12 The Influence of Speed on the Wall Thickness of Centrifugally Cast Parts DĂIAN Marcel Transilvania University of Brasov, Romania, Romania, marceldaian@yahoo.com CIOBANU Ioan Transilvania University of Brasov, Romania, ciobanu_i_bv@yahoo.com POP Mihai Alin Transilvania University of Brasov, Romania, mihai.pop@unitbv.ro Abstract The paper discusses a study on the vertical centrifugal casting of tubular parts. The necessary speed for the casting of cylindrical parts by this procedure is determined starting from the equation of the interior surface of the liquid alloy during rotation. The study of the influence of speed and part dimensions on the wall thickness of the casting is followed by a comparison of the theoretical and experimental results. The conclusions concern the determination of the optimum speed for a given part. Keywords casting, centrifugal casting, leaded aluminium-silicon alloy 1. Introduction The interest in centrifugal casting has known a steady increase due to its benefits [1, 2, 3, 4, 8]: -superior mechanical properties of the castings; - the possibility of obtaining castings of gradient structure and properties; - high utilisation efficiency of the cast liquid alloy; - simple and cost efficient installations; - cost efficient castings. The interest for this procedure is materialised in the research projects, scientific articles and patents developed in this field [5, 6, 7, 9, 10]. Lately scientific focus has shifted to vertical axis centrifugal casting [2, 10] as it allows: -the casting of parts lacking a rotational symmetry axis; - the improved obtaining of parts with gradient chemical composition, structure and properties by means of large rotation radii. 2. Aim of the Paper In centrifugal casting, the rotational speed of the mould is an important technological parameter. It influences the geometry, compactness, chemical composition and mechanical properties of the alloy in the wall of the castings. In addition, speed can be adjusted easily in practice. Modern centrifugal casting installations allow the modification of this parameter as needed. In the case of centrifugally cast tubular parts, the speed influences the geometry of the interior surface of the liquid alloy. Thus, it influences casting wall thickness. In horizontal axis centrifugal casting at a speed higher the critical, this influence is lesser as the alloy passes at high frequency through the bottom as well as through the top point of the mould. This sequence of positions yields a heat transfer along the radius practically uniform along the entire contour of the casting. Consequently, the solidification rate is uniform along the contour of the casting and a tubular part of uniform wall thickness is obtained. For this reason in centrifugal casting by means of horizontal axis installations there is but a reduced tendency of obtaining parts with unequal wall thickness. The condition for centrifugally casting parts with uniform wall thickness on horizontal axis installations is for the rotational speed to be higher that the critical one. In this case, the critical speed is calculated by equating the centrifugal force acting upon a particle of liquid at the top of the casting’s interior surface with the weight of that particle. Thus, the equation for the critical angular speed is obtained: ωCRT = (g/R1) 1/2 [rad/s] (1) The critical rotational speed results as: