Anwar Khalil Sheikh

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.

Bernstein reliability model: Derivation and estimation of parameters

A Bernstein distribution has recently been developed to model the life characteristics of machine components which deteriorate according to a scheme of non-stationary linear wear processes. The Bernstein model has been successfully used in a variety of situations such as modelling of cutting tool life, monitoring the dimensions of machine parts for statistical quality control, testing the slideways and rotating parts of machine tools using accelerated wear conditions and determining tool replacement intervals in precision machining. In spite of the adequacy of this model in such important areas, no attempt has so far been made to comprehensively investigate the statistical properties and characteristics of the model. In this paper, an attempt is made to explore the statistical characteristics and properties of the Bernstein model, and its applications in some areas of engineering science are pointed out.

A probabilistic approach to the maintenance of heat-transfer equipment subject to fouling

A probabilistic approach to characterize various fouling processes and their influence on maintenance strategies of heat exchangers is described. Some common fouling models are discussed and, in view of their performance indicators (UUc), a strategy for planned maintenance schedules is described. The effect of this strategy on overall costs of these repairable systems is outlined. Trade-offs (continued maintenance vs replacement of the equipment) are examined in terms of cost parameters. The influence of random occurrences of fouling is incorporated in the maintenance strategy.

A risk based heat exchanger analysis subject to fouling Part I: Performance evaluation

Heat exchangers operating in the power and process industries are fouled to a greater or lesser extent depending on surface temperature, surface condition, material of construction, fluid velocity, flow geometry and fluid composition. This fouling phenomenon is time-dependent and will result in a decrease in the thermal effectiveness of a heat exchanger. Once the thermal effectiveness decreases to a minimum acceptable level, cleaning of the equipment becomes necessary to restore the performance. In this paper, we present a simple probabilistic approach to characterize various fouling models that are commonly encountered in many industrial processes. These random fouling growth models are then used to investigate the impact on risk-based thermal effectiveness, overall heat-transfer coefficient and the hot- and cold-fluid outlet temperatures of a shell-and-tube heat exchanger. All the results are presented in a generalized form in order to demonstrate the generality of the risk-based procedure discussed in this paper.

Uncertainty analysis of heat-exchanger thermal designs using the Monte Carlo simulation technique

The Monte Carlo simulation technique for determining uncertainties of the thermal parameters needed in designing heat exchangers is discussed. Parameters such as the outside diameter Do, thickness t and thermal conductivity k are assumed to have Gaussian distributions, while the heat-transfer coefficients hi, and ho are considered to follow normal, log-normal and Weibull forms of distribution. An illustrative example is presented. The overall heat-transfer coefficient U obtained from the simulation is found to have a distribution that is well approximated by the Weibull model. Using the Weibull distribution of U, the additional percentage area is calculated for 80 and 99% confidence levels (CLs). The proposed approach of design analysis incorporates realistic risks associated with uncertainties of the parameters.

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.

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.

Reliability analysis of aeroplane tyres

Aeroplanes are repairable systems consisting of several non‐repairable parts; tyres are one of these. Proper record of failure data is valuable in interpreting the failure pattern, for comparative evaluation of the quality of tyres of various manufacturers, and for prediction of the future needs in a specified planning horizon or for specified operational hours. Analyses failure data of several aeroplane tyres in an aviation set‐up, and interprets these in a reliability framework. Three parameters Weibull model is found to be quite appropriate for reliability characterization of tyres. These reliability models can be effectively integrated into a computerized material requirement planning system of the aviation facility to forecast the number of tyres needed for a given planning horizon.

Truncated extreme value model for pipeline reliability

Abstract A truncated extreme value distribution has been developed to characterize pipeline reliability by considering the distribution of initial pit depths on the surface of a pipeline exposed to corrosive environments. This model can be used as an alternative to the Weibull model when a pipe is subjected to a severe corrosive environment. The model yields an exponential type hazard function with a possibility of a minor damage to the pipe surface at the time of commissioning. The probability model is analyzed thoroughly and its characteristic features such as mean, variance, median, quantile, and coefficient of variation are presented in graphical form. The model parameters, α and θ, are related to the pipe geometry and the average pit depth of the parent distribution.

Mold Design Optimization for Sand Casting of Complex Geometries Using Advance Simulation Tools

Defects data and design optimization of gating system are a great challenge in casting. Most significant factors contributing to the volumetric changes and defects are pouring temperature, time, filling pattern, gating system, and casting geometry. The optimized design of gating system facilitates the smooth filling of the mold without air entrapment, material flow hindrance. Conventionally, the foundry method includes design calculations, best practices, and the tedious hit-and-trial method to get the optimum design of pattern/mold for defect-free casting. The design iterations and improvements directly affect the development cost and time, and one of the great challenges of the foundry method is to reduce this time and cost. The numerical simulation techniques embedded in casting simulation software are a powerful tool to overcome these challenges. It predicts the factors such as filling pressure, velocity, cooling rate, hot spots, and inside porosity. This article describes the design, simulation, experimental results, and design improvements for the complex geometries in the sand casting process. Impeller is selected as test case. The gating system was initially designed by using the text book mathematical relations and best practices recommendations. To study the predicted results of effected parameters, MAGMASOFT Software was used as a simulation tool. Three design modifications of mold design were studied. The effect of design including location and size of risers, gates on parameters such as filling pattern, pressure and velocity, cooling rate, solidification and defects such as air entrapment, hot spot, and porosity were studied. The predicted results were then compared with experimental data, and an excellent agreement between them was reported. The article highlighted the effectiveness of simulation to reach an optimal design of mold by passing the costly hit-and-trial conventional practice in the foundry industry and shows the effectiveness of simulations in time compression of casting pattern/development.