Sources of variability in the free-ball micro-scale abrasion test

In the 'free-ball' version of the micro-scale abrasion or ball-cratering test the rotating ball rests against a tilted sample and a grooved drive shaft. Tests under nominally identical conditions with different apparatus commonly show small but significant differences in measured wear rate. An indirect method has been developed and demonstrated for continuous on-line measurement of the coefficient of friction in the free-ball test. Experimental investigation of the effects of sample tilt angle and drive shaft groove width shows that both these factors influence the stability of the rotation of the ball, and the shape of the abrasive slurry pool, which in turn affect the coefficient of friction in the wear scar area and the measured wear rate. It is suggested that in order to improve the reproducibility of this method the geometry of the apparatus should be specified. For the apparatus used in this work with a steel ball of 25 mm diameter, a sample tilt angle of 60-75° and a shaft groove width of about 10mm provided the most stable ball motion and a wear rate which showed least variability. © 2004 Elsevier B.V. All rights reserved.

The microstructure and mechanical properties of ball-milled stainless steel powder: The effect of hot-pressing vs. laser sintering

The microstructure and mechanical properties of sintered stainless steel powder, of composition AISI 420, have been measured. Ball-milled powder comprising nanoscale grains was sintered to bulk specimens by two alternative routes: hot-pressing and microlaser sintering. The laser-sintered alloy has a porosity of 6% and comprises a mixture of delta ferrite and tempered martensite, and the relative volume fraction varies along the axis of the specimen due to a thermal cycle that evolves with progressive deposition. In contrast, the hot-pressed alloy has a porosity of 0.7% and exhibits a martensitic lath structure with carbide particles at the boundaries of the prior austenite grains. These differences in microstructure lead to significant differences in mechanical properties. For example, the uniaxial tensile strength of the hot-pressed material is one-half of its compressive strength, due to void initiation at the carbide particles at the prior austenite grain boundaries. Nanoindentation measurements reveal a size effect in hardness and also reveal the sensitivity of hardness to the presence of mechanical polishing and electropolishing. © 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Feedback control of impact dynamics: The bouncing ball revisited

We study the the design of a tracking controller for the popular bouncing ball model: the continuous-time actuation of a table is used to control the impacts of the table with a bouncing ball. The proposed control law uses the impact times as the sole feedback information. We show that the acceleration of the table at impact plays no role in the stability analysis but is an important parameter for the robustness of the feedback system to model uncertainty, in particular to the uncertainty on the coefficient of restitution. © 2006 IEEE.

Manufacturing dish shaped rings on radial-axial ring rolling mills

Ring rolling is an established method to produce seamless rings of different cross-sectional geometries. For dish shaped rings, there are applications in different areas such as offshore, aeronautics or the energy sector. At the moment, dish shaped rings are produced by machining of rings with rectangular shaped cross section, by (open die) hollow forging on a conical mandrel or by using shaped ring rolling tools. These ways of manufacturing have the disadvantage of high material waste, additional costs for special tools, long process time and limited or inflexible geometries. Therefore, the manufacturing of dish shaped rings on conventional radial-axial ring rolling mills would expand the range of products for ring producers. The aim of this study is to investigate the feasibility of an alternative to the current manufacturing processes, without requiring additional tooling and material costs. Therefore, the intended formation of dish shaped rings-previously regarded as a form error-is investigated. Based on an analysis of geometrical requirements and metal flow mechanisms, a rolling strategy is presented, causing dishing and ring climbing by a large height reduction of the ring. Using this rolling strategy dish shaped rings with dishing angles up to 18° were achieved. In addition to the experiments finite element method (FEM)-simulations of the process have been successfully conducted, in order to analyze the local strain evolution. However, when the contact between ring and main roll is lost in the process the ring starts to oscillate around the mandrel and neither dishing nor ring climbing is observed. © 2013 German Academic Society for Production Engineering (WGP).