Abstract
The efficiency of a machining process can be measured by evaluating the quality of the machined surface and the tool wear rate. The research reported herein is mainly focused on the effect of cutting parameters and tool wear on the machined surface defects, surface roughness, deformation layer and residual stresses when dry milling Stellite 6, deposited by overlay on a carbon steel surface. The results showed that under the selected cutting conditions, abrasion, diffusion, peeling, chipping and breakage were the main tool wear mechanisms presented. Also the feed rate was the primary factor affecting the tool wear with an influence of 83%. With regard to the influence of cutting parameters on the surface roughness, the primary factors were feed rate and cutting speed with 57 and 38%, respectively. In addition, in general, as tool wear increased, the surface roughness increased and the deformation layer was found to be influenced more by the cutting parameters rather than the tool wear. Compressive residual stresses were observed in the un-machined surface, and when machining longer than 5 min, residual stress changed 100% from compression to tension. Finally, results showed that micro-crack initiation was the main mechanism for chip formation.
Highlights
Cobalt-based alloys, named ‘‘Stellite,’’ have a wide range of applications due to their higher melting temperature, superior thermal fatigue resistance, weldability, high-temperature corrosion and oxidation resistance, and wear resistance
One way to increase the product life of components subjected to aggressive environments involves the deposition of a surface layer of Stellite on a steel substrate. This process can be conducted by fusion welding techniques such as gas tungsten arc welding (GTAW), gas metal arc welding (GMAW) and plasma transferred arc (PTA) (Ref 3)
The results showed that TiAlN-coated carbide tools in general demonstrated a good machining performance at low levels of cutting speed and loads; it is not recommended for relatively heavy cutting loads; built-up-edge formation was found under some cutting conditions, that excessive tool flank typically resulted in tool breakage at the cutting edge of the uncoated tools and that at low cutting speeds, abrasive and adhesive wear were often observed while diffusion, chipping and chemical wear occurred at higher cutting speeds
Summary
Cobalt-based alloys, named ‘‘Stellite,’’ have a wide range of applications due to their higher melting temperature, superior thermal fatigue resistance, weldability, high-temperature corrosion and oxidation resistance, and wear resistance. One way to increase the product life of components subjected to aggressive environments (i.e., erosive wear) involves the deposition of a surface layer of Stellite on a steel substrate This process can be conducted by fusion welding techniques such as gas tungsten arc welding (GTAW), gas metal arc welding (GMAW) and plasma transferred arc (PTA) (Ref 3). Bagci et al (Ref 7) studied the milling surface roughness of Stellite 6 cobalt-based alloy using PVD TiN-coated carbide tools, and Benghersallah et al (Ref 8) examined the dry milling machinability of Stellite 6 alloy layers using tools with five different multilayer coatings Both groups found that when using a cutting speed between 120 and 230 m/min, a radial depth of cut of 3 mm, an axial depth of cut of 0.3 and 0.067 mm/tooth for the feed, the tool life was in general within 2 min when considering a maximum wear of VB = 0.3 mm as the tool life criterion. In order to fill the knowledge gap related to the milling of Stellite 6 coatings, a novel study related to chip formation mechanism has been undertaken as well as a study relating the influence of cutting parameters and tool wear on the quality of the machined surface to establish the optimal parameters for the efficient and economic machining of this alloy
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