Faying Surface Research Articles

Improving mechanical properties of pinless friction stir spot welded joints by eliminating hook defect

Hook defect (HD) seriously decreases the mechanical properties of friction stir spot welded (FSSW) joints. In this study, two methods were therefore used to eliminate the HD in pinless FSSW joints. The one is changing welding parameters such as rotating speed and dwell time. The other one is FSSW plus subsequent friction stir welding (FSSW-FSW), which is an innovative method proposed in this study. Experimental results showed that the HD in pinless FSSWed AA2024 joints was successfully eliminated by using FSSW-FSW, not by changing process parameters. The joints without HD exhibited a tensile–shear load of as much as 12kN, which was higher than that of 6.9kN in the joints with HD. Furthermore, it was proved that the tensile–shear load is not greatly improved only by increasing the nugget zone when HD still existed in the FSSW joints. In addition, the fracture morphology analysis demonstrated that the shear fracture of the FSSW-FSW joints took place along the boundary between the upper and lower sheets through the weld nugget, and the faying surface between the two sheets was completely sheared off.

Analysis of Fluxless, Reactive Brazing of Al Alloys Using Differential Scanning Calorimetry

During this investigation, a technique was developed, using differential scanning calorimetry (DSC), to quantitatively analyze the influence of a Ni-based electrolytic braze promotor surface deposit on the furnace brazing of aluminum alloys. The purpose of this braze promoter was to induce a large exothermic surface reaction capable of disrupting a tenacious oxide present on the aluminum braze sheet faying surface. A cyclic DSC methodology was developed which was capable of a quantitative determination of the exothermic reaction (ΔHexo) induced by the Ni plating. Samples with a small quantity of Ni plating exhibited significant “pre-reaction” between Ni and Al in the solid state which resulted in very low ΔHexo values. Samples with higher quantities of Ni plating exhibited large ΔHexo values up to 85 kJ/mole.

Fatigue life estimation of ultrasonic spot welded Mg alloy joints

Lightweight magnesium alloys are increasingly used in automotive and other transportation industries for weight reduction and fuel efficiency improvement. The structural application of magnesium components requires proper welding and fatigue resistance to guarantee their durability and safety. The objective of this investigation was to identify failure mode and estimate fatigue life of ultrasonic spot welded (USWed) lap joints of an AZ31B-H24 magnesium alloy. It was observed that the solid-state USWed joints exhibited a superior fatigue life compared with other welding processes. Fatigue failure mode changed from interfacial failure to transverse-through-thickness crack growth with decreasing cyclic load level, depending on the welding energy. Fatigue crack initiation and propagation occurred from both the notch tip inside the faying surface and the edge of sonotrode indentation-footprints due to the presence of stress concentration. A life prediction model for the spot welded lap joints developed by Newman and Dowling was adopted to estimate the fatigue lives of the USWed magnesium alloy joints. The fatigue life estimation, based on the fatigue crack growth model with the global and local stress intensity factors as a function of kink length and the experimentally determined kink angle, agreed fairly well with the obtained experimental results.

Friction stir welding of mild steel to alloy 625 – development of welding parameters

Friction stir welding parameters were developed for a 6 mm thick dissimilar butt joint made of mild steel and Ni based alloy 625. A composite W-Re/polycrystalline cubic born nitride tool was used to study the effect of tool offset (between 0 and 3·13 mm away from the alloy 625 plate) and rotational speed (between 250 and 350 rev min−1) on joint consolidation. When non-optimal parameters were used, macrovoids were observed at the advancing side and faying surface near the joint interface root. Defect free joints were obtained using 300 rev min−1, 100 mm min−1, tool offset of 1·63 mm, and axial force between 25 and 30 kN, under force controlled mode.

Analysis of Laser Weldments for Dual-Phase and Martensitic Steel Sheets for Automotive Applications

Laser weldability was investigated for advanced high-strength steel sheets for automotive applications. Dual-phase steel (DP780) and martensitic steel (MS1300) sheets were employed as base materials; laser-butt and overlap welding experiments were conducted on combinations of steels with similar and dissimilar strength. The tensile strength and metallurgical morphology were analysed for the butt-welded specimens; tensile-shear strength and bead shapes were analysed for the overlap-welded specimens. Even with laser welding, martensite in the heat-affected zone disintegrated and resulted in a softened, heat-affected zone as compared with the base materials. The tensile strength of a butt weldment was determined by the strength of the heat-affected zone. The tensile-shear strength of an overlap weldment was determined by not only the strength of the heat-affected zone but also bead shapes such as blow holes, underfill, and the bead width at the faying surface.

Improving resistance welding of aluminum sheets by addition of metal powder

In order to ensure good quality joints between aluminum sheets by resistance spot welding, a new approach involving the addition of metal powder to the faying surfaces before resistance heating is proposed. Three different metal powders (pure aluminum and two powders corresponding to the alloys AA2024 and AA7075) are investigated for the resistance spot welding of AA1050 aluminum sheets of three different thicknesses. Microstructural and mechanical analysis demonstrates that significant improvement in weld bead morphology and strength are obtained with the addition of metal powder. The improvement obtained is shown to be due to the development of a secondary bond in the joint beside the weld nugget increasing the total weld area. The application of powder additive is especially feasible, when using welding machines with insufficient current capacity for producing the required nugget size. In such cases the best results are obtained with pure aluminum powder.

Influence of interference fit size on hole deformation and residual stress in hi-lock bolt insertion

The interference fit can improve the fatigue performance of mechanical joints and is widely used in aircraft assembly. In this paper, specimens of lap plates and several interference fit sizes were designed, and then the interference fit hi-lock bolt insertion was carried out in an experimental test. Using the commercial finite element software ABAQUS, a two-dimensional axisymmetric finite element model was established to simulate the bolt insertion process. The finite element model was validated by comparison of experimental results and finite element prediction for insertion force and protuberance height. After the interference fitted bolt insertion, the changing characteristics of the non-uniform hole expansion and protuberance were presented with increases in interference fit size. Under low level of interference fit, the tensile hoop stress was produced mainly on the hole wall, and changed into compressive hoop stress when interference fit size is larger. The maximum tensile hoop stress point on faying surfaces went away from the hole wall with interference fit size increasing.

Effect of zinc interlayer on ultrasonic spot welded aluminum-to-copper joints

Dissimilar aluminum (Al) and copper (Cu) metals were joined together using ultrasonic spot welding (USW), a solid state welding technology. The welds were made with and without a zinc (Zn) interlayer to study the microstructural and mechanical properties of weld joints to analyze the effect of the Zn interlayer. USWed Al-to-Cu joints did not produce any intermetallic compounds (IMCs), and only swirls and voids were observed. It was determined through energy dispersive X-ray spectroscopy and X-ray diffraction scans that welds with a Zn interlayer placed in-between the faying surfaces of the base metals formed a composite-like eutectic structure of Al and Al2Cu at the center and Al–Zn and CuZn5 at the edges of the welded joint. Al–Cu joints welded with a Zn interlayer in-between displayed lap shear tensile strengths 25–170% greater than those of the welds without any interlayer.

Interfacial characteristics of Cu–Fe laminate fabricated by solid state welding

Solid state welding was employed to join low intersolubility metal couple Cu–Fe, and the microstructure and mechanical properties of the joints were investigated. The interface was free of defects. Scanning electron microscopy, energy dispersive spectrometry, atomic force microscopy and nanoindentation results revealed that a potential area of pseudo-binary alloy was present at the faying surface, where nanoscale particles were moved to the opposite side by facilitated diffusion. Owing to the mechanism of microcontact and facilitated diffusion, the interfacial strength was improved.

Non-parametric effects on pore formation during resistance spot welding of magnesium alloy

The effects of faying surface condition, pre-existing pores in base material and cover plate on the pore formation during resistance spot welding of magnesium alloy were investigated. The results reveal that the hydrogen rejection and the pre-existing pores in base material have little effect on the pore formation, and that the larger pore formation is mainly caused by shrinkage strain during resistance spot welding of magnesium alloy.

Effect of bonding parameters on microstructure development during TGTLP bonding of Al7075 alloy

The effect of temperature, pressure and bonding time on microstructure of temperature gradient transient liquid phase (TGTLP) diffusion bonded Al7075 alloy using liquid gallium interlayer was investigated. The selected bonding method relies on using the minimum amount of liquid gallium on faying surfaces, using a very fast heating rate to reach the joining temperature and imposing a temperature gradient across the bond region. The microstructure of the diffusion bonded joints was evaluated by light optical microscopy, scanning electron microscopy and energy dispersive spectroscopy (EDS). Results show that by increasing the temperature, pressure and time of joining, a more uniform microstructure can be obtained at the joint area. The best joint microstructure was achieved at a temperature of 460 °C, pressure of 10 MPa and time of 10 min. EDS spot analysis indicates that brittle silicon-rich precipitates form at the joint line during TGTLP bonding.

Automatic Tracking of Blind Sealant Paths in Friction Stir Lap Joining

An automatic joint-tracking technique that employs an extremum-seeking controller is evaluated as a method for automatically tracking sealant paths that have been applied in dissimilar friction stir lap joints of 2024 and 7075 aluminum alloys. Sealants are commonly used to prevent the ingress of corrosion at the faying surfaces of lap joints, and in this study, an attempt is made to exploit sealant presence to reduce necessary robotic path planning procedures. Controller parameters are tuned, and baseline tracking performance is established with milled channels, which are used to replicate the force signature of sealant, and the tracking technique is then evaluated with Pelseal® 2077 sealant applied in a prescribed fashion in both cured and uncured states. Mechanical testing is conducted to determine the implications for weld strength when welding parameters are selected primarily for successful tracking. Results are promising and demonstrate a new level of interaction between sealants and robotic control techniques.

Dissimilar joining of galvanized high-strength steel to aluminum alloy in a zero-gap lap joint configuration by two-pass laser welding

A welding procedure based on using two-pass laser scans is introduced for dissimilar joining of overlapped galvanized high-strength dual-phase (DP) steel DP590 to aluminum alloy (AA) 6061 sheets. The first pass is based on a defocused laser spot that scans across the top of the two overlapped sheets and heats the zinc coating at the faying surface to be melted and partially vaporized, while the second pass is executed with a focused laser spot in order to perform the welding. Completely defect-free galvanized steel to aluminum lap joints were obtained by using this two-pass laser welding procedure. An on-line machine vision system was applied to monitor the keyhole dynamics during the laser welding process. An energy-dispersive X-ray spectroscopy (EDS) was carried out to determine the atomic percent of zinc, aluminum, and iron in the galvanized steel to aluminum lap joints. Mechanical testing and micro-hardness test were conducted to evaluate the mechanical properties of the galvanized steel to aluminum lap joints. The experimental results showed that the lap joint of galvanized steel to aluminum obtained by the two-pass laser welding approach had a higher failure value than those joints obtained when the zinc at the faying surface was mechanically removed under the same welding speed and laser power.

Pore formation mechanism and its mitigation in laser welding of AZ31B magnesium alloy in lap joint configuration

Magnesium is one of the lightest structural metals that has been used in different industries such as automobile, aerospace and electronics. However, in fusion joining of magnesium alloys, porosity is one of the main drawbacks to achieve a weld with desirable properties. The oxide layer existing on the surface of magnesium alloy is one of the causes of pore formation in the weld bead. In the current study, a fiber laser with a power of up to 4kW is used to weld samples in a zero-gap lap joint configuration. Two groups of samples are studied: as-received (AR) surfaces (where an oxide layer remains on the surface) and treated surfaces. The surface treatment includes two techniques: mechanically removed (MR) and the use of a plasma arc (PA) as a preheating source. Also, a separate set of experiments are designed for preheating samples in a furnace for comparison with the PA-treated results. To reveal the chemical compositions of the welds and metal sheet surfaces, an energy dispersive spectroscopy (EDS) is performed. Surface chemical compositions are tested by X-ray photoelectron spectroscopy-reflected electron energy loss spectroscopy (XPS-REELS) to characterize the surface composition on AR and PA-treated samples. The dynamic behavior of the weld pool and laser-induced plasma plume is monitored in real-time using a high speed CCD camera to investigate the stability of the laser welding process. The presence of the oxide layer at the faying surface of two overlapped sheets results in an unstable process. The obtained results reveal that the preheating procedure can effectively mitigate pore formation at the interface of the two overlapped sheets.

Effects of electrode contact condition on electrical dynamic resistance during resistance spot welding

This study systematically investigates the effects of electrical resistance at the workpiece/electrode interface or electrode face on temperature dependent dynamic resistance during resistance spot welding (RSW). To evaluate temperature transport equations of mass, momentum, energy, species and magnetic field intensity in workpieces, the energy and magnetic equations in the electrode are solved. Contact resistances composed of constriction and film resistances are functions of hardness, temperature, electrode force and surface conditions. The results show that dynamic resistance is complicated due to different variations of film and constriction resistances with temperature at not only the faying surface but also the electrode face in the early stage, i.e. shorter than around 3 cycles. Dynamic resistance in this stage is fortunately insignificant to transport processes. When the power is off, dynamic resistance depends on competition between decreased bulk resistance and increased constriction resistance at the electrode face. Decreased constriction resistance at the electrode face reduces dynamic resistance and temperature.

Slip resistance of metalized–galvanized faying surfaces in steel bridge construction

The slip resistance of high strength bolted connections depends on the surface condition of the connected parts, also known as the faying surfaces. Metalizing and galvanizing are two commonly used corrosion protection solutions in steel bridge construction, providing physical barrier as well as sacrificial protection. Practical situations exist where galvanized secondary structural components are joined to primary elements that are metalized in a slip-critical connection. Contemporary bridge design standards, such as the Canadian Highway Bridge Design Code, CAN/CSA S6-06, and the American Institute of Steel Construction (AISC) specifications, ANSI-AISC 360-2010, however do not specify slip coefficient for slip-resistant connections with one faying surface metalized and the other face galvanized. Steel bridge fabricators are therefore compelled to mask off all connection faying surfaces before metalizing. This exercise is time-consuming, labor-intensive, and costly. In this investigation, the resistance of slip-critical joints having metalized–galvanized faying surfaces is characterized in view of the current North American design standards. The mean slip coefficient, determined from short-duration tension and compression test regimes, revealed slip resistance greater than the typical faying surface with roughened hot-dip galvanized finish, and in the majority of cases, greater than the typical uncoated blast-cleaned faying surfaces.

Fatigue Crack Growth in Aluminum Lithium Riveted Lap Joints

Fatigue crack initiation sites and the subsequent fatigue crack growth rate in riveted lap joints made of 2198-T8 Al-Li alloy were investigated through fractographic examinations. Two crack initiation sites were identified: 1) near the rivet holes and 2) at multiple sites along the faying surface. Fatigue crack tunneling was developed during the fatigue crack progression in all specimens, yielding final ductile fracture during the catastrophic fracture of the specimens. The fracture surface morphology exhibited shallow fatigue striations. The second phase constituents did not affect the fracture surface morphology. Multiple micro cracks and long interlaminar cracks occurred. Stereo (3-D) fracture surface morphologies indicate a meandering fracture surface, with local fatigue crack progressing along multiple directions and at different rates. Average crack growth rate (da/dN) along the width, thickness of the specimens and the feather lines were recorded, showing the effect of grain direction, alloying, and stress level on the fatigue crack growth behavior. The number of cycles to failure was also affected by the grain direction.

Optimum Design and Structural Application of the Bracing Damper System by Utilizing Friction Energy Dissipation and Self-Centering Capability

본 연구는 지진에 대한 구조물의 손상을 최소화 하기 위하여 슬립 저항력을 활용한 새로운 형태의 마찰 댐퍼형 가새 시스템의 설계와 개발을 주로 다루고자 한다. 가새 부재 내에서 전단력에 의한 마찰 거동으로 상당량의 에너지를 수동적으로 소산하기 위하여 플레이트 전단 이음부 위에 슬롯 형태의 볼트 구멍을 설치한다. 여기에 전단 마찰 거동으로 인해 발생되는 잔류변형을 줄이고자 상온에서 원형복원이 가능한 초탄성 형상합금 와이어를 꼬아서 만든 연선을 설치하여 댐퍼 시스템 내에 복원성을 증진 시켰다. 기존에 주로 사용된 수동적인 변위 제어 장치와 비교하여 본 연구에서 다루고자 하는 자동복원이 가능한 마찰 댐퍼형 가새 시스템은 중심 가새 프레임 구조물에 손쉽게 설치하여 지진발생 후에 구조물에 발생하는 층간 잔류변위를 최소화하여 유지 보수에 소모되는 비용의 대폭적인 절감을 기대할 수 있다. 본 연구에서는 자동복원이 가능한 마찰 댐퍼형 가새 시스템의 역학적인 거동 메커니즘을 살펴보고 실험값으로 보정되어 신뢰성을 확보한 스프링 모델을 사용하여 해석을 실시하였다. 시스템에 다양한 설계 변수를 적용하여 복원성과 에너지 소산 능력 측면에서 제안된 댐퍼의 성능 동향을 분석을 하고 최적의 설계 방식을 제안하고자 한다. 마지막으로 자동복원이 가능한 마찰 댐퍼를 중심 가새 프레임 구조물에 설치하여 비선형 동적 해석을 실시하고 기존의 시스템과 비교하여 성능적인 우수성을 입증하고자 한다. This study mainly treats a new type of the bracing friction damper system, which is able to minimize structural damage under earthquake loads. The slotted bolt holes are placed on the shear faying surfaces with an intention to dissipate considerable amount of friction energy. The superelastic shape memory alloy (SMA) wire strands are installed crossly between two plates for the purpose of enhancing recentering force that are able to reduce permanent deformation occurring at the friction damper system. The smart recentering friction damper system proposed in this study can be expected to reduce repair cost as compared to the conventional damper system because the proposed system mitigates the inter-story drift of the entire frame structure. The response mechanism of the proposed damper system is firstly investigated in this study, and then numerical analyses are performed on the component spring models calibrated to the experimental results. Based on the numerical analysis results, the seismic performance of the recentering friction damper system with respect to recentering capability and energy dissipation are investigated before suggesting optimal design methodology. Finally, nonlinear dynamic analyses are conducted by using the frame models designed with the proposed damper systems so as to verify superior performance to the existing damper systems.

Detection of defects in laser welding of AZ31B magnesium alloy in zero-gap lap joint configuration by a real-time spectroscopic analysis

The effect of surface oxide layer existing at the lap-joint faying surface of magnesium sheets is investigated on the keyhole dynamics of the weld pool and weld bead qualities. It is observed that by removing the oxide layer from the faying surface of the lap joint, a high quality weld can be achieved in the laser welding process. However, the presence of an oxide layer deteriorates the quality of the weld by forming pores at the interface of the two overlapped sheets. The purpose of this paper is to identify the correlation between the integrity of the weld and the interaction between the laser and material. A spectroscopy sensor was applied to detect the spectra emitted from a plasma plume during the laser welding of AZ31B magnesium alloy in a zero-gap lap joint configuration. The electron temperature was calculated by applying a Boltzmann plot method based on the detected spectra, and the correlation between the pore formation and the spectral signals was studied. The laser molten pool and the keyhole condition were monitored in real-time by a high speed charge-coupled device (CCD) camera. A green laser was used as an illumination source in order to detect the influence of the oxide layer on the dynamic behavior of the molten pool. Results revealed that the detected spectrum and weld defects had a meaningful correlation for real-time monitoring of the weld quality during laser welding of magnesium alloys.

Joint Quality Affected by Electrode Contact Condition During Resistance Spot Welding

The effects of electrical resistance at the workpiece-electrode contact surface or the electrode face on the joint quality, characterized by nugget shape, temperature, cooling rate, and solute concentration responsible for microstructure of the fusion zone during resistance spot welding (RSW), are systematically investigated. The model accounts for electromagnetic force, heat generation due to contact resistance at the faying surface and electrode face, and bulk resistance in the workpieces. Contact resistances composed of different features of constriction and film resistances are functions of hardness, temperature, electrode force, and surface condition. The computed results show that nugget growth and transport processes are independent of the film resistance at the electrode face. An increase in constriction resistance at the electrode face, however, decreases the onset time for nugget formation, cooling rates, and axial and radial heat transfer, and increases the electrode temperature. The workpiece is susceptible to surface melting if the parameter governing constriction resistance at the electrode face is high. Solute concentration and flow patterns affected by constriction resistance at the electrode face are also presented.