Weld Seam Strength Research Articles

Enabling laser transmission welding of additively manufactured thermoplastic parts using an expert system based on neural networks

In order to use laser transmission welding (LTW) for additively manufactured parts such as prototypes, small series, or one-off products, an enhanced process knowledge is needed to overcome the difficulties in the part composition resulting from the additive manufacturing process itself. In comparison to an injection molding process for thermoplastic parts, the additive manufacturing process fused deposition modeling leads to an inhomogeneous structure with trapped air inside the volume. In this paper, a neural network-based expert system is presented that provides the user with process knowledge in order to improve the weld seam quality of laser welded additively manufactured parts. Both additive manufacturing and LTW process are assisted by the expert system. First, the designed expert system supports the user in setting up the additive manufacturing process to increase the transmissivity. During welding, the additive manufacturing and LTW process parameters are used to predict the weld seam strength. To create the database for the expert system, specimens of transparent and black polylactide are additively manufactured. In order to change the transmissivity at an emission wavelength of 940 nm of the diode laser used, the manufacturing parameters for the transparent parts are varied. The transmissivity of the parts is measured with a spectroscope. The transparent samples are welded to the black samples with laser powers between 8 and 14 W in the overlap configuration and shear tensile tests are performed. In this work, the predictions of the transmissivity and the shear tensile force are demonstrated with an accuracy of more than 88.1% of the neural networks used for the expert system.

Experimental Investigation of Temperature and Contact Pressure Influence on HFI Welded Joint Properties.

This paper presents an experimental electro-thermo-mechanical simulation of high-frequency induction (HFI) welding to investigate the effect of temperature and contact normal stress on the weld seam quality. Therefore welding experiments at different temperatures and contact pressures are performed using flat specimens of 34MnB5 steel sheet. In order to characterize the weld seam strength of the welded specimens, tensile and bending tests are performed. To obtain a relative weld seam strength, the bending specimens were additionally hardened prior to testing. With the hardened specimens, it can be shown that the weld seam strength increases with increasing temperature and contact normal stress until a kind of plateau is formed where the weld seam strength remains almost constant. In addition to mechanical testing, the influence of the investigated process parameters on the weld seam microstructure is studied metallographically using light optical microscopy, scanning electron microscopy, EBSD and hardness measurements. It is shown that the weld seam strength is related to the amount of oxides in the bonding line.

Effect of ultrasonic welding process parameters on seam strength of PVC-coated hybrid textiles for weather protection

Using a lapped seam, PVC-coated hybrid textiles with uniform thickness were bonded by continuous ultrasonic welding and conventional joining method with the help of hot air tape welding technique for weather protection purposes. Three fundamental sewing parameters at two distinct levels and three primary welding parameters at three levels based on 6 and 12 mm welding widths were used. To consider the effect of welding and sewing parameters on seam strength, full factorial designs of experiments were designed, fabricated, and tested. The thermal behavior and possibility of chemical conversion in the welding zone under the influence of ultrasonic vibrations were examined. Variation in width of heat-affected zone of weld seam was measured. The seam strength of ultrasonic weld seam compared with that of conventional seams, and superior seam strength yielding parametric levels were assessed. The parametric influence of both joining techniques on seam quality and their tendencies in the relationship were analyzed statistically. The weld seam strength (1256.392 and 2116.93 N/50 mm) was optimized numerically and identified its trend with the variation of the weld seam. The discovered relationship led to the conclusion that the variation in the weld seam can be used to estimate the tensile strength of the weld seam through the developed effective numerical model as a non-destructive testing method, and its outcome was successful as a destructive testing method. The result shows that the ultrasonic weld seam provided a higher tensile strength ( > 75%) than the conventional seam for both evaluated welding widths and obtained statistically significant results.

Automatically controlled laser-based welding process for repair of CFRP parts

Due to a high strength-to-weight ratio, carbon fiber-reinforced plastics (CFRPs) have become a standard in the lightweight industry, which results in the development of new machining and repairing methods. Specifically, CFRP with a thermoplastic matrix material is more attractive for many applications as a thermosetting matrix material. Thermoplastic matrix materials enable new joining processes due to their thermoformability and weldability. Currently, the repair process is mainly executed manually. In order to reduce repair costs, an automated, reliable, and fast process is needed. For the new laser-based repair process, first a scarf joint is prepared in a CFRP laminate consisting of a thermoplastic polyphenylene sulfide matrix material. To refill the scarf, a patch with the same matrix material and fiber setup as the original laminate is cut into shape and welded into the scarf area. In order to obtain a reliable and stable welding process resulting in high weld seam qualities, an automated controlled welding process is being used, which controls the laser power via the surface temperature, which is detected by a pyrometer. For the development of the welding process, the influence of the welding time and the surface temperature on the resulting weld seam quality were evaluated. For the analysis of the joining quality, overlap samples were welded and tested in order to determine the weld seam strength. In addition, cross sections were prepared and analyzed with a microscope. Finally, the measured results were correlated in order to determine a parameter set generating a high-quality weld seam.

Investigation of Specific FDM Process Parameters to Optimize the Polymer Discharge of Carbon Fiber Reinforced PEEK

AbstractFused deposition modeling (FDM) is a widely used manufacturing process and the materials market is growing due to the increased requirements on materials. In particular, processing of high‐temperature materials such as PEEK is challenging. The aim of these studies is to investigate and improve the processability of a carbon fiber reinforced polyether ether ketone (PEEK) material in the FDM process. The first step is to define the relevant process parameters, which remain constant over the entire investigation. In particular, the material‐specific temperatures must be defined. Based on previous investigations regarding the weld seam strength and the warpage of the selected PEEK materials, a method to determine and adjust essential process parameters should be developed. One approach is the consideration of the volumetric polymer discharge at a stationary filament feed rate. The optimized adjustment of the feed rate ensures increased component densities and an improved process reliability. In this context, the component areas with non‐stationary extruder movement, such as in corner areas, have got a huge influence on the component quality. The acceleration and deceleration processes do not result in a desired uniform strand geometry due to the pressure changes within the nozzle. The acceleration can result in thinner strands while the deceleration can lead to a thickening of the polymer discharge. The aim is to adjust the pressure inside the nozzle depending on the acceleration profile to enable a homogenous strand geometry. The method should therefore allow the improvement of the processability of PEEK materials.

Small-scale resistance seam welding of 304 stainless steel with capacitor discharge welding machine

In this paper, small scale resistance seam welding (SSRSEW) of 304 stainless steel sheet with a thickness of 0.1 mm with a capacitor discharge (CD) welding machine is investigated. The effect of the main parameters such as discharged energy, electrode force, and electrode speed on the quality of the weld seam was investigated. In order to control the speed of the electrode, the electrode wheel was mounted on a CNC machine. Mechanical tests such as tensile-shear and peel tests were used to evaluate the quality of the weld seam. Also, the failure modes were investigated in different welding conditions. The results show that the discharged energy has more effect on the weld seam strength and the maximum strength in the peel test is usually lower than the strength of the tensile-shear test. Also, discharge energy level of 14 ws and electrode force of 20 N and electrode speeds of 200 and 300 mm/min are the optimum welding condition for welding the 304 stainless steel sheets with a thickness of 0.1 mm.

Vibration welding of components with angled areas in the direction of vibration

Conventional manufacturing processes for plastic products, such as injection molding or extrusion, often limit the achievable component geometries. Therefore, it is necessary to join components in order to generate highly complex geometries. Vibration welding is one way of joining components. This process is frequently used and is characterized by short cycle times, high energy efficiency, and the possibility of joining large components. In vibration welding, plastic components are heated by an oscillating friction movement of the joining surfaces, then plasticized and subsequently welded together. The joining of three-dimensional seam geometries is therefore a challenge for vibration welding, as the components can be lifted off by the linear movement and the surfaces do not plasticize sufficiently. Previous investigations have shown that angles of up to 20° can be welded in the direction of vibration, but that the deviation from the plane considerably reduces the weld strength. In order to weld three-dimensional weld seam geometries with short cycle times and simultaneously achieve a high weld seam strength, a process is being developed which is intended to extend the design freedom in vibration welding.

Research on the stripping performance during dual laser-beam bilateral synchronous welding of 2219 aluminum alloy T-joint for spacecraft

The promising dual laser-beam bilateral synchronous welding (DLBSW) technology was employed in the 2219 aluminum alloy T-joint which consisted of the skin and stringer components. The coherent weld seam (WS) is achieved under the heat affect of dual laser beam. The stripping experiment was carried out to test the stripping strength of DLBSW joint. With the purpose of discussing the relationship between the welding shape and stripping strength, H1 was measured to evaluate the shape of welded T-joint. The microstructure characteristics of T-joint were observed to discuss the crack location during striping experiment. Besides, the fracture morphology was observed by scanning electron microscope (SEM). The chemical element composition on the impure phase in the fracture was detected by energy dispersive spectrometer (EDS). The highest stripping strength is 317.46 MPa with the laser power of 5500 W and welding speed of 4.2 m/min. The stripping crack locations are mainly divided into three categories. The highest stripping strength is achieved under the second category of crack location which is taken place near the upper fusion line. It is indirectly found that the strength of WS in case 6 with the laser power of 5500 W is highest than the others. The degrading of stripping strength is mainly owing to the reducing of plasticity as well as the formation of internal defects such as pores, bonded structure and nubby impure phases. The nubby impure phase is mainly metal oxides (TiO2, Al2O3) which generally degrade the weld quality. It is notable that a new skin component accompanied with two bosses is firstly designed in the present study to replace the wire filler.

Very High Cycle Fatigue Behaviour of the 316L Weldment Fabricated By Laser Butt-Welding

In nuclear industry, 316L weldments are widely used in components fabricating, which usually suffer from very-high-cycle fatigue. Therefore, the very high cycle fatigue (VHCF) behaviour of 316L weldment fabricated by laser butt-welding joint were studied using an ultrasonic fatigue testing system. The microscopic examination indicated that an evident pore is observed in the weld seam and the fine columnar-dendrite structure is formed in the central part of the weld seam. Near the weld interface in the weld seam, the weld microstructure shows larger columnar-dendrite. The hardness distribution of the weldment is to coincide with the characteristic of the microstructure. In the central part of the weld seam, the hardness is the highest, and is the lowest in the base metal. Fatigue failure still occurred in the VHCF regime. The fatigue crack initiated from the internal pores caused by laser butt-welding. Although the fatigue strength of weld seam might be decreased due to the pore, the fatigue strength of the weldment was the same as the base metal in this research. That is, the defect with a certain size in the weld seam does not affect the strength of the weldment.

Influence of laser parameters on tensile shear strength of copper welds

The electrification of the power train in the automotive industry leads to the requirement of electrical connectors with high ampacities. These connectors are often realized using ultrasonic bonding technology. Substitution of ultrasonic welding by laser welding allows an increase of ribbon cross section by a factor of 5 and thus a similar increase of the maximum transferred current. The increased cross section leads to higher loads (at least by a factor of 5) applied to the weld seam during the process. To minimize the heat input, the weld seam strength in shear direction shall be maximized. The influence of different laser parameters on the ultimate tensile shear stress in lap joint configuration is investigated, based on representative specimen geometries (10 × 0.3 mm2). The variable parameters are: laser wavelength (IR around 1050 nm + frequency doubled 515 nm), focal diameters (42–300 μm), different temper conditions of the base material, process parameters (laser power 0.6–4 kW and feed rate 50–800 mm/s), and welding strategies (single pass welding, spatial beam modulation). The material used is Cu-ETP (>99.9% Cu). For all the investigated parameters, the observed failure mode is “fracture in the fusion zone.” The ultimate tensile shear stress for all experiments is around 188 N/mm2 even for parameter changes in the order of one magnitude. This is in contrast to the assumed relation between laser parameters and the mechanical properties. This contrast will be discussed.

Analysis of the Weld Seam Morphology of Polypropylene in Laser Transmission Welding

Laser transmission welding is a well-known joining technology for welding thermoplastics. Although the process is already used industrially, fundamental process-structure-property relationships are not fully understood and are therefore the subject of current research. One aspect of these mentioned process-structure-property relationships is the interaction between the temperature field during the welding process, the weld seam morphology of semi-crystalline thermoplastics, and the weld seam strength. In this study, the influence of the line energy on the weld seam morphology of polypropylenes is analyzed. For this purpose, the size of spherulites in the weld seam is investigated, as well as different occurring phases of polypropylene (α- and β-phase). It is shown that both the spherulite size of the α-phase and the amount of β-phase increase with increasing line energy. For the explanation and discussion of the results, a temperature-dependent thermal simulation model is used to derive characteristic attributes of the temperature field (maximum temperatures, cooling rates, temperature gradients).

Laser transmission welding of Clearweld-coated polyethylene glycol terephthalate by incremental scanning technique

Transmission laser welding using Incremental Scanning Technique(TWIST) mode and conventional contour welding mode were adopted to investigate laser transmission welding of 0.5mm thick PET plate. A 1064nm fiber laser was used to weld PET at the (TWIST) mode, and an 808nm diode laser was applied to conduct the conventional contour welding. The Clearweld coating was used as laser absorbing material. The influences of laser parameters (i.e. defocusing distance, distance between two circles) on the quality of weld seams were analyzed by optical microscopy. Moreover, geometry and shear strength of the weld zone were tested to optimize laser parameters. Additionally, the water vapor permeability (WVP) of weld seams was measured to test hermetical capacity. Results show that the shear strength and hermetic capacity of weld seam by TWIST mode are at the same level in comparison with that of the conventional contour welding.

The Influence of Laser Welding Processes on the Weld Seam Quality of Thermoplastic Composites with High Moisture Content

Presently, fiber reinforced materials are used for many industrial applications. Laser transmission welding (LTW) is a suitable method to join these materials. For multiple thermoplastics, the weld seam quality is affected by the amount of moisture content in the joining members. For LTW, heat is applied to the welding members and water in the composite changes phase from liquid to gas. Increasing the gas content can lead to detrimental cavities in the weld seam.Therefore, experiments were performed to investigate the influence of the laser focal geometry on the generation of cavities. Different focal geometries lead to different levels of process heat generation which affects the amount of vaporization. The experiments were conducted with endless glass fiber reinforced polyetherimide and endless carbon fiber reinforced polyetherimide. These samples were tested for their weld seam strength and the results were correlated to the moisture content and laser beam geometry.

Ultrasonic welding of polyamide—influence of moisture on the process relevant material properties

The welding of hygroscopic materials such as polyamide can lead to unstable conditions during the welding process. Due to changing material properties, the ultrasonic welding process is influenced heavily by the moisture level of the welding parts. To achieve stable welding processes and high weldline qualities, it is necessary to understand the influence of moisture on the material properties and the ultrasonic welding process. To perform a scientific examination of the influence of moisture on the ultrasonic welding process, the interactions between the material properties and the welding process are determined in relation to the moisture content. With the aid of welding tests, it can be shown that with constant welding parameters, the attainable weld strength decreases with increasing moisture load. Microscopy analyses show gas bubbles in the weld seam. These are formed during the welding process, because of the evaporation of water in the welding zone. Furthermore, it can be seen that the time required to reach a predetermined welding path depends on the moisture content. At first, the welding time increases with increasing moisture content and is decreasing again with further moisture load. In order to explain the welding behaviour, material tests are carried out. Especially, the material damping influences the welding process largely in case of ultrasonic welding. The material damping in dependency of temperature and frequency can be described by storage and loss modulus. To determine loss and storage modulus at the welding frequency of 20 kHz, dynamic mechanical analyses are carried out at different temperatures and frequencies. The results are extrapolated by making use of time/temperature shift to 20 kHz. The results show that at 20 kHz, the maximum material damping is not exceeded in case of the maximum moisture load but at the same moisture load as in case of the maximum welding times. Further welding tests show that the welding time can be decreased again by choosing a higher welding amplitude, which leads to a higher energy input in the specimens. However, the weld seam strength cannot be increased to the stage of the dry-welded specimens by adapting the welding parameters, because of the formation of gas bubbles in the weld seam strength.

Correlation analysis of the variation of weld seam and tensile strength in laser welding of galvanized steel

Many advantages of laser welding technology such as high speed and non-contact welding make the use of the technology more attractive in the automotive industry. Many studies have been conducted to search the optimal welding condition experimentally that ensure the joining quality of laser welding that relies both on welding system configuration and welding parameter specification. Both non-destructive and destructive techniques, for example, ultrasonic inspection and tensile test are widely used in practice for estimating the joining quality. Non-destructive techniques are attractive as a rapid quality testing method despite relatively low accuracy. In this paper, we examine the relationship between the variation of weld seam and tensile shear strength in the laser welding of galvanized steel in a lap joint configuration in order to investigate the potential of the variation of weld seam as a joining quality estimator. From the experimental analysis, we identify a trend in between maximum tensile shear strength and the variation of weld seam that clearly supports the fact that laser welded parts having larger variation in the weld seam usually have lower tensile strength. The discovered relationship leads us to conclude that the variation of weld seam can be used as an indirect non-destructive testing method for estimating the tensile strength of the welded parts.

Evaluation of Molten Zone in Micro-welding of Glass by Picosecond Pulsed Laser

Glass materials have been widely used in optics, MEMS, electronics and biomedical applications because of their excellent optical, mechanical and chemical properties. Fusion welding by ultrashort pulsed laser has been proposed as a promising technique for glass joining, since joining can be accomplished without an intermediate layer and mechanical contact. However, to apply a fusion welding technique by ultrashort pulsed laser as a reliable joining method, it is important to understand the mechanical properties of the weld seam since glass materials are very sensitive to temperature gradients. Therefore, a method of evaluating the weld seam in the fusion welding of glass by picosecond pulsed laser was discussed, and its mechanical strength was investigated. A larger optical contact area had a great influence on the shear load of the weld seam, while a smaller optical contact area could reduce the influence of the contact area on the measurement of shear load. Therefore, it was important to reduce the optical contact area to accurately evaluate the shearing strength of weld seam. The mechanical strength of the weld seam reduced at a low pulse repetition rate. High bending strength and shear stress could be obtained at a proper pulse repetition rate.

Cycletime Reduction by Forced Air Cooling for Hot Plate Welding

Hot plate welding is one of the most popular methods for joining plastics by utilizing heat conduction to plasticize and join thermoplastics. In comparison to competitive welding processes, hot plate welding generates high weld seam qualities but is characterized by a long cycle time; from 20 s, for small parts, to about 30 min. Cycle time itself is dominated by the cooling time. This paper reports on the influence of different cooling methods to reduce the cooling time for hot plate welding of polymers. In the initial investigations, water and nitrogen were used as cooling media (free convection) and were compared with compressed air (forced convection). The results of the initial investigations lead to more detailed analyses of forced cooling by compressed air. In the subsequent investigations, different parameters of compressed air were examined to reduce cooling time while retaining the weld seam quality. The parameters of compressed air: air flow, temperature and humidity were analysed to optimize the cooling time for amorphous and semi-crystalline polymers. It was found that the reduction of cooling time by compressed air leads to good results. Cooling time savings of 85 % could be achieved. Additionally, the weld seam quality was investigated by measuring the weld seam strength and analysing residual stresses. The weld seam strength is not influenced by the cooling method, but the cooling method influences the size of the stress field of the welded specimens.

Laser transmission welding of high-performance polymers and reinforced composites - a fundamental study

This article presents investigations into the weldability of high-performance polymers and carbon fiber reinforced thermoplastics (CFRP) using laser transmission welding techniques. Fundamental studies of the absorption characteristics of CFRP as joining partner and the heat distribution within the surface due to laser impact have been performed. The welding process has been analyzed with respect to the properties of the weld seam and lap shear strength tests have been realized for different material combinations.