Investigations on the continuous drive friction welding of sintered powder metallurgical (P/M) steel and wrought copper parts

Abstract

The present investigation attempts to understand the friction welding characteristics of sintered P/M steel preforms–wrought copper dissimilar parts. In order to achieve sound weld between these two materials the process parameters were optimized and inferred that the preforms of lower densities with lower process parameters yields quality weldments, established through post-weld tests. This study also envisages the influence of process parameters which include resident preform densities, friction pressure, upset pressure, and burn-off length on microstructure and mechanical properties of the welds. This work consolidates information on the aspects of joining P/M component with wrought materials for practical execution.

Introduction

Instruments functioning with components made up of dissimilar materials are becoming increasingly important in industrial applications due to their numerous advantages. These include not only the technical advantages, such as desired product properties, but also the benefit in terms of production economics. The materials employed are location dependent in the same structure for effective and economical utilization of the special properties of each material. The joining of dissimilar metals is generally more challenging than that of similar metals because of difference in the physical, thermal, electrical, mechanical and metallurgical properties of the parts to be joined. In order to take full advantage of the dissimilar metals involved, it is necessary to produce high quality joints between them.

Friction welding finds widespread industrial use as a mass-production process for the joining of similar and dissimilar materials. Friction welding can be used to join metals of widely differing thermal and mechanical properties. Several combinations that cannot be joined by other welding techniques because of the formation of brittle phases which make the joint poor in mechanical properties can be joined by friction welding. The sub-melting temperatures and short weld times of friction welding allow many combinations of work metals to be joined. There have been numerous studies related to wrought dissimilar metal welding, metal matrix composite welding and bulk metallic glasses welding using friction welding process [1], [2], [3], [4], [5], [6].

Innovation in materials and manufacturing processes make P/M components increasingly competitive in replacing wrought or cast materials for various applications. The flexibility of P/M part manufacture provides economic advantages because of the ability to press complex geometrical configurations. However, for some applications, P/M parts may have to be joined to one another or with other wrought materials as integrated components to achieve the required task. To enumerate these attributes, it becomes necessary to understand the behavior of joining or welding P/M parts.

Literature related to joining of P/M preform with wrought materials is sparse. At very beginning the sintered compacts have been joined by brazing [7]. In addition, weldments of sintered structural shapes joined to wrought materials have been successfully produced using a variety of welding techniques [8]. There have been very few articles originated related to the inertia welding of similar metals conveying that use of inertia welding eliminates porosity in the weld zone and enhanced hardness and fracture toughness at the weld interface [9]. Studies associated to the fusion welding of P/M preforms to wrought materials show that lower density parts are most often joined using processes that do not involve molten weld metal [10], [11]. The fewer particle to particle bonds at these density levels result in poor bond strength. This is because, the particle to particle bonds do not provide the ability to absorb high stresses generated by contracting weld metal. Therefore, low density components have to be joined by solid sate welding processes like diffusion bonding, sinter bonding, friction welding, adhesive bonding and brazing. Relevant study has demonstrated the feasibility of welding low density sintered steel P/M preforms using gas tungsten arc welding. P/M steel parts exhibiting higher densities (>7 g/cm³) typically have the same weldability as wrought materials and can be welded using most of the fusion welding processes [12], [13]. Recent studies on continuous drive friction welding of sintered P/M preform to wrought steel reported that inherent characteristics of P/M preform deformation have strong influence on the interface and consequently on the weld strength [14].

Many of the instruments used in automation field especially electro-hydraulic linear actuators with control systems make use of joined aluminium/copper and steel. Steel is used which addresses to strength, wear resistance while aluminium/copper to the electrical conductivity [15]. Since electrical signals for automatic and spontaneous response (for example, in safety devices) cannot be compromised, the copper in the wrought form was chosen in the present investigation. Control elements should have the basic requirements like high frequency, low inertia, and high strength. Besides they should have least relaxation time, which could ensure high response. The sintered powder metallurgical preforms have low mass, high stiffness and therefore their natural frequency is high. Having inherent porosity, they can also be good dampeners besides possessing the latent lubricant efficacy.

The present work was formulated to weld P/M steel preforms and wrought copper parts by continuous drive friction welding method. At the outset, the process conditions were optimized to attain high quality joints. Post-tests with microphotographs and energy dispersive X-ray analysis (EDAX) attached to SEM was carried out to substantiate the optimized conditions.