MIG Brazing of Galvanized Thin Sheet Joints for Automotive Industry

MIG welding of zinc-coated thin plates in the automotive industry leads to major issues, mainly zinc evaporation followed by a decrease of corrosion resistance, as well as residual strains and stresses difficult to minimize. The use of a lower heat input technique for joining galvanized steels would bring significant benefit, if the final overall mechanical properties of the joints are adequate for the application. The use of MIG brazing (MIGB) with the recently commercialized alloyed copper-based filler metal is an alternative worth considering. The present paper addresses the MIGB processes, describing the influence of the different shielding gases and the process parameters on the mechanical, corrosion, and metallurgical properties of the joint, when lower heat input procedures are targeted. The paper describes the influence of the gases on the mechanical properties of the brazed joint, both in normal conditions after joining and after corrosion in a salt water environment. Microstructural features of the different zones are discussed. Results of corrosion and tensile tests are presented and interpreted.     

Materials Behavior in Laser Welding of Hardmetals to Steel

Tool manufacturing industry faces the problem of permanently joining hardmetals to steel holders with high shear strength. The mostly used welding process still is brazing. However, brazed joints have poor lifetimes, mostly when high temperatures are achieved and often break in operation. In a previous study about the ability of CO₂ and Nd:YAG lasers to weld hardmetals to steels, it was found that Nd:YAG lasers, working in continuous wave mode, could be used especially for welding hardmetals with Co content around 12%. This article discusses the materials behavior under laser radiation and analyzes the microstructural features observed.     

Imperfections in laser clading with powder and wire fillers

Laser cladding has been increasing in laser job shops both for component repair and manufacture. The process is quite mature nowadays, from a scientific approach, but companies need manufacturing guidelines and procedures to minimise the occurrence of defects and improve service quality and productivity. Typical imperfections occurring in laser clads can be divided into two major groups: shape and microstructural defects. The first group includes the contact angle of the clad track with the substrate surface and the total height of the clad including penetration. In the second group are considered lack of fusion, porosity and cracks. It is not clear to which extent the use of powder or wire as filler materials influence defect formation, though it is well known that these are mostly related to material properties and operational procedures. This paper presents the work done in laser cladding using filler wire and powder in real productive environment in job shop cases on different substrates: AISI 316 stainless steel, H13 and P20 tool steels. An overview of the defects that are more prone to appear in these materials is presented and strategies to minimise their occurrence proposed.     

Rapid prototyping with high power fiber lasers

Laser rapid prototyping technologies comprise a set of technologies used in a wide range of materials to produce prototypes or small batches of complex shaped components. This paper presents a research work on rapid prototyping technology with laser additive manufacture of wire based alloy Ti-6Al-4V with an 8 kW fiber laser for the production of components with cylindrical geometry. For this, an engineering system was developed, a demonstration part produced and the deposition process was characterized. Two processing parameters were investigated: and these were the relative position between the wire feeding system and the substrate and the laser beam to wire width ratio. The former affects the molten metal transfer mode and the pressure exerted by the wire tip on the molten pool, while the laser beam to wire width ratio affects the process efficiency, since this is a compromise of process stability and process speed. Both parameters control surface finishing and the smoothness of the part. The melting efficiency of the process is low when compared to alternative processes involving powder pre deposition, but the density of the part is improved with homogeneous structural characteristics.     

Modelling the Retail Sales of Gasoline in a Portuguese Metropolitan Area

This paper describes the modelling approach for the sales of gasoline in service stations located in a Portuguese metropolitan area. The models were developed for planning purposes in order to assess potential sales of new sites. The estimates produced are more accurate than those made by previous models and are currently being used to support investment decisions of the largest Portuguese company. Starting from a basic multiplicative regression model, several segmentations of the service stations were obtained. Among these, the submodels for service stations located in urban areas and for service stations located along routes produced a better fit. The models reflect, without exception, the importance of the variables representing the size of service station and the "traffic" passing by, as well as variables which express location and road type. The fast growth experienced by this market explains the lesser impact on sales of the area sales potential and of competition.     

Temperature effects on the enhanced or deteriorated buoyancy-driven heat transfer in differentially heated enclosures filled with nanofluids

ABSTRACT

A two-phase model based on the double-diffusive approach is used to perform a numerical study of natural convection in differentially heated vertical cavities filled with water-based nanofluids, assuming that Brownian diffusion and thermophoresis are the only slip mechanisms by which the solid phase can develop a significant relative velocity with respect to the liquid phase. The system of the governing equations of continuity, momentum, and energy for the nanofluid, and continuity for the nanoparticles, is solved through a computational code, which incorporates three empirical correlations for the evaluation of the effective thermal conductivity, the effective dynamic viscosity, and the thermophoretic diffusion coefficient, all based on the literature experimental data. The pressure–velocity coupling is handled using the SIMPLE-C algorithm. Numerical simulations are executed for three different nanofluids, using the diameter and the average volume fraction of the suspended nanoparticles, as well as the cavity width, the average temperature of the nanofluid, and the temperature difference imposed across the cavity, as independent variables. It is found that the heat transfer performance of the nanofluid relative to that of the base fluid increases notably with increasing the average temperature, showing a peak at an optimal particle loading. Conversely, the other controlling parameters have moderate effects.     

A Demonstrative Study on the Two-phase vs. Single-phase Modeling of Buoyancy-driven Flows of Enclosed Nanofluids

A demonstrative numerical study on natural convection of water-based nanofluids in square enclosures with different boundary conditions imposed at the walls, and different orientations with respect to the gravity vector, is performed using both the single-phase and the two-phase approaches, with the main scope to evaluate in what measure the single-phase approach fails in describing the basic heat and fluid flow features, as well as in determining the thermal performance of nanofluids. The system of the mass, momentum and energy transfer governing equations is solved by way of a computational code based on the SIMPLE-C algorithm. Empirical correlations are used for the calculation of the effective thermal conductivity, the effective dynamic viscosity, and the thermophoretic diffusion coefficient. The following configurations are investigated: a tilted cavity differentially-heated at two opposite walls; a vertical cavity partially-heated at the bottom wall and cooled at both sides; and a vertical cavity differentially-heated at the vertical and horizontal walls. It is found that the non-uniform distribution of the suspended solid phase throughout the enclosure gives rise to a solutal buoyancy force, whose competition with the thermal buoyancy force results in a periodic flow detectable only if the two-phase approach is applied. Moreover, the impact of the dispersion of the nanoparticles into the base liquid, which turns out to be notably higher at higher average temperatures, is found to be systematically underestimated by the single-phase approach.     

Buoyancy-Induced Convection of Alumina-Water Nanofluids in Laterally Heated Vertical Slender Cavities

A two-phase model based on the double-diffusive approach is used to perform a numerical study of natural convection of alumina-water nanofluids in differentially heated vertical slender cavities. In the mathematical formulation, Brownian diffusion and thermophoresis are assumed to be the only slip mechanisms by which the solid phase can develop a significant relative velocity with respect to the liquid phase. The system of the governing equations of continuity, momentum and energy for the nanofluid, and continuity for the nanoparticles is solved through a computational code relying on the SIMPLE-C algorithm for the pressure-velocity coupling. The effective thermal conductivity and dynamic viscosity of the nanofluid, and the coefficient of thermophoretic diffusion of the suspended solid phase, are evaluated using three empirical correlations based on a high number of experimental data available from diverse sources, and validated by way of literature data different from those used in generating them. Numerical simulations are executed for different height-to-width aspect ratios of the enclosure, as well as different average temperatures of the nanofluid. The heat transfer performance of the nanoparticle suspension relative to that of the base fluid is found to increase as the nanofluid average temperature is increased and, at low to moderate temperatures, the aspect ratio of the enclosure is decreased. Moreover, at temperatures higher than room temperature, a peak at an optimal particle loading is found to exist for any investigated configuration.