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Sensitivity analysis in life prediction of extrusion dies

Qamar S. Z., Pervez T., Siddiqui R. A., Sheikh A. K., Arif A. F. M.

Journal of Achievements in Materials and Manufacturing Engineering

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2007

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Vol. 25, nr 1

49-54

EN

Purpose: Building up on the fracture mechanics (Paris law for crack propagation) based fatigue life prediction model developed earlier by the authors, Monte Carlo simulation has been performed to evaluate sensitivity of die life related to important geometrical and material parameters. Stochastic nature of various fatigue-related die parameters is used to reflect their variability. Design/methodology/approach: Life of the die is one of the most important factors affecting productivity and profitability in hot extrusion of metals. It has been reported in earlier works by the authors that extrusion dies most often fail by fatigue fracture. Experimental studies have shown that cracks preexist in dies due to various factors including heat treatment, machining, and surface hardening. High levels of repeated mechanical and thermal loads result in crack propagation leading to ultimate fracture failure. Findings: Findings of the sensitivity analysis are that fracture life of an extrusion die is very sensitive to initial crack size, section thickness, profile outer diameter and billet length; moderately sensitive to Paris constant and extrusion ratio; and only slightly sensitive to fracture toughness and ram speed. Practical implications: The study can be of direct utility in extrusion die design improvement, formulation of an optimum die replacement strategy, etc. Originality/value: The paper provides basis for a deeper understanding of the factors responsible for fracture failure of an extrusion die exposed to thermo-mechanical fatigue environment.

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Heat treatment of a hot-work die steel

Qamar S. Z., Sheikh A. K., Arif A. F. M., Pervez T., Siddiqui R. A.

Archives of Materials Science and Engineering

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2007

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Vol. 28, nr 8

503-507

EN

Purpose: This paper reports results of in-house experimentation and an exhaustive literature search on heat treatment of H13 tool steel. Heat treatment strategy practiced by the industry is described in detail. Effect of various types of heat treatment on fracture toughness and hardness is also analyzed. Design/methodology/approach: Because of its versatility and wide applications, aluminum has been dubbed as the metal of the millennium. Commercial extrusion of aluminum alloys is a cyclic hot-working process. The magnitude of the thermal and mechanical stresses generated in the die and relevant tooling is therefore a major factor in extrusion. The die and mandrel (used for hollow profiles) are the most important tools subject to wear and are, at the same time, the most highly stressed tools in extrusion. For reliability and durability of an extrusion die, the load carrying capacity of the tool steel, its high-temperature fatigue properties, and its wear resistance become critically important. To withstand large stresses, the steel should have high strength and toughness, and to resist wear it should have high hardness and surface integrity. This combination of high toughness and high hardness is usually achieved through specific heat treatment and surface hardening sequences. Findings: Toughness (expressed in terms of plane-strain fracture toughness KIC or Charpy impact energy CVN) and hardness (HRC) of H13 steel vary in a nonlinear manner against tempering temperature. Toughness shows a decreasing-increasing trend, while hardness exhibits an opposite increasing-decreasing pattern with increasing tempering temperature. Research limitations/implications: Optimum heat treatment strategy for commercial aluminum estrusion dies (H13 steel) appears to be tempering in the 525-550 °C temperature range, to get the best combination of high toughness and high hardness Originality/value: Experimental data from closely monitored heat treatment and mechanical testing has been added to the available published data. Careful and judicious extrapolatiopn-intrapolatioon has also been carried out to complete the data matrices. Analysis of the resulting variation pattersns provideds a good scientific foundation for devising an optimal heat treatment strategy.

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Effect of process parameters on metal flow and dead metal zone in extrusion

Qamar S. Z., Pervez T., Arif A. F. M., Sheikh A. K.

Archives of Materials Science

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2007

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Vol. 28, nr 1-4

118-123

EN

Extrusion finds increasingly more frequent and important applications in automobile, aircraft, construction and other industries. Optimization of metal flow is crucially important in both design and manufacture as it directly affects extrusion speed (essential for higher productivity) on the one hand, and mechanical properties and surface finish of the extruded product on the other. It is reasonable to assume that material flow properties, die-workpiece and container-workpiece heat transfer and friction conditions, die profile, and die design all affect metal flow. However, the mechanism of dead zone formation and how it affects metal flow is not understood well enough to forecast with an acceptable degree of accuracy what is actually happening. Predictive numerical models are limited because of this lack of perception of the physics of the process. The current paper presents some results from a study about the behavior of metal flow and dead metal zone in cold extrusion. Experiments have been performed on flat-face dies fabricated from H13 tool steel and heat treated and surface hardened to approximately the same specifications as commercial extrusion dies. Three workpiece materials have been experimented with: Al-6063 as it is the most common aluminum alloy in the construction sector, and pure aluminum and lead for their better extrudability. Extrusions have been carried out at four different speeds. Commercial finite element packages ANSYS and ANSYS-LSDYNA have been used for numerical investigation of the effect of variations in important extrusion parameters.