Estimation and control of droplet size and frequency in projected spray mode of a gas metal arc welding (GMAW) process

New estimators are designed based on the modified force balance model to estimate the detaching droplet size, detached droplet size, and mean value of droplet detachment frequency in a gas metal arc welding process. The proper droplet size for the process to be in the projected spray transfer mode is determined based on the modified force balance model and the designed estimators. Finally, the droplet size and the melting rate are controlled using two proportional-integral (PI) controllers to achieve high weld quality by retaining the transfer mode and generating appropriate signals as inputs of the weld geometry control loop.

The fatigue life of a cobalt-chromium alloy after laser welding

The aim of this study was to investigate the fatigue life of laser welded joints in a commercially available cast cobalt-chromium alloy. Twenty rod shaped specimens (40 mm x 1.5 mm) were cast and sand blasted. Ten specimens were used as controls and the remaining ten were sectioned and repaired using a pulsed Nd: YAG laser welder. All specimens were subjected to fatigue testing (30N - 2Hz) in a controlled environment. A statistically significant difference in median fatigue life was found between as-cast and laser welded specimens (p < 0.001). Consequently, the technique may not be appropriate for repairing cobalt chromium clasps on removable partial dentures. Scanning electron microscopy indicated the presence of cracks, pores and constriction of the outer surface in the welded specimens despite 70% penetration of the weld.

[Effectiveness of adjustment of bar constructions on implants by hydrogen and laser welding, metal soldering and electroerosive processing]

Marginal fit of bar constructions on dental implants was revealed. It's investigated success of correction with soldering, laser welding, "cast to" method and spark erosion.

Impact of two CO(2) laser heatings for damage repairing on fused silica surface

CO(2) laser is an interesting tool to repair defects on silica optics. We studied UV nanosecond laser-induced damage in fused silica after CO(2) laser heating. The localization of damage sites and the laser damage threshold are closely related to stress area in silica induced by heating. By applying a suitable second laser heating, we managed to eliminate the debris issued from redeposited silica and to modify the stress area. As a consequence, a significant increase of laser resistance has been observed. This process offers the possibility to improve damage repairing sufficiently to extend the lifetime of the silica components.

Hexavalent chromium exposure and control in welding tasks

Studies of exposure to the lung carcinogen hexavalent chromium (CrVI) from welding tasks are limited, especially within the construction industry where overexposure may be common. In addition, despite the OSHA requirement that the use of engineering controls such as local exhaust ventilation (LEV) first be considered before relying on other strategies to reduce worker exposure to CrVI, data on the effectiveness of LEV to reduce CrVI exposures from welding are lacking. The goal of the present study was to characterize breathing zone air concentrations of CrVI during welding tasks and primary contributing factors in four datasets: (1) OSHA compliance data; (2) a publicly available database from The Welding Institute (TWI); (3) field survey data of construction welders collected by the Center for Construction Research and Training (CPWR); and (4) controlled welding trials conducted by CPWR to assess the effectiveness of a portable LEV unit to reduce CrVI exposure. In the OSHA (n = 181) and TWI (n = 124) datasets, which included very few samples from the construction industry, the OSHA permissible exposure level (PEL) for CrVI (5 μg/m(3)) was exceeded in 9% and 13% of samples, respectively. CrVI concentrations measured in the CPWR field surveys (n = 43) were considerably higher, and 25% of samples exceeded the PEL. In the TWI and CPWR datasets, base metal, welding process, and LEV use were important predictors of CrVI concentrations. Only weak-to-moderate correlations were found between total particulate matter and CrVI, suggesting that total particulate matter concentrations are not a good surrogate for CrVI exposure in retrospective studies. Finally, in the controlled welding trials, LEV reduced median CrVI concentrations by 68% (p = 0.02). In conclusion, overexposure to CrVI in stainless steel welding is likely widespread, especially in certain operations such as shielded metal arc welding, which is commonly used in construction. However, exposure could be substantially reduced with proper use of LEV.

Flash nano-welding: investigation and control of the photothermal response of ultrathin bismuth sulfide nanowire films

Ultrathin Bi₂S₃ nanowires undergo a pronounced photothermal response to irradiation from a commercial camera flash. Controlled nano-welding was shown by using single walled carbon nanotube mats as an electrically and thermally conductive substrate. The resulting welded nanowire film is denser and has significantly lower resistance than unflashed bilayer films.

Optimization of suture-free laser-assisted vessel repair by solder-doped electrospun poly(ε-caprolactone) scaffold

Poor welding strength constitutes an obstacle in the clinical employment of laser-assisted vascular repair (LAVR) and anastomosis. We therefore investigated the feasibility of using electrospun poly(ε-caprolactone) (PCL) scaffold as reinforcement material in LAVR of medium-sized vessels. In vitro solder-doped scaffold LAVR (ssLAVR) was performed on porcine carotid arteries or abdominal aortas using a 670-nm diode laser, a solder composed of 50% bovine serum albumin and 0.5% methylene blue, and electrospun PCL scaffolds. The correlation between leaking point pressures (LPPs) and arterial diameter, the extent of thermal damage, structural and mechanical alterations of the scaffold following ssLAVR, and the weak point were investigated. A strong negative correlation existed between LPP and vessel diameter, albeit LPP (484±111 mmHg) remained well above pathophysiological pressures. Histological analysis revealed that thermal damage extended into the medial layer with a well-preserved internal elastic lamina and endothelial cells. Laser irradiation of PCL fibers and coagulation of solder material resulted in a strong and stiff scaffold. The weak point of the ssLAVR modality was predominantly characterized by cohesive failure. In conclusion, ssLAVR produced supraphysiological LPPs and limited tissue damage. Despite heat-induced structural/mechanical alterations of the scaffold, PCL is a suitable polymer for weld reinforcement in medium-sized vessel ssLAVR.

Evaluation of shear bond strength of porcelain bonded to laser welded titanium surface and determination of mode of bond failure

The aim of this study was to evaluate the shear bond strength of porcelain to laser welded titanium surface and to determine the mode of bond failure through scanning electron microscopy (SEM) and energy dispersive spectrophotometry (EDS). Forty five cast rectangular titanium specimens with the dimension of 10 mm x 8 mm x 1 mm were tested. Thirty specimens had a perforation of 2 mm diameter in the centre. These were randomly divided into Group A and B. The perforations in the Group B specimens were repaired by laser welding using Cp Grade II titanium wire. The remaining 15 specimens were taken as control group. All the test specimens were layered with low fusing porcelain and tested for shear bond strength. The debonded specimens were subjected to SEM and EDS. Data were analysed with 1-way analysis of variance and Student's t-test for comparison among the different groups. One-way analysis of variance (ANOVA) showed no statistically significant difference in shear bond strength values at a 5% level of confidence. The mean shear bond strength values for control group, Group A and B was 8.4 +/- 0.5 Mpa, 8.1 +/- 0.4 Mpa and 8.3 +/- 0.3 Mpa respectively. SEM/EDS analysis of the specimens showed mixed and cohesive type of bond failure. Within the limitations of the study laser welding did not have any effect on the shear bond strength of porcelain bonded to titanium.

Immediate definitive rehabilitation of the edentulous patient using an intraorally welded titanium framework: a 3-year prospective study

OBJECTIVE

The aim of this prospective study was to evaluate the concept of intraoral welding as a suitable technique for the placement of a final restoration in the edentulous patient on the same day as surgery.

METHOD AND MATERIALS

Any patient with a completely edentulous arch who was considered eligible received a fixed restoration supported by an intraorally welded titanium bar. Definitive abutments were connected to the implants and then welded to a titanium bar using an intraoral welding unit. This framework was used to support the definitive acrylic restoration, which was fitted on the same day as implant placement. Restoration and implant success, mean marginal bone loss, pocket probing depth, and bleeding on probing were assessed over a 36-month follow-up period.

RESULTS

Twenty-six patients with an edentulous maxilla and 34 patients with an edentulous mandible, with a mean age of 57.1 years (SD = 17.9, n = 60), were consecutively treated with 324 immediately loaded implants. No fractures or radiographically detectable alteration of the welded framework was evident. A total of 321 (99.1%) implants osseointegrated and were clinically stable at the 6-month follow-up. At the 36-month follow-up, the accumulated mean marginal bone loss was, respectively, 0.967 mm (SD = 0.361) for the maxillary cases and 1.016 mm (SD = 0.413) for the mandibular cases.

CONCLUSIONS

It is possible on the same day of surgery to successfully rehabilitate the edentulous patient with a fixed definitive prosthesis supported by an intraorally welded titanium framework.

Exploring infrared sensoring for real time welding defects monitoring in GTAW

This paper presents an evaluation of an infrared sensor for monitoring the welding pool temperature in a Gas Tungsten Arc Welding (GTAW) process. The purpose of the study is to develop a real time system control. It is known that the arc welding pool temperature is related to the weld penetration depth; therefore, by monitoring the temperature, the arc pool temperature and penetration depth are also monitored. Various experiments were performed; in some of them the current was varied and the temperature changes were registered, in others, defects were induced throughout the path of the weld bead for a fixed current. These simulated defects resulted in abrupt changes in the average temperature values, thus providing an indication of the presence of a defect. The data has been registered with an acquisition card. To identify defects in the samples under infrared emissions, the timing series were analyzed through graphics and statistic methods. The selection of this technique demonstrates the potential for infrared emission as a welding monitoring parameter sensor.

Modelling the attenuation in the ATHENA finite elements code for the ultrasonic testing of austenitic stainless steel welds

Multipass welds made in austenitic stainless steel, in the primary circuit of nuclear power plants with pressurized water reactors, are characterized by an anisotropic and heterogeneous structure that disturbs the ultrasonic propagation and makes ultrasonic non-destructive testing difficult. The ATHENA 2D finite element simulation code was developed to help understand the various physical phenomena at play. In this paper, we shall describe the attenuation model implemented in this code to give an account of wave scattering phenomenon through polycrystalline materials. This model is in particular based on the optimization of two tensors that characterize this material on the basis of experimental values of ultrasonic velocities attenuation coefficients. Three experimental configurations, two of which are representative of the industrial welds assessment case, are studied in view of validating the model through comparison with the simulation results. We shall thus provide a quantitative proof that taking into account the attenuation in the ATHENA code dramatically improves the results in terms of the amplitude of the echoes. The association of the code and detailed characterization of a weld's structure constitutes a remarkable breakthrough in the interpretation of the ultrasonic testing on this type of component.

Joining and forming using torsional ultrasonic principles

Developments in torsional ultrasonic joining mean that it is now used in a diverse range of joining, forming, selective weakening and "break-off" applications in the medical device industry.The principles and benefits of the technique are described together with application examples.

A method for joining polymers and metals or polymers and ceramics

A recently developed laser fusion technique allows polymers to be attached to metals and ceramics in short cycle times. The process and its capabilities are reviewed together with its growing range of applications.

Bottom-up nanoconstruction by the welding of individual metallic nanoobjects using nanoscale solder

We report that individual metallic nanowires and nanoobjects can be assembled and welded together into complex nanostructures and conductive circuits by a new nanoscale electrical welding technique using nanovolumes of metal solder. At the weld sites, nanoscale volumes of a chosen metal are deposited using a sacrificial nanowire, which ensures that the nanoobjects to be bonded retain their structural integrity. We demonstrate by welding both similar and dissimilar materials that the use of nanoscale solder is clean, controllable, and reliable and ensures both mechanically strong and electrically conductive contacts. Nanoscale weld resistances of just 20Omega are achieved by using Sn solder. Precise engineering of nanowelds by this technique, including the chemical flexibility of the nanowire solder, and high spatial resolution of the nanowelding method, should result in research applications including fabrication of nanosensors and nanoelectronics constructed from a small number of nanoobjects, and repair of interconnects and failed nanoscale electronics.

A review of micro/nano welding and its future developments

A review is given about the current state-of-the-art surrounding micro/nano scale welding/joining used in miniaturization or microsystems. Except for reviewing the applications of micro/nano welding/joining in various fields, such as microelectromechanical system (MEMS), micro total-analysis systems (microTAS), carbon nanotubes (CNTs) etc., the optimal equipments for micro/nano welding/joining, its corresponding future applications and research directions are discussed. Moreover, an extensive part of the paper is devoted to the introduction of new techniques, micro/nano laser and electron beam welding/joining. In turn, it is anticipated these techniques will be used in future micro/nano welding/joining applications. Meanwhile, because even smallest particles of dust can influence the results, more considerations are given to machine improvements that are required for its corresponding nano welding/joining. It will be one challenge for R&D efforts to combine the progress in micro/nano scales with that in miniaturization. This review provides a timely explanation of the literature surrounding the factors required for successful micro and nano welding/joining.

Spatially resolved ultrasonic attenuation in resistance spot welds: implications for nondestructive testing

Spatial variation of ultrasonic attenuation and velocity has been measured in plane parallel specimens extracted from resistance spot welds. In a strong weld, attenuation is larger in the nugget than in the parent material, and the region of increased attenuation is surrounded by a ring of decreased attenuation. In the center of a stick weld, attenuation is even larger than in a strong weld, and the low-attenuation ring is absent. These spatial variations are interpreted in terms of differences in grain size and martensite formation. Measured frequency dependences indicate the presence of an additional attenuation mechanism besides grain scattering. The observed attenuations do not vary as commonly presumed with weld quality, suggesting that the common practice of using ultrasonic attenuation to indicate weld quality is not a reliable methodology.

Effects of voltage and wire feed speed on weld fume characteristics

Welding generates high concentrations of ultrafine particles, which research suggests may be more toxic than larger particles. Fume characteristics were measured in a controlled apparatus as a function of voltage level and wire feed speed. Particles were sampled close to the welding process on mixed cellulose ester membrane filters and analyzed for iron, manganese, and total particulate matter at an accredited industrial hygiene laboratory. An ultrafine condensation particle counter measured the particle number concentration, and an optical particle counter measured the particle size distribution. Submicrometer particle number concentrations and iron, manganese, and total particle mass concentrations all depended on voltage levels but not on wire feed speed at a constant voltage. Ultrafine particle concentrations were more than three times greater at 23.5 V than at 16 V. Particles 0.5-0.7 microm in diameter counted by the optical particle counter increased from 9800 particles/cm(3) at 16 V to 82,800 particles/cm(3) at 23.5 V. Manganese concentration was 1.7 mg/m(3) at 16 V vs. 6.4 mg/m(3) at 23.5 V. The data suggest that welders should use lower voltage levels whenever possible.

Pial and arachnoid welding for restoration of normal cord anatomy after excision of intramedullary spinal cord tumors

A significant postoperative problem in patients undergoing excision of intramedullary tumors is painful dysesthesiae, attributed to various causes, including edema, arachnoid scarring and cord tethering. The authors describe a technique of welding the pia and arachnoid after the excision of intramedullary spinal cord tumors used in seven cases. Using a fine bipolar forcep and a low current, the pial edges of the myelotomy were brought together and welded under saline irrigation. A similar method was used for closing the arachnoid while the dura was closed with a running 5-0 vicryl suture. Closing the pia and arachnoid restores normal cord anatomy after tumor excision and may reduce the incidence of postoperative painful dysesthesiae.

In vitro toxicity evaluation of silver soldering, electrical resistance, and laser welding of orthodontic wires

The long-term effects of orthodontic appliances in the oral environment and the subsequent leaching of metals are relatively unknown. A method for determining the effects of various types of soldering and welding, both of which in turn could lead to leaching of metal ions, on the growth of osteoblasts, fibroblasts, and oral keratinocytes in vitro, is proposed. The effects of cell behaviour of metal wires on osteoblast differentiation, expressed by alkaline phosphatase (ALP) activity; on fibroblast proliferation, assayed by the 3-(4,5-dimethylthiazol-2-yl)-5-(3-carboxymethoxyphenyl)-2-(4-sulphophenil)-2H-tetrazolium-phenazine ethosulphate method; and on keratinocyte viability and migration on the wires, observed by scanning electron microscopy (SEM), were tested. Two types of commercially available wires normally used for orthodontic appliances, with a similar chemical composition (iron, carbon, silicon, chromium, molybdenum, phosphorus, sulphur, vanadium, and nitrogen) but differing in nickel and manganese content, were examined, as well as the joints obtained by electrical resistance welding, traditional soldering, and laser welding. Nickel and chromium, known as possible toxic metals, were also examined using pure nickel- and chromium-plated titanium wires. Segments of each wire, cut into different lengths, were added to each well in which the cells were grown to confluence. The high nickel and chromium content of orthodontic wires damaged both osteoblasts and fibroblasts, but did not affect keratinocytes. Chromium strongly affected fibroblast growth. The joint produced by electrical resistance welding was well tolerated by both osteoblasts and fibroblasts, whereas traditional soldering caused a significant (P < 0.05) decrease in both osteoblast ALP activity and fibroblast viability, and prevented the growth of keratinocytes in vitro. Laser welding was the only joining process well tolerated by all tested cells.

Evaluation of welded titanium joints used with cantilevered implant-supported prostheses

STATEMENT OF PROBLEM

Early failure of laser-welded titanium implant frameworks in clinical practice has prompted an investigation of the strength and durability of welded cantilevered titanium sections.

PURPOSE

The purpose of this study was to determine the effect that the use of filler wire in laser welding of titanium cantilever frameworks had on the flexural strength and fatigue resistance of the welded joints.

MATERIAL AND METHODS

Sixty titanium implant-supported frameworks with 12-mm cantilevers were fabricated in 4 groups (n=15), using 3 different laser welding protocols with 0, 1, and 2 weld passes with filler wire, and 1 conventional tungsten inert gas welding method. The volume of filler wire used (mean volumes 0, 1.7, 3.4, and 8.3 mm(3)) was determined by measurement of the length of wire before and after welding each joint. Ten frameworks from each group were tested for ultimate flexural strength by loading the cantilevers 10 mm from the abutment. The remaining 5 frameworks from each group were similarly tested under a simulated masticatory load of 200 N until failure, or to 1 million cycles. A 2-way analysis of variance was used to examine the flexural strengths, and log-rank statistics were applied to cyclic test data (alpha=.05).

RESULTS

There were significant differences between the 4 groups for ultimate flexural strength (P<.001) and resistance to cyclic loading (P=.002). The volume of filler wire added was a significant predictor of ultimate flexural strength (P=.03), and was a borderline determinant of the number of cycles to failure at 200 N (P=.05). Each laser weld pass with filler wire roughly doubled the ultimate flexural strength and fatigue resistance of the joint relative to the previous weld. Tungsten inert gas welding with efficient argon shielding deposited the most filler wire and produced the strongest and most fatigue-resistant joints.

CONCLUSION

The ultimate flexural strength and fatigue resistance of cantilevered joints in laser-welded titanium prostheses are improved by the use of filler wire. Tungsten inert gas welding with efficient argon shielding can be used in situations when a high-strength joint is required.