High-strength Joints Research Articles

Strengthening spot weld joint by autotempering acceleration at heat affected zone

High tensile strength steel sheets in automotive body are composed of high weight fraction carbon and other additional elements. Therefore, securing the joint strength is difficult in spot welding. This study proposes a new technique for improving the spot weld joint strength. Typically, high joint strength can be obtained by prolonging the post-heating duration. This post-heating is expected to prevent propagation of crack into the nugget and improve the toughness of the heat affected zone. Compared to the conventional spot welding and other methods, the heat input is higher, thereby increasing the highest reaching temperature of heat affected zone. Hence, the austenite grain size becomes larger and martensite start temperature increases. Consequently, autotempering is promoted, resulting in stronger joint strength.

The quantitative investigation of mechanical properties and characterization of fractured position for friction stir lap welded A6111/A5023

Abstract The effects of plunge depth (PD) revolutionary pitch (RP) and materials arrangement on deformation and softening material at the fractured position of a friction stir welded AA6111 to AA5023 lap joint were investigated. As a result, the main factor determining tensile shear load and fracture was different with the location of soft material. When the soft material was located on top, the softening material and the deformed surface height occurred by friction heat generation of the rotating shoulder. The main factor was the deformed surface height rather than the softening material, and the deformed surface height was decreased with increasing RP. On the other hand, when the soft material was located at the bottom, the movement of the un-bonded line and hooking occurred due to the vertical flow of the rotating probe. The crucial factor was the position of the un-bonded line rather than the height of the hook. The un-bonded line occurred along the interface between two materials affected by the lowest strength, which were deformed toward the hard material affected by a higher joint strength.

Influence of pin geometry and process parameters on friction stir lap welding of AA5754-H22 thin sheets

The influence of pin geometry and process parameters on the morphology and strength of friction stir lap welds, in very thin sheets, is analysed. The base material is the AA5754-H22 work hardened aluminium alloy. Three different pin geometries were tested, one cylindrical and two conical, with different taper angles. For each pin geometry, varying welding speeds, ranging from 350 to 1000mm/min, were tested in order to simultaneously weld properties and process productivity. Weld defects assessment was performed using metallographic analyses. strength was by performing mechanical tests both monotonic and cyclic loading conditions. Monotonic tests included performing transverse and lap tensile-shear tests. Strain data acquisition using Digital Image Correlation (DIC) enabled to determine local weld properties as the joints failure mode monotonic loading. The results show that the use of unthreaded conical pin tools, with a low shoulder/pin diameter relation, is the more suitable solution for the production of good quality welds. Irrespective of the pin geometry, very high joint strength efficiencies (close to 90%) were obtained when loading the retreating side of the welds. It is also shown that good quality welds may be produced at very high welding speeds.

Friction Stir Welding Tool Geometries Affecting Tensile Strength of AA6063-T1 Aluminum Alloy Butt Joint

Friction Stir Welding (FSW) is a solid state welding that could successfully weld the difficult-to-weldmaterials such as an aluminum alloy. In this welding process, the stirrer of the welding tool is one of the important factors for producing the perfect sound joint that indicates the higher joint strength. So, this report aims to apply the friction stir welding using various stirrer geometries to weld the AA6063-T1 aluminum alloy butt joint, investigates the mechanical properties of the joint and then compares the mechanical properties with the microstructure of the joint. An experiment was started by applying the friction stir welding process to weld a 6.3 mm thickness of AA6063-T1 aluminum alloy butt joint. A study of the stirrer geometries effect such as a cylindrical geometry, a cone geometry, a left screw geometry and a right screw geometry at a rotational speed of 2000 rpm and a welding speed of 50-200 mm/min was performed. The mechanical properties such as a tensile strength and a hardness of the joint were also investigated and compared with the microstructure of the joint. The results are as follows. A variation of FSW Stirrer shape directly affected the quality AA6063-T1 aluminum alloy butt joint. A cylindrical stirrer shape and a cone stirrer shape produced the void defect at the bottom part of the weld metal and initiated the failure of the joint when the joint was subjected to the load during the tensile test. Left and right screw stirrer shapes gave the sound joint with no void defect in the weld metal and affected to increase the joint strength that was higher than that of the aluminum base metal.

Joining Al 5052 alloy to aluminized steel sheet using cold metal transfer process

The arc braze welding process for Al/Fe dissimilar metal joints was investigated to meet the needs for multi-material mixed structures in the automotive industry. In this study, the Al 5052 alloy was joined to hot-dip aluminized steel sheets using low-heat-input cold metal transfer (CMT) arc welding. Four kinds of filler wires (Al 4043, 4047, 5356, 5183) were examined in the dissimilar metal joint. Using aluminized steel sheets, although the wettability was relatively poor compared with galvanized steel, a lower intermetallic compound (IMC) layer thickness was observed between the dissimilar metals, which resulted in a higher joint strength. The Si content in the filler metal restricted the growth of IMC layer, namely, the growth of the trapezoidal Fe2Al5 IMC layer into the steel base metal. When AlMg filler wires were used, a thinner IMC layer was observed, but a relatively low tensile shear strength was measured due to micro cracks and porosities formed along the interface between braze welds and steel sheet. The joint strength of the combination of Al 5052/aluminized steel/AlMg filler wire was equal or higher than that of the heat-affected zone in the Al 5052 alloy. The integrity of the Al 5052/aluminized steel/AlMg filler dissimilar metal joint was guaranteed using both a high-speed tensile test and salt spray corrosion test.

Plasma chemical modification of polymer surface. Polyethylene terephthalate

Methods of electron microscopy, atomic force microscopy, contact angles, multiple attenuated total internal reflection IR spectroscopy, and X-ray photoelectron spectroscopy were used to study the molecular-chemical, structural and morphological, and energy characteristics of polyethyleneterephthalate (PET) surfaces before and after treatment in plasma of high-frequency air discharge. It is shown that modification of PET surface results in a change in the structural-morphological surface properties and the formation of oxidized near-surface layers. The peel test was used to determine the adhesion characteristics of the substrate with respect to a standard adhesive. It is shown that the decisive role in providing high strength of adhesive joints in the course of plasma chemical modification is played by a significant increase in the surface energy of the substrate, namely its polar component. The kinetics of relaxation of surface characteristics of the substrate are studied in the case of long-term storage under laboratory conditions.

Joinability of different thermoplastic polymers with aluminium AA6082 sheets by mechanical clinching

The joinability of different rigid thermoplastic polymers with aluminium AA6082-T6 alloy sheets by mechanical clinching is investigated. The polymers were heated using a convective source prior of clinching. Experimental tests were conducted by varying die geometry and pre-heating time. The joinability window for different thermoplastic polymers including polystyrene (PS), polycarbonate (PC) and poly(methyl methacrylate) (PMMA) was determined. The mechanical behaviours of clinched connections were assessed by means of single lap shear tests. To understand the effect of process parameters on the mechanical behaviours of clinched joints, dimensional analysis was also carried out. It was turned out that mechanical clinching could be successfully employed for joining thermoplastic polymer sheets with aluminium sheets being characterized by a very short processing time and producing high strength joints as compared to competitive technologies. The formability of the aluminium alloy and the toughness of the polymer component are crucial for the success of the process. However, even materials with reduced formability were joined by modifying conveniently the pre-heating time and die geometry.

Research and Preparation of Silver Free Amorphous Active Brazing Alloy

A silver free amorphous Cu-35Ti-12Ni active brazing alloy was successfully prepared in this work. The crystallinity, microstructure, and chemical composition were characterized with X-ray diffraction (XRD), scanning electron microscope (SEM), and energy-dispersive spectrometry (EDS), respectively. A typical characteristic peak of amorphous material was observed in the XRD pattern. The microstructures and chemical compositions of the silver free amorphous alloy were uniform. Differential scanning calorimetry (DSC) result shows that the amorphous silver free brazing alloy has higher melting temperature than commercial silver brazing alloy (Ag-26.5Cu-1.5Ti). Wetting contact angle and spreading area on Si3N4 ceramic substrate were used to evaluate the wetting ability of brazing alloy. The wetting angle was smaller than 5o, and the spreading area was 141.6 mm2 at 1100°C. The bending strength of silver free brazing alloy/Si3N4 was also carried out. The mechanical test shows that the amorphous Cu-35Ti-12Ni/Si3N4 has higher joint strength (304.7MPa) than the crystal Cu-35Ti-12Ni/Si3N4 (294.7MPa) at room temperature.

Laser surface pre-treatment of aluminum for hybrid joints with glass fiber reinforced thermoplastics

Lightweight construction is a major trend in the automotive industry. The connection of fiber reinforced plastics with aluminum is consequently seen as a promising prospect. In this regard, thermal joining is one potential joining technology for bonding of such hybrid joints. Yet a pretreatment of the aluminum surface is necessary to create a load bearing metal-plastic joint. Recent research has shown that a laser surface pretreatment provides high joint strengths. However, there is a variety of laser sources and possible surface topographies with typical structure sizes ranging from macroscopic to nanoscopic profiles. Within this work, two different laser induced structures were generated on aluminum sheets which were subsequently joined to glass fiber reinforced thermoplastics. One of the two laser structures was created with a continuous wave laser using a modified remote ablation cutting process. The other structure was manufactured by a pulsed laser source with a pulse width in the nanosecond-range. The two laser processes were compared with a conventional abrasive blasting process and they were characterized in terms of surface topography and chemical composition of the surface. Furthermore, the achievable joint strength for joining with glass fiber reinforced thermoplastics was investigated. It is shown that the properties of the thermoplastic as well as the surface topography and chemical composition of the metal strongly influence the resulting joint strength.

Microstructural and Mechanical Observations of Galvanized TRIP Steel after Friction Stir Spot Welding

Friction stir spot welding was done in transformation-induced plasticity steel sheets coated with zinc. The influence of tool rotational speed and dwell time on the microstructure and mechanical properties of lap-joints were investigated. After processing, different zones were formed in the joints. Microstructures in each zone depended on the welding conditions employed. Higher dwell time coupled with higher rotational speed promoted the deposition of a large amount of allotriomorphic ferrite beside the keyhole left by the pin. Coalesced bainite formation was stimulated by the deformation. Mechanical and chemical stabilization of the austenite occurred in different welding zones. Some zinc from the coating remained in the joint, in the stirring zone, representing a partial bonding between the steel sheets. The strength of the welds depended on a complex interaction between geometrical features, such as bonding ligament length and distance between the zinc and the keyhole left by the pin and the resultant microstructure in the stirring zone. The highest joint strength was observed for the “lowest tool rotational speed–highest dwell time” combination of welding parameters.

Bonding and high-temperature reliability of NiFeMo alloy/n-type PbTe joints for thermoelectric module applications

PbTe is an extremely important thermoelectric (TE) material, due to its high TE conversion efficiency. Consequently, our effort focuses on developing PbTe-based TE modules, which requires developing novel approaches for bonding metallic contacts to PbTe. In this study, Fe, Mo, and NiFeMo alloy foils were directly bonded to n-type PbTe using a rapid hot press at 600, 700, or 800 °C under a pressure of 40 MPa and for various holding times. We find that in the case of Fe and Mo, it is difficult to form a metallurgically bonded high strength joint with PbTe. However, we find that NiFeMo alloy does effectively bond to PbTe at 700 °C, but not at 600 °C. Significant liquid Pb, which might be due to the reaction of PbTe with Ni, is found that penetrates along the NiFeMo grain boundaries near NiFeMo/PbTe joints during bonding at 700 °C where the extent of liquid Pb penetration can be controlled with the time of bonding. Furthermore, the Seebeck coefficient of bulk PbTe with NiFeMo contacts is similar to that without NiFeMo contacts. Finally, the accelerated thermal aging of NiFeMo/PbTe elements at 600 °C for 240 h shows that the failure mechanism of NiFeMo/PbTe joints under operating conditions is the continued formation and penetration of eutectic liquid NiFeMo–PbTe and liquid Pb along the NiFeMo grain boundaries.

Technology of brazing and heat treatment of brazed joints in creep-resisting deformable (EP975) and cast single crystal intermetallic (VKNA-4U) alloys

The problems of producing high-strength permanent joints in dissimilar nickel-based alloys are investigated. The results of investigation of the effect of the brazing and heat treatment conditions on the structure of the brazed joints in the EP975 deformed alloy and VKNA-1U intermetallic single crystal alloy are presented.

К созданию клеевых соединений древесины повышенной прочности

К созданию клеевых соединений древесины повышенной прочности

Experimental Research on Ultrasonically Welded Copper

Ultrasonic processing proves particularly effective in leading branches of technology, such as: precision mechanics, optics, electrical engineering, nuclear technology etc. High-strength joints, short machining time and low energy consumption are only three of the main ultrasonic welding advantages that determined research intensification in this field. This paper presents experimental data that illustrates the interdependence between some process parameters and their influence on the joint resistance of the copper sheet ultrasonically welded. Due to the surface friction energy, which is generated in the ultrasonic welding process, some intermolecular connections are broken and other are new are created. In order to demonstrate the quality of the ultrasonically welded copper, there were made macro and microstructural analysis that are proving an excellent metallurgical bonding. The results of microhardness tests, that were made in five different points, on each welded copper sample, revealed the ultrasonic welding process quality showing that the welded material hardness doesn't change. The copper welding experiments were made at the National Research and Development Institute for Welding and Material Testing (ISIM) Timişoara.

On the von Mises elastic stress evaluations in the bi-adhesive single-lap joint: a numerical and analytical study

Bi-adhesive joints are an alternative stress-reduction technique for adhesively bonded joints. The joints have two types of adhesives in the overlap region. The stiff adhesive should be located in the middle and the flexible adhesive at the ends. This study is the extension of our previous paper to the von Mises stress evaluation and discusses the values and importance of the von Mises stresses in the bi-adhesive single-lap joint. Both analytical and numerical analyses were performed using three different bi-adhesive bondline configurations. The Zhao’s closed form (analytic) solution used includes the bending moment effect. In the finite element models, overlap surfaces of the adherends and the adhesives were modeled using surface-to-surface contact elements. The contribution levels of the peel and shear stresses for producing a peak von Mises stress are also studied. It is concluded that the contribution level of the shear stress at where von Mises stress becomes peak is more than that of the peel stress. Joint strength analyses were performed based on the peak elastic von Mises stresses. It is seen that joint strength can be increased using bi-adhesive bondline. The analytical and numerical results show that the appropriate bond-length ratio must be used to obtain high joint strength.

HARDNESS AND TENSILE BEHAVIOR OF FRICTION STIR WELDED ALUMINUM AA6351 ALLOY

Friction Stir Welding (FSW) is a solid state welding process gaining more applications in various industries due to better quality of the joint as it has no effect on parent metal. In FSW process a non consumable rotating welding tool is used to generate frictional heat between tool and abutting surface of work piece and plastic dissipation of energy to accomplish the weld. Being a solid state joining process, friction stir welding process offers various advantages like low distortion, absence of melt related defects, high joint strength etc. as compared to other conventional fusion welding techniques.Experiments were conducted on 6 mm thickness Aluminum AA6351-T4, commercially available plates. The seplates are joined by FSW along the rolling direction (longitudinal weld orientation) and across the rolling direction (transverse weld orientation). The hardness and tensile strength results of the weldments are presented. Results show superior mechanical properties for the joints with plates along the rolling direction as compared with the joints obtained by across the rolling direction.DOI: http://dx.doi.org/10.3329/jme.v44i1.19494

Properties of X80 Pipeline Girth Welds for Different Welding Procedures

The X80 girth welds were produced by solid-wire gas metal arc welding (GMAW) and shield metal arc welding (SMAW) using two welding consumables respectively, which contained different mass fraction of C, Mo and Ni. The tensile properties, notch toughness, hardness, and microstructures of welded joints were evaluated. The results indicate that high strength and good toughness of welded joints can be achieved. But the tensile properties of all weld metal of GMAW and SMAW process were evidently different because of the difference of mass fraction of C, Mo, Ni. The strength reduced slightly in softening zone of HAZ. Using welding consumable which contain higher Mo additions, the microstructure in filler weld and fusion zones were IAF+GB and GB+M respectively. Furthermore, the mechanical properties of X80 pipeline welded by single wire welding and double wire welding respectively have been compared. The double wire welds exhibited lower yield strength but higher toughness compared to the corresponding single wire welds.

Friction stir keyholeless spot welding of AZ31 Mg alloy-mild steel

Friction stir keyholeless spot welding (FSKSW) using a retractable pin for 1.0 mm thick galvanized mild steel and 3 mm thick AZ31B magnesium alloy in a lap configuration was investigated. The process variables were optimized in terms of the joint strength. The effects of the stacking sequence on joint formation and the joining mechanism of FSKSW AZ31B-to-mild steel joints were also analyzed. It shows that the process window and joint strength are strongly influenced by the stacking sequence of the workpieces. While the process window is narrow and unstable for FSKSW of a magnesium-to-steel stack-up, a desirable process was established for the steel-to-magnesium stacking sequence, a desirable process and higher strength joint can be got when the steel-to-magnesium stacking sequence. XRD phase and EPMA analyses of the FSKSW joint showed that the intermetallic compounds are formed at the steel-to-magnesium interface, and the element diffusion between the mild steel and AZ31B magnesium alloy revealed that the joining methods for FSKSW joints is the main mechanical joining along with certain metallurgical bonding.

Weldability of titanium and nickel with alloy filler addition and different electrodes tip shapes by using micro spot brazing method

The weldability of titanium and nickel using micro spot brazing technology was successfully investigated. Brazing was performed using a 4-kA welding current, 2-V voltage, 60-N load, and a 25–50-ms welding time. An alloy filler metal, specifically a 0.1-mm thick foil with a composition of 71Ag-28Cu-1Mg, was sandwiched between Ti and Ni. The three electrode copper tip shapes that were investigated in this study were circular-, rectangular-, and ring-shaped tips. The weldability of the joint was evaluated in terms of the fracture load, weld size, shear strength, and micro hardness. A high joint strength was obtained in the case of the 71Ag-28Cu-1Mg filler when a rectangular electrode tip face was used.

Influence of Surface Pre-treatments on Laser Welding of Ti6Al4V Alloy

In the present study, Ti6Al4V samples have been welded under conduction regime by means of a high power diode laser. The main objective of the work has been to determine the actual influence of the surface pre-treatments on the laser welding process. Thus, six different pre-treatments were applied to Ti6Al4V samples before performing bead-on-plate and butt welding treatments. The depth, width, microstructure, and microhardness of the different weld zones were deeply analyzed. Grinding, sandblasting, and chemical cleaning pre-treatments lead to welds with the highest depth values, presenting high joint strengths. Treatments based on the application of dark coatings generate welds with lower penetration and worse mechanical properties, specially the graphite-based coating.