FOR HIGH-ENERGY GRINDING OF POWDER MIXTURES OF THE Ti-Al-Nb SYSTEM IN THE PRESENCE OF STEARIC ACID AND TOLUENE

In this work, the influence of process control agents on the morphology and structural-phase state of powder mixtures of the Ti-Al-Nb system was studied. To improve the technique for grinding powder mixtures of the Ti-Al-Nb system in a planetary ball mill, two types of powders were prepared with the addition of process control agents based on stearic acid and liquid toluene. The process of grinding the compositions with the addition of agents was carried out at 550 rpm/m for 30, 60, 120, 180 minutes.During the studies, it was found that the evolution of particle morphologies using stearic acid and liquid toluene was similar. The effect of the agent as a process regulator of stearic acid on the evolution of powder morphology was greater than that of liquid toluene, since it inhibited the cold welding process of powders more effectively. During high-energy grinding using toluene for 120 and 180 minutes, undesirable carbide phases are formed.

Mechanical Properties of Cold‐Welded CoCrFeCuNi Nanowires with Side‐by‐Side Contact: A Molecular Dynamics Study

Cold welding at the nanoscale is a promising technique for bottom‐up fabrication and assembly of nanostructured materials. Herein, the cold welding process of the CoCrFeCuNi high‐entropy‐alloy (HEA) nanowires in the form of side‐by‐side contact using molecular dynamics simulation is inestigated. The effects of overlap length, crystal orientation, and temperature are taken into consideration. The results demonstrate that strength is positively correlated with the overlap length. Fracture strain first increases up to a maximum and then decreases with the increase in overlap length. When the temperature increases from 300 to 900 K, the ultimate stress of the welded nanowires decreases from 1.18 to 0.87 GPa, and the welding stress decreases from −0.54 to −0.26 GPa. The crystal orientation significantly influences the deformation mechanism. For samples welded by nanowires with the same crystal orientation, the primary deformation mechanisms are twinning and dislocation slip. However, for samples welded by nanowires with different crystal orientations, the deformation is primarily mediated by the grain boundary slip. The research can enhance the understanding of the cold welding behavior for low‐dimensional materials and is hopeful to provide some valuable guidance for the bottom‐up fabrication and assembly of HEA nanocomponents.

Cold welding of ultrathin metallic glass nanowires with side-to-side contact using molecular dynamics simulations

Cold welding can effectively join a wide variety of components, forming high quality assemblies. In this work, the cold welding behaviors of Cu-Zr metallic glass nanowires (MGNWs) in the form of side-to-side contact are investigated by molecular dynamics simulation. The mechanical performance of the welded MGNW is inspected by uniaxial tensile tests. Cold welding processes of MGNWs with various sizes and under different welding conditions are explored. Our results show that the strength of the welded MGNW at 50 K is less than 60% of that of the crude nanowire. At the same welding velocity, the change of welding time within a certain range does not influence the welding quality obviously. The increase in overlap width makes the welded MGNW stronger. The enhancement of welding temperature facilitates the atom diffusion activity, and thus strengthens the contact regions, improving the welding quality. We also compare the side-to-side contact and head-to-head contact cold welding. It is found that the former endows the welded MGNW with lower strength but better plasticity. This work provides a preliminary theoretical guideline on the cold welding of MGNWs for further experiments and to facilitate the engineering practice of cold welding.

The characterization of cold welding process in CuZr metallic glasses with dissimilar alloying compositions

This work aimed to investigate the cold welding process of dissimilar Cu60Zr40/Cu50Zr50 metallic glass (MG) system through the molecular dynamics (MD) simulation. The mechanical properties and plasticity behavior of the joint zone were also studied with the consideration of stress variations and shear atomic strains under the nanoindentation. The results indicated that the joining evolution was accompanied by the accumulation of shear events in the joint zone. However, one side of the joint was more affected by the atomic rearrangements, which were due to the dissimilar mechanical features of Cu60Zr40 and Cu50Zr50 alloys. The nanoindentation results also showed that the joint zone was considerably softer than the base alloys; however, no sudden shear propagation appeared in this part of the assembly. The strain maps of shear atomic strain under the nanoindentation indicated that the atomic rearrangement and shear band interactions were unsymmetrical in the joint zone so that the Cu50Zr50 side tended to form finer shear bands with more interactive features. Finally, it was concluded that a sound metallurgical bond was attained in the Cu60Zr40/Cu50Zr50 system; however, the good plasticity may come at the expense of strength in the joint zone.

Design and fabrication of bimetallic plasmonic colloids through cold nanowelding.

The integration of Au and Ag into nanoalloys has emerged as an intriguing strategy to further tailor and boost the plasmonic properties of optical substrates. Conventional approaches for fabricating these materials via chemical reductions of metal salts in solution suffer from some limitations, such as the possibility of retaining the original morphology of the monometallic substrate. Spontaneous nanowelding at room temperature has emerged as an alternative route to tailor Au/Ag nanomaterials. Herein, we perform a thorough study on the cold-welding process of silver nanoparticles onto gold substrates to gain a better understanding of the role of different variables in enabling the formation of well-defined bimetallic structures that retain the original gold substrate morphology. To this end, we systematically varied the size of silver nanoparticles, dimensions and geometries of gold substrates, solvent polarity and structural nature of the polymeric coating. A wide range of optical and microscopy techniques have been used to provide a complementary and detailed description of the nanowelding process. We believe this extensive study will provide valuable insights into the optimal design and engineering of bimetallic plasmonic Ag/Au structures for application in nanodevices.

Influence of process control agent and Al concentration on synthesis and phase stability of a mechanically alloyed AlxCoCrFeMnNi high-entropy alloy

This paper reports the effects of different process control agents (PCA) (methanol, n-heptane, and stearic acid) and aluminum concentration (x = 0.5, 1.0, and 1.5 at%) on the particle size distribution, structure, microstructure, stability, and prediction of phases in AlxCoCrFeMnNi high-entropy powder synthesized by mechanical alloying. The results showed that the vapor pressure, the polar nature of the different process control agents used, and type of milling (dry/wet) generating changes in the particle size distribution, microstructure, and crystalline structure since the use of PCA modifies the balance between the cold-welding and fracture processes. Quantitative analysis showed that the PCA chemical decomposition generated residual oxygen and carbon in all systems. X-ray diffraction showed that all systems present a mixture of crystal structures BCC-FCC with nanometric crystallite size, and the peak intensities changed in function of lattice deformation generated by type milling employed. On the other hand, the results also showed that the boundary between BCC-FCC and FCC regions is even broader and extends to higher Al concentration than those reported by the valence electron concentration criterion.

Dispersion and Damage of Carbon Nanotubes in Carbon Nanotube/7055Al Composites During High-Energy Ball Milling Process

High-energy ball milling (HEBM) combined with powder metallurgy route was used to fabricate carbon nanotube (CNT) reinforced 7055Al composites. Two powder morphology evolution processes (HEBM-1 and HEBM-2) were designed to investigate the dispersion and damage of CNTs during HEBM process. HEBM-1 evolution process involved powder flattening, cold-welding and fracture, while HEBM-2 evolution process consisted of powder flattening and fracture. For HEBM-1, the repetitive fracture and cold-welding process was effective for dispersing CNTs. However, the powder flattening process in HEBM-2 was unsuccessful in dispersing CNTs due to two reasons: (1) the thickness of flaky Al powders exceeded the critical value, and (2) the clustered CNTs embedded in flaky Al powders could not be unravelled. Because of the broadening of D band and the appearance of a new defect-related D’ band, product of ID/IG and full width half maximum of D band, rather than ID/IG, was used to evaluate the actual damage of CNTs. It indicates that the damage of CNTs was severe in powder flattening and fracture stages, while the damage of CNTs was small in powder cold-welding stage.

Molecular dynamics study of tensile behaviour for cold and linear friction welded single crystal tungsten

In the present paper, single crystal tungsten model has been established by using molecular dynamics simulation. The joining of two single-crystal tungsten has been shown using two different welding techniques namely Cold welding and Linear Friction Welding (LFW) at room temperature. The tungsten atom interactions have been developed using the Embedded Atom Method potential. The Radial Distribution Function (RDF) simulation has been performed for two different layer thickness around the welded region for structural analysis. It has been found that as the layer thickness increases the probability of getting perfect structure atoms in long range order decreases. It has also been observed that few dislocations are generated during welding. The tensile properties have also been investigated for the sample joined by two processes for 50 ps, 100 ps and 150 ps at a strain rate of 1.5 × 109 sec−1. The results show that the joint developed by cold welding process at 100 ps resulted in a yield strength of 3588 MPa and ultimate strength of 6678 MPa. During LFW yield strength (5152 MPa) and ultimate strength (7172 MPa) is achieved in less welding time i.e. 50 ps.

Study and evaluation of cold joining on metals and non metals combinations (ss, al, acr) using polymer based adhesive

The world innovated the different generation and different joining process for fabricating the metals using different methods. It reaches the ultimate level in the welding of similar and dissimilar metals, in different phases also. All joining processes, proofs its mechanical properties. But, the metal joining methods are in a condition to change to compete in market. So that now a day’s metal joining methods take a different move towards polymer based epoxy adhesive cold welding process. This epoxy is the convenient alternative for cold welding of metals, and nonmetals. The mechanism of bonding is mutually reactive chemical groups with coupling agent create chemical bonds and adhere the both materials. Based on this concept, this research work is carryout. In this research work combinations of different metals and nonmetals such as Stainless Steel (SS), Aluminium (Al), and Acrylic (ACR) are joined with polymer based epoxy resin, in the background of same metals like SS + SS, AL + Al. ACR + ACR. These combinations of cold joining process were carryout with two conditions, finished condition and roughed condition using A394emery sheet. The adhesive thickness maintained using feeler gauge size of 0.5 mm. This fabrication process were carried out using specially designed and fabricated fixture to avoid misalignments while fabrication. To fine the character and behavior of adhesive welding flexural test were conducted onUTM. The result shows that, on finished condition samples and prove adhesive mechanism character in flexural test by increasing order such as ACR + ACR increases 9.63 times, Al + Al increases 4.82, SS + SS increases 1.54 times.

Numerical simulation of periodic cracking mechanism in microscopic surface films during roll bonding processes

AbstractThe microscopic surface films existing on the top of metallic layers play an important role in the process of joining by plastic deformation. The bond formation during cold welding processes is basically associated with the fracturing of surface films to produce intimate metallic contacts. The present paper aims at providing a numerical model to describe the cracking pattern of brittle surface films bonded to the ductile substrates. A microscale finite element model is developed which takes into account the fracturing mechanisms of thin surface films in roll bonding processes. The presented model is calibrated by using the existing experimental data for an aluminum alloy covered by a thin layer of oxide film. The model is also validated against a well‐known analytical model for periodic cracking. The distribution of stresses within the fractured surface film demonstrates that the generated cracks in the surface film have essentially a periodic pattern. Moreover, it is shown that the crack spacing is highly dependent on the properties of the surface film. Finally, the obtained results for the roll bonding show that a crack density saturation takes place at the entry of the roll bite where a small surface expansion is applied to the rolled samples.

Effect of milling time and presence of Sn on the microstructure and porosity of sintered Ti-10Ta-8Mo and Ti-10Ta-8Mo-3Sn alloys

The aim of the present study was to assess the influence of the milling time (10 h, 15 h and 20 h) on the structure and properties of Ti-10Ta-8Mo (wt.%) and Ti-10Ta-8Mo-3Sn (wt.%) alloys for potential biomedical application. In addition, the influence of the addition of 3 wt% Sn on the structure and properties of the milled and sintered materials was investigated. X-ray diffraction confirmed the partial synthesis and formation of the β phase during the milling process. The transmission electron microscope analysis of the bright and dark field images and the diffraction image proved the nanocrystalline nature of the material after being subjected to 20h milling duration. The scanning electron microscope observation of the sintered samples confirmed the influence of the milling time and presence of Sn on the microstructure of the obtained material. Furthermore, a small addition of Sn led to better size homogenization by establishing a better balance between the cold-welding and agglomeration processes. At the same time yielding different structures of pores for the sintered samples. The dry sliding tests revealed the excellent friction properties and wear rate of the samples previously milled for 15 h, with the addition of Sn.

Finite Element Modeling of Bond Formation in Cold Roll Bonding Processes

The paper aims to present a finite element model for the bond strength evolution in cold roll bonding processes. To accomplish this, first, the micro-mechanisms taking place along the cold welded joint interfaces are explained. Then, based on the microscopic description of cold welding processes, a bonding interface model is employed to describe the bond formation between the rolled metallic layers. The obtained bond strength is calculated based on the governing parameters of the bonding such as the degree of plastic deformation and the surface cleanness. The numerical simulation given in this paper includes the modelling of joining during cold roll bonding followed by the debonding process in Double Cantilever Beam (DCB) peeling test. Finally, the effects of two important factors on the bond formation, i.e. (1) the degree of plastic deformation and (2) the surface cleanness, are numerically investigated.

Preparation and cold welding of silver nanowire based transparent electrodes with optical transmittances >90% and sheet resistances

In this article, silver nanowires (AgNWs) with aspect ratios of 1000 and lengths up to 200 μm are obtained by a modified polyol approach. These very long AgNWs are then utilized to prepare transparent electrodes (TEs) displaying a transmittance of 91.3% at a sheet resistance of 8.6 ohm/sq without any post-treatment. Furthermore, we also demonstrate a process for the cold welding of Ag NWs by simply dipping the AgNWs films into CTAB solutions, resulting in a further improvement for the optoelectronic performance. After the post-treatment, the AgNW-based TEs can achieve a transmittance of 93% at a sheet resistance of 9.5 ohm/sq. In addition, the electric behaviors of AgNW-based TEs are investigated. In the bulk-like regime, for the as-prepared AgNW-based TEs, the Figure of merit (FOM), DC to optical conductivity ratio reaches up to 566.8. After the cold welding process, the DC to optical conductivity ratio can reach even higher values (631.6). In the percolative regime, the as-prepared and welded AgNW-based TEs can achieve Π (FOM with percolative-like behavior) values of 166.8 and 242.1, respectively.

Artificial neural network-based prediction technique for coating thickness in Fe-Al coatings fabricated by mechanical milling

ABSTRACTThe objective of this study was to evaluate the effect of milling time, milling speed and particle size of initial powders on the coating thickness of Fe-Al intermetallic coating by using artificial neural network (ANN). Coating morphology and cross-section microstructures were evaluated using a scanning electron microscope (SEM). It was found that an increase in the milling time provided an increase in the coating-layer thickness due to the cold welding process between particles and the steel substrate. The microstructure of the coating surface was refined by ball impacts in the milling process. As a result of this study, the ANN was found to be successful for predicting the coating thickness of Fe-Al intermetallic coatings. The correlation between the predicted values and the experimental data of the feed-forward back-propagation ANN was quite adequate. The mean absolute percentage error (MAPE) for the predicted values didn’t exceed 7.46%. The ANN model can be used for predicting the coating thickness of Fe-Al intermetallic coating produced for different milling time, milling speed and particle size.

Experimental and Simulation Studies on Cold Welding Sealing Process of Heat Pipes

Sealing quality strongly affects heat pipe performance, but few studies focus on the process of heat pipe sealing. Cold welding sealing technology based on a stamping process is applied for heat pipe sealing. The bonding mechanism of the cold welding sealing process (CWSP) is investigated and compared with the experimental results obtained from the bonding interface analysis. An orthogonal experiment is conducted to observe the effects of various parameters, including the sealing gap, sealing length, sealing diameter, and sealing velocity on bonding strength. A method with the utilization of saturated vapor pressure inside a copper tube is proposed to evaluate bonding strength. A corresponding finite element model is developed to investigate the effects of sealing gap and sealing velocity on plastic deformation during the cold welding process. Effects of various parameters on the bonding strength are determined and it is found that the sealing gap is the most critical factor and that the sealing velocity contributes the least effect. The best parameter combination (A 1 B 3 C 1 D 3, with a 0.5 mm sealing gap, 6 mm sealing length, 3.8 mm sealing diameter, and 50 mm/s sealing velocity) is derived within the experimental parameters. Plastic deformation results derived from the finite element model are consistent with those from the experiment. The instruction for the CWSP of heat pipes and the design of sealing dies of heat pipes are provided.

Experimental Investigation and Numerical Modeling of the Bond Shear Strength of Multi-layered 6000 Series Aluminum Alloys

The use of high-strength aluminum alloys is one key factor for the realization of lightweight design in industrial applications. Besides alloying, it is possible to produce high-strength sheet materials using the Accumulative Roll Bonding (ARB) process. Based on a repetitive stacking and roll bonding process, multi-layered sheets with a nanocrystalline grain structure are produced. The bond formation is created by a cold welding process due to the high pressure in the rolling gap and cracks in the oxide film of the aluminum sheet material caused by the thickness reduction. The experimental investigation of the bond strength is crucial for the assessment of the formability of the multi-layered sheet material. High bonding quality is necessary in order to prevent failure mechanisms like delamination in subsequent forming operations. Furthermore, a numerical model, which contains discrete layers over the sheet thickness as well as their bonding mechanisms will be implemented in finite element analysis. This enables a process layout for forming operations by the prediction of delamination effects. Within this investigation, an experimental test method is presented to characterize the bond shear strength of multi-layered sheet material. The use of an optical strain measurement system enables the identification of the material behavior over the sheet thickness during the shear test. For this investigation a 16-layered aluminum alloy AA6014 is used. However, the bond shear strength of the last bonded respectively middle layer of the sheet material is investigated. The 16-layered material has 15 bonds over the thickness but the one in the middle has the lowest bond strength as it was only roll bonded once compared to the others, which were rolled at least twice. The bonding mechanisms are modelled using a tiebreak contact formulation in the finite element software LS-DYNA. Concluding, the numerical model is validated by comparison with the experimental results.

Side-to-Side Cold Welding for Controllable Nanogap Formation from “Dumbbell” Ultrathin Gold Nanorods

Cold welding has been regarded as a promising bottom-up nanofabrication technique because of its ability to join metallic nanostructures at room temperature with low applied stress and without introducing damage. Usually, the cold welding process can be done instantaneously for ultrathin nanowires (diameter <10 nm) in "head-to-head" joining. Here, we demonstrate that "dumbbell" shaped ultrathin gold nanorods can be cold welded in the "side-to-side" mode in a highly controllable manner and can form an extremely small nanogap via a relatively slow welding process (up to tens of minutes, allowing various functional applications). By combining in situ high-resolution transmission electron microscopic analysis and molecular dynamic simulations, we further reveal the underlying mechanism for this "side-to-side" welding process as being dominated by atom kinetics instead of thermodynamics, which provides critical insights into three-dimensional nanosystem integration as well as the building of functional nanodevices.