Jorge A. Olortegui-Yume

Crater Wear Evolution in Multilayer Coated Carbides During Machining Using Confocal Microscopy

Abstract Steady-state turning experiments were carried out with multilayer coated inserts consisting of TiN/Al2O3/TiCN deposited on a carbide substrate. Confocal microscopy was used for the first time to observe the topography of crater wear evolution in multilayer coated inserts. A hump made of TiN coating next to a growing crater of Al2O3, traces of attached steel, and the maximum depth regions have been identified. Scoring marks were also detected in the TiN layer, indicating the presence of abrasion wear. Interestingly, the crater depth was stagnant once it reached the Al2O3 layer, and the wear progresses by broadening the area of exposed Al2O3. It was concluded that the effectiveness of multilayer coated tools comes from the dissolution resistance of the Al2O3 layer, which delays depth growth and develops the wear front into a wider area. Confocal microsocopy was found to be a valuable tool to obtain wear topography for multilayer coated tools.

Tool wear mechanisms in machining

Most of tool wear studies are classified as empirical (e.g. Taylors equation); thus, they do not bring out the physical nature of the wear phenomenon. Consequently, tool life in general cannot be predicted by extending the result from one study. By understanding the physics behind the process, the important wear mechanisms can be identified. By constructing a wear model for each wear mechanism with more fundamental quantities such as materials properties, these models can be combined and extended to estimate tool life. This should be the ultimate goal of tool wear research in machining. However, a major gap exists between the current understandings of tool wear and the ultimate goal of tool wear research. This paper will describe how cutting tools are being worn down during machining based on the physics behind tool wear.

Measurement of Droplet Size and Distribution for Minimum Quantity Lubrication (MQL)

Cutting fluid in machining is important due to their role in reducing tool wear and improving surface finish. However, controlling the usage is imperative due to the ecological and/or economical and reasons. Minimum quantity lubrication (MQL) has recently emerged as a viable option to minimize the amount of fluid used in machining without sacrificing its lubrication and cooling in machining. This paper presents our effort to understand the parameters in the MQL process in order to expand its effectiveness in cutting operations. Although some literature regarding the influence of flow rates and nozzle directions is available, this information is insufficient. This paper studies three new parameters in the MQL process, namely the droplet size, the droplet size distribution, and the wetting angle. Confocal laser scanning microscopy (CSLM), wavelet filtering, and image processing algorithms were used to calculate the droplet geometry and distribution for various commercially available MQL oils. The wetting angles of the oils were measured by depositing oil droplets onto TiSiN and TiAIN coated inserts. The parameters studied are important to estimate the comparative performance of lubricant-coating pairs.

Strength and dynamic characteristics analyses of wound composite axial impeller

A low cost, light weight, high performance composite material turbomachinery impeller with a uniquely designed blade patterns is analyzed. Such impellers can economically enable refrigeration plants to use water as a refrigerant (R718). A strength and dynamic characteristics analyses procedure is developed to assess the maximum stresses and natural frequencies of these wound composite axial impellers under operating loading conditions. Numerical simulation using FEM for two-dimensional and three-dimensional impellers was investigated. A commercially available software ANSYS is used for the finite element calculations. Analysis is done for different blade geometries and then suggestions are made for optimum design parameters. In order to avoid operating at resonance, which can make impellers suffer a significant reduction in the design life, the designer must calculate the natural frequency and modal shape of the impeller to analyze the dynamic characteristics. The results show that using composite Kevlar fiber/epoxy matrix enables the impeller to run at high tip speed and withstand the stresses, no critical speed will be matched during start-up and shut-down, and that mass imbalances of the impeller shall not pose a critical problem.

Local Crater Wear Prediction Using Physics-Based Models

A physics-based, pointwise model is developed to predict crater profiles of multilayer coated carbides after a series of turning experiments. Dissolution and abrasion mechanisms, which are identified to be the dominant wear mechanisms at the crater, are reformulated into a pointwise or local quantity to predict the crater profiles based on the temperature and pressure profiles from finite element (FE) simulations. The crater profiles predicted by the proposed model have to be adjusted, however, due to the creep deformation of the carbide substrate occurring under the machining conditions employed in our experiment. The crater predictions correlate pretty well with the crater profiles experimentally observed in the multilayer (TiN―Al 2 O 3 ―TiCN) coated carbides until the wear front reached the middle of the Al 2 O 3 layer. At this point, the Al 2 O 3 coating undergoes the κ-to-α-phase transformation, which makes the wear prediction difficult due to substantial changes in the thermomechanical properties of the Al 2 O 3 coating.

The genesis of tool wear in machining

This paper presents a series of experimental and theoretical efforts that we have made in unraveling the tool wear mechanisms under steady state conditions in machining for the last few decades. Two primary modes of steady state tool wear considered in this paper are flank and crater wear. We preface this paper by stating that flank wear is explained as abrasive wear due to the hard phases in a work material while crater wear is a combination of abrasive wear and generalized dissolution wear which encompasses both dissolution wear as well as diffusion wear. However, the flank wear was not a function of the abrasive cementite content when turning low alloy steels with pearlitic microstructures. The machined surfaces of these alloys are examined to confirm the phase transformation (ferrite to austenite), which diminishes the effect of cementite content. In particular, the cementite phase present in low alloy steels dissociates and diffuses into the transformed austenitic phase during machining. Dissolution wear is claimed to describe the behavior of crater wear at high cutting speeds. The original dissolution mechanism explains the crater wear in the machining of ferrous materials and nickel alloys at high cutting speeds, but the generalization of the dissolution wear is necessary for titanium alloys. In machining titanium alloys, the original dissolution mechanism did not show a good correlation with experimental results; generally the diffusivity of the slowest diffusing tool constituent in titanium limits the wear rate. The phase transformation from alpha (HCP) to beta (BCC) phases can also take place in machining titanium alloys, which drastically increases the crater wear due to the few orders of magnitude increase in diffusivity. The most puzzling issue is however the presence of the scoring marks even though no hard inclusion is typically present in titanium alloys. This is finally explained by the heterogeneity in the microstructure due to the anisotropic hardness of alpha (HCP) phase (the hardness in c-direction is 50% higher than the hardness in other directions) and the presence of lamellar microstructure (alternating layers of alpha and beta). The lamellar microstructure has not only the in-plane anisotropic hardness but also a greater hardness than other phases. Even though we cannot claim to fully understand the physics behind tool wear, our combined approaches have unveiled some elementary wear mechanisms.Copyright

Stress and Vibration Analysis for Woven Composite Axial Impeller

A stress and vibration analysis procedure is developed to assess the static and dynamic characteristics of the Woven Composite Axial-Impeller under loading conditions particular to centrifugal force. This procedure is based on Finite Element Analysis (FEA, commercial software ABAQUS) results. A low-cost, light-weight, high-performance, composite turbomachinery impeller with uniquely designed blade patterns are evaluated. Understanding the stress-strain behavior of fiber composite laminates to ultimate failure and the ability to predict ultimate strength is critical in the design of safe and lightweight impellers. To determine failures, the maximum stress failure criterion is used. In order to avoid resonance which can make impellers suffer a significant reduction in the design life, the designer must calculate the natural frequency and modal shape of impeller to analyze the dynamic characteristics. The results show that composite material Kevlar fiber/epoxy matrix enables the impeller to run at high tip speed of 450m/s and withstand the stresses; no critical speed will be matched during start-up and shut-down and that mass imbalances of the impeller shall not pose a critical problem.Copyright

Local Tool Wear Prediction Using Physics-Based Models

A semi-empirical model based on the physics behind tool wear that can depict the tool wear locally is developed to predict crater profiles of multilayer coated carbides. Dissolution and abrasion relationships are recast into a point-wise or local version to predict directly based on the temperature and pressure profiles from FE simulation. The approach is reasonable to explain the crater profiles observed in multilayer coated carbides. However, the model deviates from the real profiles due to the κ-to-α phase transformation in the middle Al2 O3 layer, the change in the friction conditions as each layer is exposed, and the combined wear resistance of multi-layers of the cutting tool.Copyright