Chip Back Surface Research Articles

An Investigation of the Effect of Novel Mono/Bi-Layered PVD-Coated WC Tools on the Machinability of Ti-5Al-5V-5Mo-3Cr.

The Ti-5Al-5V-5Mo-3Cr (Ti-5553) alloy is a relatively novel difficult-to-cut material with limited machinability and tool life analysis available in the literature, and hence requires further investigation. This study focuses on the machining and tribological performance of Ti-5553 under high-speed finish turning (150 m/min, 175 m/min, and 200 m/min) via novel mono/bi-layered PVD-coated WC tools. A base AlTiN coating is used as the reference monolayer coating, with AlCrN, diamond-like ta-C, and TiAlSiN coatings each deposited on top of a base AlTiN coating, totaling four separate coated tools (one monolayer and three bi-layer). Tool life, cutting forces, workpiece surface quality, and tribological chip analysis are among the subjects of investigation in this study. Overall, the AlTiN/AlCrN coated tool outperformed all the other combinations: an improvement of ~19% in terms of tool life in reference to the base AlTiN coating when averaging across the three speeds; lowest surface roughness values: ~0.30, 0.33, and 0.64 µm; as well as the lowest chip back surface roughness values: ~0.80, 0.68, and 0.81 µm at 150, 175, and 200 m/min, respectively. These results indicate that the AlTiN/AlCrN coating is an excellent candidate for industrial applications involving high-speed machining of Ti-5553.

Numerical investigation on serrated chip formation during high-speed milling of Ti-6Al-4V alloy

Due to several inherent properties, Ti-6Al-4V alloy is a kind of typical difficult-to-machine material, especially during high-speed milling processes. The combined action of translation and rotation motion, as well as the short and variable undeformed chip thickness generation, make the milling process different from the turning process. Hence, it is essential to investigate the thermo-mechanical effect on serrated chip formation mechanism during the high-speed milling of Ti-6Al-4V alloy. The present paper investigates numerical methodology (i.e., the combined action of material constitutive with an energy-based ductile failure mechanism) for serrated chip formation in high-speed milling of Ti-6Al-4V alloy. First, a simplified 2D milling model was adopted based on the J-C constitutive equation combined with the energy density-based failure material model. Secondly, the proposed model was corroborated through experimentally obtained results. A good correlation was found between the numerically adopted model and experimentally obtained results. Thirdly, the physical phenomenon of the serrated chip formation and the effect of friction coefficient on shear stress is highlighted and discussed. Finally, the chip back surface's microstructural changes and phase transformation were investigated. The present investigation is beneficial to well understanding the serrated chips formation during high-speed milling of Ti-6Al-4V alloy as well as to optimizing process parameters and maintaining desirable machined surface integrity.

Modelling of phase transformations induced by thermo-mechanical loads considering stress-strain effects in hard milling of AISI H13 steel

Hard milling of difficult-to-machine materials has emerged as a popular manufacturing technology in the hot forging and mould manufacturing industries. Local phase transformation of steels often occurs under deeply coupled thermomechanical effects during the hard milling process. This work focuses on the prediction of phase transformations in hard milling of AISI H13 steel. First, a rapid heat phase transformation model that considers stress-strain effects is proposed and described based on phase transformation kinetics. Second, the proposed model is implemented into a validated finite element model (FEM) as a user subroutine of Abaqus/Explicit to describe phase transformations during chip formation in hard milling of AISI H13 steel. The predicted results indicate that martensite transforms into austenite in the chip back surface while no austenite is produced in the machined surface. The volume fraction of austenite in different zones increases when cutting speed is increased from 200 m/min to 400 m/min. Finally, the proposed metallo-thermomechanical coupled finite element model is verified by comparing the simulated results with experimental data. Good agreement is achieved, demonstrating that the proposed model can be used for prediction of phase transformations during hard milling of AISI H13 steel. The experimental and predicted results help to promote understanding of the phase transformations and microstructure evolution mechanisms of AISI H13 steel in hard milling processes. This study also contributes to optimizing the machining parameters to acquire the desired surface integrity in hard milling of AISI H13 steel.

Effect of edge hone radius on chip formation and its microstructural characterization in hard milling of AISI H13 steel

Chip formation as a key factor during the machining process can affect cutting stability and machined surface integrity, specifically the formation of saw-tooth chip. The saw-tooth chip formation with respect to edge hone radius during hard milling of AISI H13 steel (50 ± 1 HRC) was investigated in this paper involving chip morphology, metallurgical and mechanical property evolution, chemical composition distribution, and the mechanism of saw-tooth chip formation. The chip microstructural characterization was determined by means of optical microscopy, scanning electronic microscopy (SEM), a nano-tester, and X-ray diffraction (XRD). First, the effect of edge hone radius on chip morphology and evolution of chip color has been investigated. The results highlight that saw-tooth pitch decreases while the degree of segmentation is opposite with edge hone radius and the chip back surface color evolves gradually from purple to blue. Secondly, larger edge hone radius induces a thicker white layer generated on chip back surface and a higher value of nano-hardness identified inside an adiabatic shear band (ASB) as well as on chip back surface. In addition, the occurrence of phase transformation which occurs on chip back surface is observed which can be semi-quantified by the chip back surface color. Furthermore, the detected carbon content almost keep unvaried both in adiabatic shear band and on chip back surface, showing no impact on nano-hardness variation. Finally, it is indicated that the mechanism of saw-tooth chip formation in hard milling of AISI H13 steel is mainly attributed to adiabatic shearing accompanying with cyclic crack formation which facilitates the degree of segmentation. This research is a basic study for tool edge geometry preparation as well as controlling hard milling process.

Shear band formation and wear mechanism of titanium alloy powder metallurgy material prepared by HIP using Ti-6Al-4V pre-alloy powder

In this paper, wear mechanism model and shear plane formation model were built based on the influence of residual micro-void of powder metallurgy material on chip-tool interaction and serrated chip shear plane. In the wear mechanism model, the micro-void in the chip back surface tore and accumulated adhered workpiece material in sticking and sliding zones, respectively. In the occasional random shear model, the micro-void increased the change rate of the speed component near the micro-void to cause the shear plane shifts to the micro-void. Ti-6Al-4V powder metallurgy material was prepared with hot isostatic pressing technology to carry out machining experiment. The wear mechanism model and the occasional random shear model were verified with the research on micro-fluctuation of cutting force, wear morphology of cutting tool, and cross section of serrated chip.

Material Testing of Copper by Extrusion-cutting

An investigation was carried out on the use of extrusion-cutting as a material test method operating at severe conditions of strain, strain-rate and temperature, such as in machining. In extrusion-cutting, a shoe constrains the chip back surface producing a geometrically defined orthogonal cutting process which can be modelled using methods from the theory of plasticity such as, e.g., slip-line and upper-bound. The process was previously proposed for use as a material testing technique to determine the shear flow stress of materials under strains, strain rates and temperatures relevant for analytical modelling of metal cutting. This work represents a new step where the final objective is the generation of stress-strain curves that can be used in analytical models as well as using Finite Element Method (FEM) simulations. A new experimental setup for extrusion-cutting using discs as workpieces was developed and implemented on a CNC lathe. An investigation was carried out extrusion-cutting copper discs using high-speed-steel cutting tools at 100 m/min cutting speed. Flow stress values for copper under machining-relevant conditions were obtained from measurement of the extrusion-cutting force on the tool and application of a simple upper-bound model for the extrusion-cutting process. An attempt to extend the validity of test data to cover a range of cutting conditions was made, and suggestions for improvement of the simple theoretical model given.

Study on dynamic characteristics of serrated chip formation for orthogonal turning Ti6Al4V

A 2D orthogonal cutting process of Ti6Al4V was simulated to study the dynamic process of serrated chip formation and to establish the relationship between the tool-chip contact length and the saw-tooth pitch. All orthogonal cut experiments were carried out using a machined Ti6Al4V titanium alloy bar with PCBN tools. The features of the chip back surface and the cross-section surface were analyzed. The factors that affect the formation of adiabatic shear band were investigated. For the same chip length, the variation of the number of adiabatic shear band with cutting speed was investigated. The tool-chip contact length and saw-tooth pitch were measured to explore both the variation trends with cutting speed and their internal relations. When the cutting speeds are over 110 m/min, the whole tool-chip contact area can be treated as sticking. The shear stress was used to represent the tool-chip interface characteristics, and the trend of shear stress with cutting speed was studied.

Experimental study on cutting temperature and cutting force in dry turning of titanium alloy using a non-coated micro-grooved tool

Various non-coated micro-grooves with 7–149μm in depth and 0.14–0.50 in aspect ratio are proposed on tool rake surfaces along cutting chip flowing. The objective is to understand how the micro-groove shape and size influence cutting temperature and cutting force in dry turning of titanium alloy. First, the micro-grinding with a diamond wheel V-tip was employed to fabricate accurate and smooth micro-grooves; then dry turning experiments were performed with regard to material removal rate; finally, cutting temperature, cutting force and tool wear were investigated. It is shown that these micro-grooves patterned on tool rake surface contribute to decreasing cutting chip frictions and excluding cutting heat. The micro-grooved tool decreases cutting temperature by 103°C and more against traditional plane tool. The predicted cutting tip temperature reaches about 1322°C for traditional plane tool, but it does only about 500°C for micro-grooved tool. Moreover, the cutting temperature decreases and the shear angle increases with decreasing micro-groove depth except for the case of 7μm in micro-groove depth. The 25-μm-depth micro-grooved tool decreases cutting temperature and cutting force more greatly in larger material removal rate turning. In the dry turning with a micro-grooved tool, it is required that the micro-groove width is less than cutting chip width and its aspect ratio is enough large to maintain the air spaces between chip back surface and tool rake surface.

Machinability Studies in Hot Machining of Ti-6Al-4V Alloy

This work presents the influence of workpiece preheat temperatures on the machinability of Ti-6Al-4V alloy and chip formation. Machinability has been studied in terms of cutting forces, surface roughness and tool wear. Influence of preheat temperatures on chip morphology and roughness of chip back surface has been studied using scanning electron microscope and atomic force microscope respectively to get better insight of tribology at tool-chip interface. Based on overall observations, preheat temperature of 300 0C was found as the most appropriate parameter.

Chip morphology study in high speed drilling of Al–Si alloy

Chip formation during drilling operation is greatly influenced by the cutting parameters such as cutting speed, feed rate, and drill geometry. However, not many studies have focused on the direct observation of the chip formation during high speed drilling. This paper investigates the effect of cutting speed and feed rate on the chip morphology during high speed drilling of aluminum–silicon alloys using carbide drill. Observation on the multiview characterization of the chips was carried out which includes free surface, back surface, and cross-section of top surface. Structure and shape alterations of the free and back surfaces were analyzed using a scanning electron microscope. Finally, different geometrical parameters of chip cross-section were measured in order to study the effect of cutting parameters on chip compression ratio and thrust force. It was found that increase in cutting speed significantly affects the chip morphology especially on the structure of free surface and cross-section of the chips. Results also showed that built-up edge on the rake face of tool played an important role on the formation of irregular pattern on the chip back surface.

Probabilistic approach to evaluate improvements in the reliability of a chip-substrate (chip-card) assembly

A probabilistic approach is used to evaluate improvements in the reliability of the chip-substrate (chip-card) assembly against cracking of the integrated circuits (IC's). Such cracking is typically caused by bending deformations of the substrate. The suggested model is aimed at the assessment of the effect of the improvements in the IC preparation and encapsulation on the probability of the IC fracture. Examples of possible improvements are: application of low modulus encapsulant and die attach materials, change in the IC thickness and its positioning on the substrate, polishing of the chip's back surface, etc. The developed model enables one to evaluate the in-service variance of the substrate curvature from the reported IC failure statistics and additional experimental data obtained by bending the assembly on a series of circular mandrels. Then, one can evaluate the improvements in the reliability of the chip-substrate assembly from the changes in the mean and variance of the substrate curvature at the point of IC fracture, i.e., to assess the decrease in the probability of fracture for the improved design.

Extrusion cutting of brass strips

Extrusion cutting is a newly introduced process which can be described as cutting where the chip back surface is constrained by a shoe in such a way that the material is extruded through the gap between shoe and cutting tool. In contrast to what happens in conventional cutting, the chips obtained by extrusion cutting are of very good quality, being shiny on both sides and ductile. In the present work, an investigation was carried out about the feasibility of using extrusion cutting as a continuous manufacturing process for brass strip production. Orthogonal extrusion cutting was established on a lathe by radially feeding the tool into a disc clamped on a mandrel. It was shown that the process can be carried out in a stable way. The influence from varying tool geometry and work material was investigated.