Jan Vatavuk

Evaluation of Fatigue Behavior of SAE 9254 Steel Suspension Springs Manufactured by Two Different Processes: Hot and Cold Winding

The fatigue resistance is a property that exerts a strong influence on the suspension spring performance in vehicles. The choice of SAE 9254 steel was due to its wide use in the manufacture of these springs and their fatigue properties and toughness. The manufacture of SAE 9254 steel springs has been made by the hot winding process and the heat treatment by conventional quenching and tempering or by cold winding process and induction hardening and tempering. The shot peening induced a compressive residual stress which increased the fatigue life of the SAE 9254 steel. The residual stress profile from the surface of springs showed a peak in the values of the compressive stress for both manufacturing processes. The maximum residual stress in the cold processed spring was higher than the hot processed spring and maintained much higher values along the thickness of the spring from the surface, resulting from manufacturing processes. The fatigue cracking of the springs, without shot peening, started by torsional fatigue process, with typical macroscopic propagation. The fracture surface showed stretch marks with high plastic deformation.

Computational Simulation in Centrifugal Casted Aluminum–Silicon Engine Cylinder Liner

Automotive cylinder liners are mechanical components with the function of internal coating of the cylinder automotive engines. The replacement of parts made of steel/cast iron by aluminum alloys has been made with advantages not only in reducing weight as well as fuel consumption and emission of pollutants. This study was aided by the finite element software Hyperworks, where the mesh was generated and the simulation was performed in Abaqus. The mesh for the engine block was defined with elements of four nodes of tetrahedrons. The liners were designed with hexahedron elements of six nodes. Due to the manufacturing process of the cylinder liners (centrifugal casting), the finite elements model was created in layers to meet the variation in the amount of silicon along the wall thickness. The variation in the amount of silicon affects the physical properties of the liners along the wall thickness. With this model, it was possible to show the viability of application of aluminum liners in engine blocks made of cast iron. The modal analysis showed that the model does not contain its first natural frequency within the range of work of the engine, approving its application with this concept.

Failure Analysis of an Automotive Component (Cardan Yoke) by Scanning Electron Microscopy

Two SAE 1541 (0.39%C; 1.44%Mn; 0.23%Si; 0.16%Ni; 0.16%Cr) carbon steel cardan yokes that were forged, machined, quenched and tempered, as part of the manufacturing process to ensure long term operation under specific loading conditions, failed during its manufacture. The cardan yokes ruptured in the bearing seat region while these were being straightened by bending. This study deals with fracture analysis that was carried out by visual inspection and scanning electron microscopic examination. The focus of this study was to investigate the fracture mechanism associated with the failures. Fractographs of the broken components indicated that the rupture initiated at the edges of the component, from preexisting cracks, due to the bending stresses during the straightening process. The initial stage of rupture was predominantly intergranular in the tempered martensite surface layer, revealing the brittle nature of the component. Cracks were observed at regions prone to stress concentration. Eventual rupture of the component probably initiated at these cracks. This behavior is probably related to metallurgical processing steps like quenching, that causes the formation of a banded structure and promotes circumferential and radial cracking before the tempering. The fracture surface revealed regions with micro dimples and a large smooth area with some elongated inclusions. The morphology of these inclusions was cellular and originated at the grain boundaries of the primary austenite. These inclusions are probably MnS with a dendrite structure, capable of causing brittle intergranular rupture.

Effect of Copper and Magnesium on the Microstructure of Centrifugally Cast Al-19%Si Alloys

Hypereutectic Al-Si alloys can be used in applications that require high wear resistance. Such wear resistance is achieved by the presence of hard primary silicon particles, allied to the formation of Mg2Si intermetallic phase when magnesium is added in this alloy. Centrifugal casting generates a gradient in the microstructure of hypereutectic Al-Si alloys that can favor such applications. Cylindrical components of Al-19%Si alloy containing added copper and magnesium contents were processed by centrifugal casting. The purpose of this study is to investigate the formation and segregation of particles of primary silicon (β) and Mg2Si in Al-19%Si alloy containing additions of copper and magnesium. Because the density of silicon (2.33 g/cm3) and Mg2Si (1.88 g/cm3) is lower than that of aluminum (2.67 g/cm3), centrifugal casting causes primary silicon (β) and Mg2Si particles to concentrate more at the outer wall of the centrifuged pipe. In this study, primary silicon (β) and Mg2Si particles were found to be retained at the outer wall of the pipe. It is believed that the rapid cooling of the molten metal in the region of contact with the mold, whose temperature is lower than that of the molten metal, allied to the centrifugal force, prevented the particles from migrating to the inner wall of the pipe. The microstructure shows a gradient in the distribution of these phases, enabling the production of a functionally graded material. The addition of copper and magnesium leads to the formation of Mg2Si and Al5Cu2Mg8Si6 phases, reducing the amount of primary β phase (Si) particles. In all the evaluated conditions, a tendency is also observed for a gradual increase in the segregation of silicon towards the inner wall along the entire length of the centrifuged pipe.

Study of the utilization of polyamide composite with fiberglass reinforcement in automotive engine mounts

The aim of this work is to study the replacement of the support of the engine mounts from aluminum to commercial polyamide composite (PA 6.6) reinforced with 30, 35 and 50 % of fiberglass. The purpose of this replacement is to reduce the weight of the component. Flexion and fatigue tests were performed at 120 °C utilizing the Staircase Method. The results have shown the excellent adherence of polyamide in fiberglass. The polyamide composite with 50 % fiberglass presented the lowest deflection. The comparison of the tension distribution utilizing the Finite Element Method between the supports of the bearing engine made of polyamide with fiberglass and Al alloy have shown almost the same results. However, the polyamide composite presented higher values of deflection. Finally, due to the weight reduction of 32 %, it was possible to confirm that the polyamide composite is viable to change the Al alloys in automotive engine bearings.

The Effect of Strain Hardening in Stainless Steels Submitted to Nitriding Treatment

The present study shows an evaluation of the effects of strain hardening on the kinetics of the nitriding thermochemical treatment in alloyed ferritic stainless steels AISI 430 and AISI 439 for different degrees of plastic deformation. The processes of application of the gas nitriding, in salt baths and plasma were performed on these alloys. The layer thickness was larger for the process of nitriding gas. Note also that the nitrogen concentration reduces with the depth of the nitrided layer as it comes near to the base metal. On the other hand, carbon is pushed inside of the sample, promoting decarburization in the region enriched in nitrogen and an accumulation in the regions just below to the nitride layer. In general, note that the ferritic matrix (Fe–Cr) and the formation of chromium nitride (CrN) in samples of AISI 430 and AISI 439 which suffered plasma nitriding and gas. In addition to this compound, it was possible to identify the presence of Fe4N and Fe3N. In the samples of AISI 430 and AISI 439 nitrided in salt baths, it can be observed that the nitrided layer consists in the ferritic matrix (Fe–Cr) with predominance of precipitates (FeCr)2N and without presence of peaks of CrN.

Avaliação da resistência mecânica de três diferentes pinos de Schanz às forças de torção em montagens do fixador externo monolateral

OBJECTIVE: The objective of this study was to evaluate the mechanical force of three different assemblies of a linear external fixator using 4.5 mm and 5.5 mm Schanz screws with a 3.2 mm-diameter root, and another screw with 5.5 mm diameter with a 4,5mm root. MATERIAL AND METHODS: The linear external fixator was assembled in a 500 mm-long polypropylene tube, with a 45-degree oblique angle cut in the center of the cylinder, with two Schanz screws in each segment. Eighteen assemblies were studied, divided into three groups of six pieces each. In the group 1, 4.5mm-diameter Schanz screws were used, in the group 2, 5.5mm-diameter Schanz screws, both with 3,2mm root were used. In the group 3, 5.5mm-diameter Schanz screws with a diameter root of 4.5mm were used. The mechanical tests were performed in a MT-100 torsion machine, and the resistance measurements were performed at 4.5o, 9.0o, 13.5o, and 18o of torsion. RESULTS: The 4.5mm Schanz screws with 3,2 mm root showed an average torsion resistance at 4.5o, 9.0 o, 13.5 o and 18.0 o, respectively of: 12.0 N/mm, 21.0 N/mm, 33.0 N/mm, and 46.0 N/mm. The 5.5mm Schanz screw with 3.2 mm root, showed as average resistance: 13.2 N/mm, 25.3 N/mm, 40.0 N/mm, and 51.2 N/mm, respectively. The tests with 5.5 mm Schanz screws with 4.5 mm root showed an average resistance of: 15.2 N/mm, 33.5 N/mm, 53.0 N/mm, and 70.0 N/mm. Statistical analysis with the Variance Analysis test and the Bonferronis test showed the absence of statistically significant differences among groups with root diameter of 3.2mm (4.5 mm and 5.5 mm). There was a statistically significant difference between the group with the 5.5mm Schanz screws with 4.5 mm root and the other two groups. CONCLUSIONS: 1- There was no statistically significant difference among assemblies of linear external fixator using 4.5mm and 5.5mm Schanz screws with 3.2mm root. 2- The linear external fixator with 5.5mm Schanz screws with 4.5mm root, showed a stronger mechanical resistance to the torsion, statistically significant, regarding the other two groups.