Serafino Caruso

MODELING OF WHITE AND DARK LAYER FORMATION IN HARD MACHINING OF AISI 52100 BEARING STEEL

In machining of hardened materials, maintaining surface integrity is one of the most critical requirements. Often, the major indicators of surface integrity of machined parts are surface roughness and residual stresses. However, the material microstructure also changes on the surface of machined hardened steels and this must be taken into account for process modeling. Therefore, in order for manufacturers to maximize their gains from utilizing hard finish turning, accurate predictive models for surface integrity are needed, which are capable of predicting both white and dark layer formation as a function of the machining conditions. In this paper, a detailed approach to develop such a finite element (FE) model is presented. In particular, a hardness-based flow stress model was implemented in the FE code and an empirical model was developed for describing the phase transformations that create white and dark layers in AISI 52100 steel. An iterative procedure was utilized for calibrating the proposed empirical model for the microstructural changes associated with white and dark layers in AISI 52100 steel. Finally, the proposed FE model was validated by comparing the predicted results with the experimental evidence found in the published literature.

Finite Element Modeling of Microstructural Changes in Hard Turning

The material grain size changes significantly during machining of hardened steels, and this must be taken into account for improved modeling of surface integrity effects resulting from machining. Grain size changes induced during orthogonal cutting of hardened AISI 52100 (62 HRC) are modeled using the Finite Element (FE) method; in particular, a user subroutine involving a hardness-based flow stress model is implemented in the FE code and empirical models are utilized for describing the phase transformation conditions to simulate formation of white and dark layers. Furthermore, a procedure utilizing the Zener-Hollomon relationship is implemented in the above-mentioned user subroutine to predict the evolution in material grain size at different cutting speeds (300, 600, 900 SFPM). All simulations were performed for dry cutting conditions using a low CBN-content insert (Kennametal KD050 grade, ANSI TNG-432 geometry). The model is validated by comparing the predicted results with experimental evidence available in the literature.

Analysis of Surface Integrity in Dry and Cryogenic Machining of AZ31B Mg Alloys

Surface integrity of machined products can have a critical impact on their performance, such as corrosion, wear and/or fatigue resistance. It has been reported that reducing the grain size of AZ31B Mg alloys could significantly enhance its corrosion resistance, which is often the limiting factor for its wide application. Severe plastic deformation (SPD) has proved to be an effective way to induce grain refinement. In this study, the potential of cryogenic machining as a novel SPD method to induce grain refinement on the surface of AZ31B Mg alloys was investigated. The microstructures of the workpiece surface/sub-surface and the machined chips after both dry and cryogenic machining were studied. A surface layer where nanocrystallized grains exist was found in the machined surface under cryogenic conditions. Increasing the edge radius of the cutting tool resulted in a thicker grain refinement layer. In addition to the experimental study, an FE model based on the Johnson-Cook constitutive equation was developed and validated using experimental data in terms of chip morphology and forces. The capability of this model to predict critical deformation parameters for dynamic recrystallization (DRX), such as strain, strain-rate and temperature, was demonstrated. With further development, the model can be used to predict the onset of DRX and the grain size on the machined surface.

Finite element modelling of microstructural changes in dry and cryogenic machining AISI 52100 steel

The paper presents the results of investigations of grain size and hardness variation induced by orthogonal machining of hardened AISI 52100 bearing steel in the white layer (WL). The experiments were performed under dry and cryogenic cooling conditions using honed cubic boron nitride tool inserts. The experimental results are compared with a newly developed finite element model to describe microstructural changes and dynamic recrystallisation. Furthermore, the numerical model will be able to suggest if the WL formation is mainly thermally induced through phase transformation or mainly mechanically induced through severe plastic deformation. An iterative procedure was utilised to experimentally calibrate and then validate the proposed model at varying cutting speeds and cutting cooling conditions (dry and cryogenic). This paper is part of a Themed Issue on Recent developments in bearing steels.

Tool Engage Investigation in Nickel Superalloy Turning Operations

This paper reports the results of an experimental study of the tool wear and cutting forces in turning of Inconel 718 with coated carbide inserts. Inconel 718 is a difficult-to-cut nickel-based super-alloy commonly used in aerospace industry. The effects of cutting speed, feed rate and cutting tool geometry on tool wear have been widely analyzed in literature. Turning operations on complex components such as aircraft engines casings require the insert replacement at the end of each geometric feature manufacturing, independently from the actual tool wear level. For this reason, it is important to preserve tool integrity mainly in the most critical phase of operation (i.e., when the tool engages the workpiece). In fact, if the tool is damaged in this stage the quality of the whole operation is compromised. The attention has been focused on engage cutting conditions because the phenomenon that appears in this critical step plays a wide influence on tool integrity and, consequently, on the quality of the operation. For this purpose a nickel-based super alloy ring-workpiece, (Inconel 718), has been machined in lubricated cutting conditions by using a CNC lathe with carbide coated tools. Two variables have been investigated in this study: the Depth Of Cut (DOC) and the approaching Engage angle (En). In the studied working conditions Speed (S), Feed-rate (F) and removed volume (Vrim) were kept constant. Both tool wear and cutting forces evolution during cutting have been analyzed.

Experimental comparison of the MIG, friction stir welding, cold metal transfer and hybrid laser-MIG processes for AA 6005-T6 aluminium alloy

In this study, the mechanical properties of welded joints of AA 6005-T6 aluminum alloy obtained with hybrid laser-MIG and cold metal transfer (CMT) welding were analyzed. The performance of hybrid laser-MIG and CMT welded joints were identified using tensile, bending, shear and fatigue life tests. Taking into account the process conditions and requirements, hybrid laser-MIG and CMT welding processes were compared with friction stir welding (FSW) and conventional metal inert gas (MIG) welding processes, shown in a previous work, to understand the advantages and disadvantages of the processes for welding applications of studied Al alloy. Better tensile, bending and shear strength and fatigue life behavior were obtained with hybrid laser-MIG and FSW welded joints compared with conventional MIG processes.

Experimental Comparison of the MIG and Friction Stir Welding Processes for AA 6005 Aluminium Alloy

In this study, the mechanical properties of welded joints of AA 6005 aluminum alloy obtained with friction stir welding (FSW) and conventional metal inert gas welding (MIG) are studied. FSW welds were carried out on a semi-automatic milling machine. The performance of FSW and MIG welded joints were identified using tensile and bending impact tests, as far as the environmental aspects are also included in the discussion. The joints obtained with FSW and MIG processes were also investigated in their microstructure. The results indicate that, the microstructure of the friction stir weld is different from that of MIG welded joint. The weld nugget consists of small grains in FSW than those found in MIG weld. Taking into consideration the process conditions and requirements, FSW and MIG processes were also compared with each other to understand the advantages and disadvantages of the processes for welding applications of studied Al alloy. Better tensile and bending strength were obtained with FSW welded joints.

Dry vs. Cryogenic Orthogonal Hard Machining: an Experimental Investigation

Friction, and consequently heat generation in the cutting zone, significantly affects the tool life, surface integrity and dimensional accuracy, apart from other machining results. Application of a coolant in a cutting process can increase tool life and dimensional accuracy, decrease heat generation, and consequently cutting temperatures, reduce surface roughness and the amount of energy consumed in cutting process, and thus improve the productivity. Furthermore, coolant application also affects the surface microstructural alterations (i.e., white and dark layers) due to a machining operation, which have a significant influence on product performance and life. This paper presents the results of an experimental investigation to determine the effects of cryogenic coolant application on tool wear, cutting forces and machined surface alterations during orthogonal machining of hardened AISI 52100 bearing steel (54±1 HRC). Experiments were performed for dry and cryogenic cutting conditions using chamfered PCBN ...

Residual Stresses in Machining of AISI 52100 Steel under Dry and Cryogenic Conditions: A Brief Summary

Residual stress is one of the most important surface integrity parameter that can significantly affect the service performance of a mechanical component, such as: contact fatigue, corrosion resistance and part distortion. For this reason the mechanical state of both the machined surface and subsurface needs to be investigated. Residual stress induced by dry and cryogenic machining of hardened AISI 52100 steel was determined by using the X-ray diffraction technique. The objective was to evaluate the influence of the tool cutting edge geometry, workpiece hardness, cutting speed, microstructural changes and cooling conditions on the distribution of the residual stresses in the machined surface layers. The results are analysed in function of the thermal and mechanical phenomena generated during machining and their consequences on the white layer formation.

Experimental analysis of influence of cutting conditions on machinability of waspaloy

Taking into account the main importance of nickel-based superalloy in aerospace, marine and chemical industries concerning the production of high performance artifacts, in this paper experimental results of cutting forces, chip morphology, tool wear and temperatures were investigated during orthogonal machining of Waspaloy (45 HRC). All the experiments were performed in dry and lubricated cutting conditions, analyzing and comparing the collected results for a range of different cutting parameters: cutting speed and feed rate. The results show a good trends coherence, highlighting the influence of lubrication during machining.