Increase In Joint Strength Research Articles

Mechanical Properties of Friction Stir Welded Carbon Steel Joints — Friction Stir Welding with and without Transformation

Low temperature friction stir welding (including below A1 point) was successful for three types of carbon steels with different carbon contents (IF steel, S12C, S35C). For all the carbon steels, the strength of the FSW joints increased when compared to that of the normal structure (ferrite + pearlite) of base metal. Compared with IF steel, the microstructures and mechanical properties of the carbon steel joints are significantly affected by the welding conditions due to the phase transformations. For the S12C steel, the welding produces a ferrite-pearlite structure, and the strength slightly increases with the increasing welding speed (decreasing the heat input) due to the refined microstructure. For the S35C steel, under the conditions exceeding the lower critical cooling rate, martensite was formed, resulting in a significantly increased joint strength; while under the conditions when martensite is not formed, the strength of the joint increased with the increasing welding speed (decreasing the heat input) due to the refined microstructures. Thus, friction stir welding enables us to control both the maximum temperature and the cooling rate in order to produce higher strength joints.

High-performance nanoadhesive bonding of titanium for aerospace and space applications

In this investigation, attempts are made to prepare high-performance nanoadhesive bonding of titanium for its essential applications to aviation and space. The high-performance nanoadhesive is prepared by dispersing silicate nanoparticles into the ultra-high-temperature-resistant epoxy adhesive at 10 wt% ratio with the matrix adhesive followed by modification of the nanoadhesive after curing under high-energy radiation for 6 h in the pool of SLOWPOKE-2 nuclear reactor with a dose rate of 37 kGy/h to promote crosslink into the adhesive. Prior to bonding, the surfaces of the titanium sheets are mechanically polished by wire brushing, ultrasonically cleaned by acetone and thereafter the titanium sheets are modified by plasma ion implantation using plasma nitriding. The titanium surface is characterized by X-ray photoelectron spectroscopy (XPS). The thermal characteristics of the epoxy adhesive and the high-performance nanoadhesive are carried out by thermal gravimetric analysis (TGA). The TGA studies clearly shows that for the basic adhesive there is a weight loss of the adhesive, however, in the case of epoxy–silicate nanoadhesive, there is almost 100% retention of weight of the adhesive, when the adhesive is heated up to 350 °C. Lap shear tensile strength of the joint increases considerably, when the titanium surface is modified by plasma-nitriding implantation. There is a further massive increase in joint strength, when the plasma-nitriding implanted titanium joint is prepared by nanosilicate–epoxy adhesive and further modification of the adhesive joint under high-energy radiation results a further significant increase in joint strength. In order to simulate with aviation and space climatic conditions, the joints are separately exposed to cryogenic (−196 °C) and elevated temperature (+300 °C) for 100 h and thermal fatigue tests of the joints are carried out under 10 cycles by exposing the joint for 2 h under the above temperatures. When the joint completely kept at ambient condition and the joint strength compared with those joints exposed to aviation and space climatic conditions, it is observed that the joint could retain 95% of the joint strength. Finally, to understand the behavior of the high-performance silicate–epoxy nanoadhesive bonding of titanium, the fractured surfaces of the joints are examined by scanning electron microscope.

Influence of Mixed Field Radiation and Gamma Radiation on Nano Adhesive Bonding of HP Polymer

In this investigation, attempts are made to fabricate high performance polymer such as polybenzimidazole (PBI) (service temperature ranges from -260°C to +500°C) by nano silicate epoxy adhesive and to see its performance under space radiation. The polybenzimidazole sheets are fabricated by high performance nano adhesive i.e., by dispersing silicate nano powder into the ultra high temperature resistant epoxy adhesive (DURALCO 4703, the service temperature of the adhesive is -260°C to +350°C) with 2 to 20% weight ratios with the matrix adhesive. Prior to fabrication of polybenzimidazole sheet, the surface of the polybenzimidazole is ultrasonically cleaned by acetone followed by its modification through low-pressure plasma by using 13.56 MHz RF Glow Discharge with 30, 60, 120, 240 and 480 seconds at 100 W of power using nitrogen as process gas. It is observed that polar component of surface energy leading to total surface energy of the polymer increases with exposure time of low pressure plasma up to 120 seconds and then it saturates. Nano adhesive bonding of high performance polymer is exposed to two types of radiations (i) mixed field radiation for 24 hours at a dose rate of 37kGy/hr in the pool of a SLOWPOKE-2 (safe low power critical experiment) nuclear reactor and (ii) Co-60 irradiation with 100 % gamma radiation at a dose rate of 4 kGy/hr for 60 hours. Tensile lap shear strength reveals that when the polymer surface is modified by low pressure plasma, joint strength increases from 1 MPa to 13 MPa and increases further up to 23 MPa when the polymer is fabricated by nano silicate epoxy adhesive with increasing weight ratios of silicate nano powder up to 10% and then it deteriorates with the increasing weight ratio of silicate nano powder. When this nano silicate epoxy adhesive joint is exposed to high-energy radiation of mixed field, there is a further considerable increase in joint strength up to 30 MPa. However, when the nano silicate epoxy adhesive joint is exposed to 100 % gamma rays condition, joint strength deteriorates. Therefore, this is possible that mixed field radiation; basically increase the crosslink density of the adhesive resulting in increase in adhesive joint strength and in the second case: gamma radiation is detrimental and which could essentially makes chain scission to the basic adhesive and resulting in significant deterioration of joint strength. Finally, to understand the behaviour of nano silicate epoxy adhesive bonding of Polybenzimidazole, the fractured surfaces of the joints are examined by scanning electron microscope.

Adhesives for automotive windscreen replacement: Kinetics of cure and bond strength

Cure depth (z) has been measured for three water-hardening isocyanate adhesives, which are used in replacing automotive windscreens, at a range of temperatures. Swelling the cured materials in toluene showed that temperature does not affect the extent of crosslinking. Depth of cure conforms to the equation z=(2PpVt)1/2, where P is the water permeability coefficient of the cured adhesive, p the vapour pressure of water, V the volume of adhesive which reacts with 1 mol of water, and t is time. The actual vapour pressure of water is the dominant factor in causing the rate of cure to vary with temperature. There was a tendency for joint strength to increase with cure time; however, this was subject to much scatter and generally the increase in joint strength was less than would be expected from consideration of the increase in cured adhesive area. This was attributed to the complex failure paths observed, with combinations of failure in the adhesive, in the metal primer and at the aluminium–metal primer interface. The lowest joint strengths were associated with high proportions of failure at the aluminium–metal primer interface. Joint strength prediction was reasonable for those joints failing predominantly in the adhesive, but strength was over-predicted for joints failing predominantly at the aluminium–metal primer interface. This was because the failure criteria used were based on failure in the adhesive.

Surface Modification of a Polyurethane Film by Low Pressure Nitrogen Plasma for Improved Adhesion to Polyethylene Foam for Automotive Industry Laminates

A low pressure glow discharge nitrogen plasma has been used to modify wettability of a polyurethane film and, consequently, improve adhesion properties to polyolefin foam in order to obtain technical laminates for uses in automotive industry. A significant increase in surface wettability is achieved after the plasma treatment as observed by contact angle measurements. Surface changes have been studied by X-ray photoelectron spectroscopy (insertion of polar groups) and atomic force microscopy (changes in surface roughness and topography). Laminates based on plasma-treated polyurethane film and polyolefin foam show a considerable increase in the T-peel strength of the adhesion joint; furthermore we can observe a slight ageing process under aggressive conditions (high temperature and relative humidity).

Ductile double-lap joints from brittle GFRP laminates and ductile adhesives, Part II: Numerical investigation and joint strength prediction

Joint analysis using a non-linear finite element model has been performed to analyze the effects of adhesive ductility on the stiffness and strength of full-scale adhesively-bonded double-lap joints composed of brittle pultruded GFRP laminates. Experimental and numerical results of joint and specimen elongations and axial strains in the bondline compared well. Calculated stress states at failure location inside the adherends showed that plastification of ductile adhesives provide uniform load transfer leading to increased joint strength. Joint strength increases almost linearly with increasing overlap length. Flexible and stiff joints are defined depending on the ratio of adhesive-to-adherend modulus. Flexible joints exhibit lower stiffness than the adherends, while stiff joints provide continuity of structural stiffness. The strength of ductile adhesively-bonded joints was predicted by extending an existing through-thickness shear-tensile-interaction failure criterion developed for brittle joints with epoxy adhesive.

Properties of thermoplastic polyurethane adhesives containing nanosilicas with different specific surface area and silanol content

Thermoplastic polyurethane (TPU) adhesives containing nanosilicas with different specific surface area and silanol group content were prepared and characterized by FTIR spectroscopy, differential scanning calorimetry (DSC), thermogravimetry (TGA), X-ray diffraction, plate-plate rheology, dynamical–mechanical–thermal analysis (DMTA), transmission electron microscopy (TEM), and strain–stress test. Adhesive strength was obtained from T-peel tests of PVC/polyurethane adhesive joints. Formation of agglomerates of nanosilica particles within the polyurethane matrix were favoured by increasing the silanol content likely due to stronger hydrogen bond interactions between the silanol groups on the nanosilica over those between the polyurethane and the nanosilica. As a consequence, inter-urethane bonds formation rather than ester-urethane bonds were favoured, leaving the soft segment chains more free to interact between them. Thus, addition of nanosilica favoured the phase segregation in the thermoplastic polyurethane. The increase in specific surface area and silanol content in the nanosilica, generally enhanced the degree of phase separation in the polyurethane, being less marked for nanosilicas with more than 200 m 2/g and 0.60 mmol SiOH/g silica. On the other hand, the addition of the nanosilica improved the tensile strength and elongation at break, and the viscoelastic properties of the polyurethane. The immediate adhesive strength of PVC/polyurethane adhesive joints increased in the filled adhesives and it was determined by the rheological properties of the polyurethane–nanosilica mixtures. By increasing the time after joint formation, the crystallization of the polyurethane was produced giving higher adhesive strength and although a cohesive failure in the PVC was always obtained, a slight though progressive increase in joint strength was found with the passage of time with the ordering of the three systems (PU-0.45, PU-0.60 and PU-0.90) remaining unchanged with the PU-0.60 system the stronger and the PU-0.90 system the weaker. This is in agreement with the trends in the viscoelastic and mechanical properties of the filled adhesives.

Mechanical behaviour of self-piercing riveted multi-layer joints under different specimen configurations

A study examining the effect of specimen configuration on the mechanical behaviour of self-piercing riveted, multi-layer joints in aluminium alloys was conducted. It has observed that the specimen configuration had a significant effect on the strength and failure mechanism of a self-piercing riveted multi-layer joint. During shear testing, when secondary bending was present interlock failure was observed, while in the absence of secondary bending, sheet material failure occurred. Interlock and sheet material failure modes were also observed in T-peel tests, where depending on the configuration, either substrate failure led to peeling the top sheet away from the rivet head, or interlock failure resulted in pulling the rivet out of the bottom sheet. In the cases where the sheet material failed, an increase in sheet material strength led to an increase in joint strength. Under identical setting conditions, joints with double shear and no secondary bending effect exhibited significantly higher shear strength and more energy absorption than joints in single shear.

코로나 방전처리에 의한 반도전-절연 실리콘 고무의 표면특성 및 접착특성

In this work, the effects of the corona treatment on surface properties of semiconducting silicone rubber were investigated in terms of contact angles, ATR-FTIR(Attenuated total reflection fourier transform infrared spectroscopy) and XPS(X-ray photoelectron spectroscopy). And the adhesive characteristics were studied by measuring the T-peel strengths. Based on chemical analysis, the surface modification can be mainly ascribed to the creation of chemically active functional groups such as C-O, C=O and C-OH on semiconducting silicone surface. This oxidized rubber layer is inorganic silica-like structure of Si bound with three to four oxygen atoms (<TEX>$SiOx,\;x=3{\sim}4$</TEX>. The Corona treatment produces an increase in joint strength that is maximum for 10 min treatment. However, due to brittle property of this oxidized layer, the highly oxidized layer from too much extended treatment could be act as a weak point, decreasing the adhesion strength.

Modification of high‐performance polymer composite through high‐energy radiation and low‐pressure plasma for aerospace and space applications

AbstractIn this investigation, attempts are made to modify a high‐performance polymer such as polybenzimidazole (PBI) (service temperature ranges from −260°C to +400°C) through high‐energy radiation and low‐pressure plasma to prepare composite with the same polymer. The PBI composites are prepared using an ultrahigh temperature resistant epoxy adhesive to join the two polymer sheets. The service temperature of this adhesive ranges from −260°C to +370°C, and in addition, this adhesive has excellent resistance to most acids, alkalis, solvents, corrosive agents, radiation, and fire, making it extremely useful for aerospace and space applications. Prior to preparing the composite, the surface of the PBI is ultrasonically cleaned by acetone followed by its modification through high‐energy radiation for 6 h in the pool of a SLOWPOKE‐2 (safe low power critical experiment) nuclear reactor, which produces a mixed field of thermal and epithermal neutrons, energetic electrons, and protons, and γ‐rays, with a dose rate of 37 kGy/h and low‐pressure plasma through 13.56 MHz RF glow discharge for 120 s at 100 W of power using nitrogen as process gas, to essentially increase the surface energy of the polymer, leading to substantial improvement of its adhesion characteristics. Prior to joining, the polymer surfaces are characterized by estimating surface energy and electron spectroscopy for chemical analysis (ESCA). To determine the joint strength, tensile lap shear tests are performed according to ASTM D 5868–95 standard. Another set of experiments is carried out by exposing the low‐pressure plasma‐modified polymer joint under the SLOWPOKE‐2 nuclear for 6 h. Considerable increase in the joint strength is observed, when the polymer surface is modified by either high‐energy radiation or low‐pressure plasma. There is further significant increase in joint strength, when the polymer surface is first modified by low‐pressure plasma followed by exposing the joint under high‐energy radiation. To simulate with spatial conditions, the joints are exposed to cryogenic (−196°C) and high temperatures (+300°C) for 100 h. Then, tensile lap shear tests are carried out to determine the effects of these environments on the joint strength. It is observed that when these polymeric joints are exposed to these climatic conditions, the joints could retain their strength of about 95% of that of joints tested under ambient conditions. Finally, to understand the behavior of ultrahigh temperature resistant epoxy adhesive bonding of PBI, the fractured surfaces of the joints are examined by scanning electron microscope. It is observed that there is considerable interfacial failure in the case of unmodified polymer‐to‐polymer joint whereas surface‐modified polymer essentially fails cohesively within the adhesive. Therefore, this high‐performance polymer composite could be highly useful for structural applications in space and aerospace. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 1959–1967, 2006

Improvement of Interfacial Performances on Insulating and Semi-conducting Silicone Polymer Joint by Plasma-treatment

In this paper, we investigated the effects of short-term oxygen plasma treatment of semiconducting silicone layer to improve interfacial performances in joints prepared with a insulating silicone materials. Surface characterizations were assessed using contact angle measurement and x-ray photoelectron spectroscopy (XPS), and then adhesion level and electrical performance were evaluated through T-peel tests and electrical breakdown voltage tests of treated semi-conductive and insulating joints. Plasma exposure mainly increased the polar component of surface energy from <TEX>$0.21\;dyne/cm^2$</TEX> to <TEX>$47\;dyne/cm^2$</TEX> with increasing plasma treatment time and then leveled off. Based on XPS analysis, the surface modification can be mainly ascribed to the creation of chemically active functional groups such as C-O, C=O and COH on semi-conductive silicone surface. This oxidized rubber layer is inorganic silica-like structure of Si bound with three to four oxygen atoms (<TEX>$SiO_x,\;x=3{\sim}4$</TEX>). The oxygen plasma treatment produces an increase in joint strength that is maximum for 10 min treatment. However, due to brittle property of this oxidized layer, the highly oxidized layer from too much extended treatment could be act as a weak point, decreasing the adhesion strength. In addition, electrical breakdown level of joints with adequate plasma treatment was increased by about <TEX>$10\;\%$</TEX> with model samples of joints prepared with a semi-conducting/ insulating silicone polymer after applied to interface.

Damage and Failure in Silicon-Glass-Metal Microfluidic Joints for High-Pressure MEMS Devices

The design, fabrication, and testing of microfluidic joints consisting of Kovar metal tubes attached to silicon using borosilicate glass for high pressure microelectromechanical systems devices are presented. The MIT microrocket, which requires microfluidic joints to sustain pressures of at least 12.7 MPa and temperatures in excess of 700 K, is used to demonstrate the feasibility of the glass sealing methodology. A key concern in such joints is the occurrence of cracks due to residual stresses during fabrication, which can affect the load-carrying capability. To obtain a better understanding of the damage and failure characteristics, a hierarchical approach was taken. First, two types of joint configurations with several glass compositions and geometries were considered at the joint-level. Axial tension and pressure tests were performed, and finite element models were used to obtain the residual stress field and to predict failure loads based on linear elastic fracture mechanics. Subsequently, tests were performed on actual and dummy microrockets to validate the methodology at the device-level. Key observations include the importance of bonding between the Kovar tube and the silicon sidewall, which can help increase joint strength, and the detrimental effects of joint proximity under differential pressure loading and manufacturing defects in multiple joint specimens. In addition to specific experimental and analyses results that allow a physical understanding of the damage and failure mechanisms, another key contribution of this work is the overall insight of the design and analysis of reliable glass-sealed microfluidic packages. This insight will help one make better design and process selections for packages in other high-pressure silicon-based MEMS applications.

Pre-stressed adhesive strap joints for thick composite sandwich structures

A simple but effective design to improve the strength of thick adhesive composite strap joints is validated with experiment and finite element method. The strap joint under investigation, with a particular application to naval ship structures, consists of two thick woven E-glass/vinyl ester laminates joined together with two steel doublers. Longitudinal tensile loads are applied to the joints, resulting in large concentrated shear and peel stresses near the free edges of bondlines. The new design intends to reduce the adhesive peel stress by application of through-the-thickness compressive pre-stress along the bondline and thus leads to an increase of joint strength. Experiment results show that all the joint failures are delamination of the top layer of the laminated adherends. The test further confirms that joint strength increases significantly by applying the transverse pre-stress. Finite element analysis reveals that the pre-stress can effectively reduce the magnitude even reverse the sign of the peel stress in the adhesive layer and the adherends. Recessing the adhesive leading edge could magnify the pre-stress effect and reduce the adhesive peel stress, but would increase the shear stress. For those composite joints with low transverse interlaminar strength and susceptible to delamination, this simple design/technique can considerably improve their joint strength.

Quasi-static strength and fatigue life of hybrid (bonded/bolted) composite single-lap joints

The strength and fatigue life of hybrid (bonded/bolted) joints with carbon-fibre reinforced plastic adherends have been investigated. The effect of adhesive material properties and laminate stacking sequence on the joint structural behaviour and failure modes were determined experimentally. Hybrid joints were shown to have greater strength, stiffness and fatigue life in comparison to adhesive bonded joints. However, the benefits were only observed in joints with lower modulus adhesives which allowed for load sharing between the adhesive and the bolt. Hybrid joints with high modulus adhesives showed no significant improvement in strength although increased fatigue life was observed due to the presence of the bolt. Three distinct stages in the fatigue life of hybrid joints were observed where the adhesive, the bolt and their combination were all contributing to the load transfer. Fatigue crack initiation was found to occur later in the hybrid joints where the bolt transferred a significant portion of the load. The failure mode of the joints was shown to be dependent upon the relation between the hybrid joint strength and the bearing strength of the laminate. Hybrid joints in which the strength greatly exceeded the laminate bearing strength failed catastrophically in net-section mode. In contrast, laminates with higher bearing strength failed in a non-catastrophic bearing mode. Careful comparison of the material bearing strength and adhesive bond strength is necessary to ensure increased joint strength and non-catastrophic failure.

Influence of reflow and thermal aging on the shear strength and fracture behavior of Sn-3.5Ag solder/Cu joints

The mechanical behavior of Sn-rich solder/Cu joints is highly sensitive to processing variables such as solder reflow time, cooling rate, and subsequent thermal aging. In this article, we focus on the lap shear behavior of Sn-3.5Ag/Cu joints as a function of solder yield strength and intermetallic thickness. Experimental results showed that the shear strength of the solder joints is primarily controlled by the mechanical properties of the solder, and not the intermetallic thickness. The thickness of intermetallic, however, controlled the fracture mode of the solder joints. At intermetallic thicknesses greater than 20 µm, brittle fracture between Cu6Sn5 and Cu3Sn was the most common failure mechanism. Finite-element simulations were carried out to evaluate the effect of solder properties and of intermetallic thickness and morphology on lap shear behavior. The finite-element simulations corroborated the experimental findings, i.e., that increased solder strength results in increased joint strength. The simulations also showed that thicker intermetallics, especially of nodular morphology, yielded higher local plastic shear strain and work hardening rate.

セラミックス／金属接合部品の残存接合強度に及ぼす接合構造の影響

An actual joint structure of ceramics-metal in the airtight seal parts for a neutron detector was studied on the residual stress generated in the joint process due to the difference of the thermal expansion coefficient between ceramics and metal. The influences of shape and dimension of the joint structure on the residual stress were clarified analytically by finite element method (FEM) and verified experimentally by checking cracks generated in prototype samples. Based on fracture mechanics, equivalent stress intensity factor KIeq was calculated in consideration of a latent flaw in the residual stress fields, and optimal geometries of the joint structure were proposed for reducing the residual stress. The residual joint strength was discussed by using the difference ΔKIeq between fracture toughness KIC and KIeq. The main results obtained are summarized as follows : the residual joint stress increases with the thickness of a Cu-Ti eutectic reaction layer; the residual joint stress decreases with the reduction in taper angle of the end of Cu interlayer; the residual joint stress is hardly influenced by wall thickness in the end of SUS304 pipe. On the basis of these results, the optimization in the structural factors can decrease the residual stress because of reducing stress concentration field near the interface of ceramic and metal, and so the residual joint strength increase with decreasing of the residual stress depending in the structural factors as the thickness of a Cu-Ti eutectic reaction layer, taper angle of the end of Cu interlayer and wall thickness in the end of SUS304 pipe.

Adhesive and composite failure prediction of single-L joint structures under tensile loading

In this article, single-L composite peel joints are investigated under tensile loading. The adhesive peel fillet is the critical component regarding joint failure, with sub-critical damage developing in the composite. Optimisation of the joint can be achieved by changing the fillet shape. A method of adhesive injection was applied to guarantee repeatable joint designs. Experimental testing shows an increase in joint strength of up to 100% by altering the fillet shape. Numerically, sub-critical composite failure, composite delamination and adhesive failure are investigated. Adhesive failure criteria are reviewed and compared to the experimental outcome. A zone criterion was found to predict the failure load best for different fillet shapes, while a damage mechanics approach could be applied to simulate the crack growth during adhesive failure.

Adhesive and composite failure prediction of single-L joint structures under tensile loading

In this article, single-L composite peel joints are investigated under tensile loading. The adhesive peel fillet is the critical component regarding joint failure, with sub-critical damage developing in the composite. Optimisation of the joint can be achieved by changing the fillet shape. A method of adhesive injection was applied to guarantee repeatable joint designs. Experimental testing shows an increase in joint strength of up to 100% by altering the fillet shape. Numerically, sub-critical composite failure, composite delamination and adhesive failure are investigated. Adhesive failure criteria are reviewed and compared to the experimental outcome. A zone criterion was found to predict the failure load best for different fillet shapes, while a damage mechanics approach could be applied to simulate the crack growth during adhesive failure.

Damage and Failure Mechanisms in High Pressure Silicon-Glass-Metal Microfluidic Connections

ABSTRACTThe characteristics and mechanisms of damage and failure in microfluidic joints consisting of Kovar metal tubes attached to silicon using borosilicate glass seals have been investigated. These joints are representative of seals for the MIT microrocket which is a silicon-based MEMS device. A key concern in such joints is the occurrence of cracks in silicon and glass due to residual stresses caused by a large thermal excursion during processing and the dissimilar coefficients of thermal expansion of the constituent materials. Joints with two types of glass compositions and joint configurations were fabricated, tested, and inspected. Axial tension tests were performed to investigate load carrying capability and the effect of thermally-induced cracks. Finite element models were used to obtain residual stresses due to the fabrication, and the location of the cracks from the experiments were found to coincide with the locations of the maximum principal stresses. The current work shows that the certain types of thermally-induced cracks are more detrimental to joint strength than others and a good bond between the Kovar tube and the silicon sidewall can help increase joint strength via shear load transfer.

Metallic glass fluid flow during welding using self-propagating reactive multilayer foils

ABSTRACTWe use a fluid mechanics model to analyze glass fluid flow during the welding of bulk metallic glasses with reactive multilayer foils acting as local heat sources. The resulting welded joints were shear tested, and fracture surfaces were analyzed by optical microscopy. Fracture surfaces of failed metallic glass joints show distinct regions of metal-metal veins that indicate effective metallurgical bonding. We observe a monotonic increase in the failure strength of the joints with the fraction of the joint composed of such veins. For the strongest joint tested (shear strength of 420 MPa), nearly 60% of the fracture surface is comprised of metal-metal veins. We have developed a qualitative fluid mechanics explanation of the welding process, in which shear stresses (due to pressure applied during joining) push the reactive foil from the joint interface and create the metal-metal veins. The welding process is more effective at higher joining pressure and greater foil thickness, leading to increased joint strength.