A.F.M. Arif

Numerical prediction of plastic deformation and residual stresses induced by laser shock processing

Abstract Laser shock processing (LSP) involves a high-energy laser beam combined with suitable overlays to generate high pressure pulses on the surface of the metal. This stress wave propagates into the material, causing the surface layer to yield and plastically deform, and thereby, develop a significant residual compressive stress in the surface of the metallic material. This compressive stress field is beneficial for surface mechanical properties such as fatigue, wear or corrosion. After briefly reviewing the mechanism of shock wave formation, a numerical algorithm to predict this stress field is presented. To calculate the surface compressive stress, several experimental and analytical formulations with simplified assumptions have been reported in the literature. The proposed method uses a finite difference algorithm to simulate propagation of stress wave in the material. This module is explicitly coupled at each time step with a finite element module to predict deformation and stresses. The proposed method is implemented in a computer code and several problems are tested. The comparison of the calculations using the proposed numerical model with experimental results as well as with results obtained by analytical solutions shows a very good correlation.

A study of die failure mechanisms in aluminum extrusion

Abstract A very important factor contributing to the performance and economics (efficiency and quality) of any hot metal-forming process is the service life of tooling. Product rework and rejects can be traced back to various defects spread over the die life-cycle: die design, die manufacture, heat treatment and die service. Initiation and propagation of die damage can be caused by a number of mechanisms. Analysis of tool and die failure thus plays an important role in the prediction and prevention of die failure, and subsequently in improving process economics. This depends to a large extent on the knowledge of the manufacturing and service history of the failed tool and die. Such information is generally not very easily available, and especially not for a large number of die failures and a large spectrum of die profiles. Very few articles are available in literature that present failure analysis based on a substantial sample size of real die breakdowns. The three most commonly reported modes of die failure are fatigue-based fracture, wear, and plastic deformation/deflection. Shape complexity of the die profile plays an important role in hot extrusion of aluminum alloys. The paper presents results of an ongoing study about the relationship between die profile and modes of die failure. A total of 616 die failures involving 17 different die profiles were studied, in collaboration with a local industrial setup. All dies were made of H-13 steel, while the billet material was Al-6063 in all the cases. The analysis presented here reflects three different perspectives: (a) overall and class-wise break-up of failure modes, (b) failure analysis for dies of different complexities, and (c) shape-wise breakdown of each failure mode.

Material response to thermal loading due to short pulse laser heating

Abstract The laser-induced thermal stresses are important during heating of substrate surfaces, since stress levels above the yield stress of the substrate material occur. In this case, the plastic deformation and/or material defects occur in the irradiated region. In the present study, laser short pulse heating of gold is considered. The electron kinetic theory approach is employed to model the heating process. This approach accounts for the non-equilibrium energy transport in the surface vicinity of the substrate material. The thermal analysis and temperature predictions from an electron kinetic theory approach were presented in a previous study. Therefore, the results of thermo-elastic analysis due to the temperature field predicted in the previous study are given here. The elastic stresses developed in the substrate material due to the temperature field are modelled, and the temporal and spatial variations of the stress field and von Mises stresses are computed. It is found that the stress field in the substrate material does not follow the temperature field. A stress level on the order of 109 Pa occurs in the surface vicinity.

Laser gas assisted nitriding of alumina surfaces

Abstract The laser control melting can be applied to improve the structural and tribological properties of alumina surfaces. In this case, the laser remelting provides a homogeneous structure while the assisting gas diffuses into molten layer forming the nitride compounds in the irradiated region. Since the process is rapid, local heating with controllable depth and cost effective, the laser remelting provides several advantages over the conventional surface treatment techniques. In the present study, laser melting and gas assisted nitriding of alumina pellets are carried out. Morphological and metallurgical changes after the laser treatment process are examined using scanning electron microscopy (SEM), optical microscopy and X-ray diffraction (XRD), and indentation tests. It is found that two regions are formed in the laser irradiated zone. The first region below the surface is dense and composes of α-Al2O3 and AlN while in the second region, which is below the first region, randomly stacked lamellae structure is observed.

VARIATION OF PRESSURE WITH RAM SPEED AND DIE PROFILE IN HOT EXTRUSION OF ALUMINUM-6063

Prediction of extrusion pressure, especially in the case of complex die geometries, is an area of continued research interest. Die complexity, usually defined by “shape factor” (the ratio of the perimeter to the cross-sectional area of the profile), critically affects the flow of metal and the pressure required to extrude a given product. Applied strain rate (related directly to the ram speed of the extrusion press) also alters the product quality significantly. The current paper presents results of an ongoing study about effects of ram speed and die profile on extrusion pressure. Experiments were conducted using dies of different complexity to track the effects of ram speed variation and changing die profiles on extrusion pressure. Al-6063, the most popular commercial variety of structural aluminum, was used as the billet material for all experiments.

Thermal Analysis and Optimization of Orthotropic Pin Fins: A Closed-Form Analytical Solution

Analytical solutions for temperature distribution, heat transfer rate, and fin efficiency and fin effectiveness are derived and presented for orthotropic two-dimensional pin fins subject to convective-tip boundary condition. The generalized results are presented and discussed in terms of dimensionless variables such as radial and axial Biot numbers (Bi r , Bi z ), fin aspect ratio, L/R, and radial-to-axial conductivity ratio k * . Several special cases are derived from the general solution, which includes the insulated-tip boundary condition. It is also demonstrated that the classical temperature distribution and heat transfer rate from the two-dimensional isotropic pin fin introduced earlier in literature can easily be recovered from the general solutions presented in this paper. Furthermore, dimensionless optimization results are presented for orthotropic pin fins that can help to solve many natural and forced convection pin fin problems.

Laser shock processing of aluminium : model and experimental study

Laser shock processing is involved with high amplitude pressure wave propagation into the specimen. The magnitude and duration of the pressure generated across the evaporating surface during the laser rapid evaporation defines the depth of the deformed region. In the present study, laser shock processing of aluminium is considered and the material response to the laser-induced pressure wave is modelled. The recoil pressure generated at the irradiated surface is formulated while the stress field in the specimen is predicted using finite element analysis. An experiment is carried out to examine the metallurgical changes in the plastically deformed region using electron scanning microscopy (SEM). It is found that the prediction of the depth of the deformed region agrees well with the experimental results. The deformed region is free from cracks, which is observed from the SEM micrographs.

Characterization of Nanoreinforcement Dispersion in Inorganic Nanocomposites: A Review

Metal and ceramic matrix composites have been developed to enhance the stiffness and strength of metals and alloys, and improve the toughness of monolithic ceramics, respectively. It is possible to further improve their properties by using nanoreinforcement, which led to the development of metal and ceramic matrix nanocomposites, in which case, the dimension of the reinforcement is on the order of nanometer, typically less than 100 nm. However, in many cases, the properties measured experimentally remain far from those estimated theoretically. This is mainly due to the fact that the properties of nanocomposites depend not only on the properties of the individual constituents, i.e., the matrix and reinforcement as well as the interface between them, but also on the extent of nanoreinforcement dispersion. Therefore, obtaining a uniform dispersion of the nanoreinforcement in the matrix remains a key issue in the development of nanocomposites with the desired properties. The issue of nanoreinforcement dispersion was not fully addressed in review papers dedicated to processing, characterization, and properties of inorganic nanocomposites. In addition, characterization of nanoparticles dispersion, reported in literature, remains largely qualitative. The objective of this review is to provide a comprehensive description of characterization techniques used to evaluate the extent of nanoreinforcement dispersion in inorganic nanocomposites and critically review published work. Moreover, methodologies and techniques used to characterize reinforcement dispersion in conventional composites, which may be used for quantitative characterization of nanoreinforcement dispersion in nanocomposites, is also presented.

Analysis of Product Defects in a Typical Aluminum Extrusion Facility

Abstract Aluminum extrusions are popular in the automobile, aircraft, and construction industries. They are highly versatile, have relatively modest prototyping costs, possess good strength and corrosion resistance, and yield a high benefit–cost ratio. Technical and economic viability of an extrusion plant depends on the minimization of defects that lead to product rejection. Attempts at improvement of extrusion quality and productivity thus translate straightaway into an analysis of product defects. Product rejection may be traced back to material defects, tooling defects, processing anomalies, and postextrusion and surface finishing defects. The first part of the current paper gives a brief description of extrusion defects generally encountered in a commercial setup. The second part deals with collection and categorization of real world rejection data (spanning 9 years) from a local aluminum extrusion facility, plant activities being divided into three major cost centers: press, anodizing, and painting. The last part presents a statistical analysis of defects from three different viewpoints: (1) plantwise defects breakdown, (2) annual rejection scenario, and (3) defects breakdown in each cost center. Rejection and acceptance percentages at each cost center have been worked out relative to individual cost center production and in relation to total plant production.

THERMAL ANALYSIS OF A COLD ROLLING PROCESS — A NUMERICAL APPROACH

The deformation of material and friction between the roll and deforming material contact region produce a large amount of heat. This heat energy is conducted toward the roll and the workpiece (strip). A well-designed cooling system is needed to control the material properties and grain structure of the rolled product. Therefore, complete knowledge of the temperature distribution in both the roll and strip is necessary to design an efficient cooling system to control the material properties. In this work, both the roll and strip have been modeled as a coupled heat transfer problem to predict the temperature distribution. Using a finite-volume approach, the governing differential equations as well as the boundary conditions are discretized, which are then solved numerically to predict the temperature distributions. The stability of the solution was examined by changing the grid sizes in the bite region; in addition, the numerical results are validated against published work in the literature for certain special operating conditions. The impact of roll speed and heat transfer coefficient on the distribution of heat flow in both the roll and workpiece are demonstrated through the temperature contour plots.