Similar notch wear and chipping were reported by tool—workpiece interface to reduce notch wear in whisker Ezugwu et al. CBN inserts showing a , b depth of cut notch wear ; c chipping  3. Ti-based alloys following turning of Ti—6A—l4V. Kuljanic et al. This was attributed to the formation of a stable TiC layer that shields the tool surface from dissolution of PCD into the workpiece, similar to that outlined previously by Hartung and Kramer .
Ugarte et al. In reference to machining of cutting edge than coarse grain PCD . Poor adhesion of CVD diamond coating carbide drills outperformed HSS drills due to their higher hot on tungsten carbide substrate under practical cutting conditions is hardness . This and multifaceted carbide drills during ultra-high speed drilling up weakens the edge and accelerates edge wear. This increased the contact area thickness . Face milling of Nevertheless, workpiece quality can be affected in its integrity CFRP with PCD tools produced acceptable surface quality, even by the appearance of so-called surface anomalies that could be with cutting length as high as mm.
In the following, faces of the chisel edge, the primary cutting edge, the corner of the the main types of surface anomalies with their possible causes of drill, and the secondary cutting edges Fig. Tool wear by appearance are discussed. Ni-based superalloys of the WC grains and soft abrasion mode. Material drag or plastic deformation Fig. Coarse Of course, for continuous operations e. Note that material drag can also occur at low chip thicknesses e.
TEM and diffraction patterns revealing the partially recrystallized nanostructure of the SPD layer in RR after abusive drilling . Recently, some studies focused on the sensitivity of various Ni- based superalloys when being abusive machined i. The alloys with higher ultimate tensile strength UTS at high temperature i. Pluckings material pull-out and laps material re-deposition Fig.
Hughes et al. Ti-based alloys turned Ti—6Al—4V surfaces, with random instances of minor grain Analysis of machined subsurface typically reveals plastically pull, Fig. This was in line with results presented by Li et al. Similar defects were also observed on dry milled TiS workpieces together with tearing of the surface . Klocke et al. A negative rake angle, however, tends to result b crack-free surface with deformed lamellae . As seen in Fig. The formation of burrs at both entry and exit of the cutting edge from the workpiece are key considerations when drilling Ti alloys.
Additive Manufacturing of Ti-6Al-4V Alloy Components for Spacecraft Applications
Dornfeld et al. Unlike edge dissipation through the cutting edge material. For example, to low cutting temperature low cutting speed and high feed rate , with the evolution of tool wear, higher level and deeper even if the forces are high, result in best surface integrity, with no compressive residual stresses in turning of Inconel was noticeable damages [31,]. The reverse is also true when the reported —Fig.
It has also been observed that a smaller depth of cut generates less subsurface damage . Quantitative evaluation of workpiece surface integrity and the performance of machined parts 3.
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A much higher increase in near surface nanohardness was observed for RR and Alloy Li when compared to Alloy and Fig. This was correlated with the formation of thicker SPD nanostructured layers. Several researchers however reported that microhardness at the machined surface is above the material bulk value  due to the dominance of strain hardening, while cyclic heating and cooling has been suggested as the reason for harder regions observed deeper in the workpiece subsurface after milling [70,].
The likely variation in mechanical properties as a result of the material phase change however is unacceptable for aerospace applications. The latter was attributed to thermal quenching of the with minimal anomalies. Sridhar et al. The worn tools 0. The effect of the orbital workpiece material. This contradicted data reported by Chen et al.
This workpiece residual stress values [,,]. This diameter of the circle that encompasses the damaged area and Dh is effect was found to increase with both spindle speeds and feeds. Hole circularity was found to increase with speed but decrease with feed rate. These dissimilar materials introduced a unique Dh Dh set of problems during drilling. Similar approach has also been used by observed when dry drilling.
However, with the use of minimum Sadek et al. Extensive work has CFRP composites. Capabilities of various NDE techniques to assess different machining- induced defects . Interlaminate delamination internal defects around the circumference of the hole at a depth of 1. C-scan at a depth of 1.
Composite materials damage. For the exit side, severe delamination is seen with close Drilled holes generally reduce the bearing impact and fatigue agreement between the OCT and the OM measurements. Similar conclusion was drawn by 3. Relationship between surface integrity and part functional Damir et al. It can be seen that aerospace materials, there is very limited published information thermal damage has more detrimental effect in initiating failure on how these affect the functional performance of the parts .
Ni-based superalloys While some studies focused on the effect of surface roughness [,] and grain size  on specimen life, some studies to relate particular surface integrity characteristics e. Supported by surface integrity Fig. Such an effect can ca. Advanced machining technologies 3. Yao et al. Moreover, the ongoing demand for continuous increase in highlighted a marked decrease in average fatigue life at maximum productivity and process stability is the main driver for research nominal stress of MPa of high speed milled TB6 specimens as and development work.
High performance cooling strategies 4. High-pressure lubricoolant supply Due to the high mechanical and thermal tool load in cutting of 4. For that reason, tive industry . In aerospace industry, dry machining and MQL the application of advanced cooling strategies becomes more and cannot be used in the same extent as in automotive industry up to more important.
However, in this way alloys, such as titanium and nickel based alloys. Therefore, these materials are heat generation is not penetrated by the coolant . Thus, a liquid wedge more. The major aim is to develop adapted tool geometries and is formed which effectively cools and lubricates the cutting zone. Regarding a rake face high-pressure lubricoolant with emulsion, whereas dry machining showed worse results supply these effects can be traced back to three major mechanisms . Besides nickel based alloys, several research studies reported results of dry and MQL machining of titanium alloys [71,,,].
By optimizing the tool coating, Liu et al. Xie et al. Mechanisms of action of a rake face high-pressure lubricoolant supply reduced alternating thermal load and hence, thermally induced . It has been reported in the past that high temperatures at the In contrast to nickel based and titanium alloys, the application cutting zone leads to a vaporization of the lubricoolant and the of dry and especially MQL machining of aluminum alloys is widely generation of a vapour barrier which, during conventional used in aerospace industry and has been topic of various research lubricoolant supply, prevents effective cooling of the tool in the studies.
Main task in machining these materials is to handle the region of the cutting edge. The application of a high-pressure adhesion tendency by adapting the tool geometry, material and lubricoolant supply may displace the vapour barrier and enable coating, as well as the cutting conditions. This moisture absorption on the shear fracture toughness of FRPs.
In this regard it has to be prevented high velocity. One example is shown in Fig. Palanisamy et al. Moreover, they detected an increased frequency of chip serration and decreased average chip thickness . The reduction of tool temperature led to Fig. For cutting nickel based alloys with high-pressure lubricoolant supply, different and contradictory effects have been observed depending on the boundary conditions.
Ezugwu and Bonney  Fig. In contrast, at a cutting speed of only 4. For these with rising coolant pressure due to a reduced tool—chip contact materials, an alternative for removing the generated heat from the length. In addition, the decrease of the tool temperature gets only cutting zone is cryogenic cooling. This cooling strategy offers huge marginal after a critical value of the coolant pressure is reached.
Similar carbon dioxide CO2 due to their good availability and relatively results as Ezugwu and Bonney  were achieved in other studies safe handling. It is important to note that CO2 is technically process stability. This is due to the is often counted as cryogenic medium. In general, the main fact that in contrast to titanium alloys, the amount of mechanical advantage of cryogenic cooling is higher productivity by reduced wear mechanisms is higher when turning nickel based alloys. A lot of research studies on cryogenic machining have been Besides turning operations, extensive research has been performed in the last two decades, mainly using titanium, steel and conducted with cryogenic cooling in milling.
A comprehen- reported a positive effect of CO2 and particularly LN2 on tool life in sive review on cryogenic machining and its induced surface milling Ti6Al4V, Fig. Similar results for milling Inconel were reported by creasing hardness on the surface and generated compressive Shokrani et al. In the above-mentioned studies [3,,], residual stress. Besides, the state of the art on cryogenic processing the cryogenic media was supplied externally onto the tool.
Recent and machining was recently summarized by Shokrani et al. Cordes et al. Truesdale et al. In most research studies in the past, this supply method was preferred. In further studies of Hong et al. Bermingham et al. The tool holders were similar to the ones used at high-pressure lubricoolant supply. A Fig. Milling tool for internal supply of CO2 and aerosol . Similar results were reported by Klocke diffuser cases, shafts etc.
Tool life and surface to dry machining. The internal supply of LN2 through the spindle and the tool Using CO2 as cryogenic media, positive results on tool life and causes higher technical problems due to the low temperature of surface integrity during turning titanium alloys has been reported liquid nitrogen. The major research focus is therefore set on the by several authors [,,].
Regarding turning Inconel with LN2 cooling, extensive For external as well as internal supply of the cryogenic media, research has been reported by Pusavec et al. One major the combination of the cooling effect achieved by cryogenic cooling focus was to analyze the sustainability and economic feasibility and the lubrication effect of minimum quantity lubrication MQL of cryogenic cooling. The authors showed that the purchase has been in focus of research work in the last years and offers a of nitrogen can be compensated by the loss of conventional huge potential .
Hereby it has to be divided into different kinds coolants.
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In tool life tests they determined a comparable tool wear of supply systems. The most common way is to supply CO2 resp. One of the main advantageous of VAM is the reduction of In this state, the CO2 has good solubility cutting forces which is explained by the progressive removal of for aliphatic and most aromatic hydrocarbons and thus can carry smaller divisions of the uncut chip thickness in each of the metalworking lubricants in solution .
Another possibility is to intermittent cutting cycles. Tool life in end away from the workpiece. In end milling Ti6Al4V, Su et al. The main ing. Stephenson et al. The Inconel Silberschmidt et al. For drilling processes, the for vibration assisted turning of Ti6Al4V frequency 19 kHz, internal supply of the cryogenic media through the tool is even amplitude 0. Moreover, Koshimizu  observed a more important compared to milling. Muhammad  investigated hot ultrasonically assisted Overall, it can be summarized that cryogenic cooling offers a turning of b-titanium alloy Ti The vibration motion was high potential for increasing the productivity, as well as surface combined with hot machining in order to achieve combined integrity when machining aerospace alloys.
However, there is still advantages of both technologies.
Machining of Titanium Alloys and Composites for Aerospace Applications
The workpiece was pre-heated by a huge need for further research, amongst others focusing on a band resistance heater. Besides turning, the technology of VAM was comprehensively investigated in drilling aerospace alloys. In contrast to turning, only 1D VAM systems are in use for drilling. The vibration motion can be realized either at the workpiece or at the tool.
Pujana et al. In other studies, similar results in terms of better chip breakage, force and surface roughness reduction as well as tool life prolongation were achieved for drilling titanium alloys Fig. They proved a reduction of chip size, an increase of chip segmentation, a decrease 4.
Hybrid cutting processes of thrust force as well as a prolongation of tool life by 2. Moreover, lifting the drill during A comprehensive review on hybrid processes in manufacturing drilling high aspect ratios was not necessary. Best results were was recently published by Lauwers et al.
Baghlani et al. The latter is in focus of the separate The controlled application of low-frequency modulation fm to Section 4. Modulation Assisted Machining MAM have been successfully applied to enhance material removal process in the 4. The method Vibration-assisted machining VAM can be divided into relies on a precisely controlled sinusoidal oscillation that is ultrasonic assisted machining UAM , at which small-amplitude superimposed onto the cutting tool during the machining process and high-frequency tool displacement is added to the cutting thus enabling a small gap to form intermittently at the tool—chip, motion of the tool, and modulation assisted machining MAM tool—workpiece interfaces.
The cutting is momentarily disrupted applying high amplitudes and low frequency . The tool is and the otherwise continuous chip formation is converted to a either stimulated one dimensionally in a reciprocating motion series of discrete cutting events. In laser assisted machined chips, reduced strain, reduced temperature, reduced turning of b-titanium alloy Ti, Rahman et al. Brecher et al. In milling, vibration assisted machining is less used compared  investigated LAM of Inconel using a newly developed to turning and drilling due to a higher complexity of kinematics concept for milling, Fig.
In this concept, the laser spot is superposition depending on the workpiece geometry . Denkena et al. Novel concept for laser assisted milling . Although the preparation process of direct addition is relatively quick and simple, the luster of titanium brings a series of issues, such as the degree of wetting [ 8 , 9 , 10 ], the reaction interface [ 11 , 12 ], a higher manufacturing cost than that of ordinary materials, and others [ 13 , 14 ]. In situ synthesis can generate one or several enhanced phases through an in situ reaction under certain conditions.
This can be used to prepare particle-reinforced composites with a small particle size, a stable thermodynamic performance, no interface pollution, and a high bonding strength. It is a promising particle-reinforced composite material manufacturing process [ 15 , 16 , 17 ]. Research shows that TiB whiskers have as good physical properties and mechanical properties as in situ particles of TMCs. The interface between TiB and the Ti matrix introduced using the in situ, autogenous method is clean and has no interface reaction [ 18 , 19 , 20 ]. Chandravanshi et al.
Its good compatibility makes TiB an ideal reinforcement for titanium matrix composites. TMCs are mainly prepared by powder metallurgy PM , the casting solidification molding method, the spray molding method, and the laminated composite method [ 22 , 23 , 24 ].
For the base metal, a type and size of more than two kinds of second phase particle can be chosen according to the technical demands of the powder metallurgy composite method. However, the reaction between the matrix metal and strengthening particles is hard to achieve through the traditional hot pressing process [ 25 ]. Plasma activated sintering PAS is a technology developed in recent years for the synthesis of materials. Using the DC pulse voltage of the switch to generate an instantaneous, high-temperature plasma between the powder particles or the gap, it is possible to quickly eliminate the impurity and gas adsorbed on the surface of the powder particles, and promote the high speed diffusion and migration of the material.
Ghasali et al. The materials can be consolidated at lower temperatures, in a short period of time using the PAS process [ 28 , 29 , 30 ]. In this paper, the plasma activated sintering method was applied to the preparation of in situ titanium matrix composites by powder metallurgy. The influence of the PAS process and second phase particles used for the in situ reaction on the microstructure and mechanical properties of the prepared titanium matrix composites were investigated. In this work, spherical TC4 titanium alloy powders 6. The milled powders were compacted into a graphite die with an inside diameter of 25 mm.
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Before sintering, the chamber was evacuated to a high vacuum. Vickers hardness was measured on the polished sections of the samples using Vickers indentation Tukon , Buehler, Binghamton, NY, USA at a load of g held for 15 s. Each sample was tested three times with nominally identical specimens to obtain an average value of the yield strength. Figure 2 shows the relative densities of TMCs sintered at different temperatures.
The densities of the two kinds of composites are The relative densities of titanium-based metal matrix composites TMCs sintered at different temperatures. Figure 3 shows the microscopic morphology of the samples with the addition of C-B at different sintering temperatures, and the combination of the particles with the matrix.
The thickness of the reaction layer was 1. TiB whiskers were dispersed in the matrix; these were the complete reaction product of C-B and the Ti matrix. The reaction layer between the residual elemental boron particles and the matrix became thicker, measuring up to 3. There were no obvious existing defects at the interface between the C-B particles and the matrix.
EDS analysis was carried out on the micro-area; the distribution of elements is shown in Figure 4. The main elements in the diffusion layer were titanium, vanadium, and boron; the content of aluminum was low. Therefore, the diffusion layer was mainly formed by the diffusion and mutual reaction of these three elements.
When the titanium element is in a supersaturated state, the reactant TiB 2 will continue to react with the titanium to form TiB; the saturated state of the C-B elements near the side of the C-B particles does not provide the necessary conditions for any further reaction. Figure 6 shows the microstructure of the composites with A-B addition at different sintering temperatures, and the microstructure of the matrix without the addition of the reinforcing phase at the corresponding sintering temperature.
However, the blank control group sample was fully saturated.
However, the agglomeration of boron could still be observed in some areas, while reactant TiB whiskers were distributed in the matrix material. The grain shape of the material was a mixture of short plate and equiaxed, while the control group sample transformed into a whole lamellar tissue.
The microstructure of the composites was still of an equiaxial and short-plate shaped mixed state. These phenomena were due to the fact that, in the control group samples, the powder was in contact with the same species that facilitates the formation of the sintered neck and the inter-diffusion of the elements. With the addition of the enhanced phase, the boron particles were uniformly wrapped around the matrix powder, which hindered the diffusion between the matrix powders, making it difficult to achieve sintering densification.
Research showed that TiB whiskers could also pin the grain boundary effectively, preventing the grain from growing at high temperature. When its major axis is parallel to the grain boundary, TiB whiskers have the most remarkable pinning effect on the grain growth [ 35 ]. The XRD patterns are displayed in Figure 7. The sample with A-B addition showed higher TiB diffraction peaks than the C-B—TC4 sample at the peak positions of the , , and crystal faces. The complete reaction of A-B and the titanium matrix led to the higher content of TiB, while the typical B diffraction peaks in the C-B—TC4 composite sample are due to the amount of residual crystalline boron.
Figure 8 shows the Vickers hardness value of samples sintered at different temperature. The addition of A-B led to a more pronounced upward trend. A-B was more fully involved in the reaction under the same conditions, resulting in a higher volume fraction of TiB whiskers, and leading to an increase in grain boundaries. The high-density boundaries block the movement of dislocations because of the dislocation entanglement phenomenon; this improved the deformation resistance. Because the value of Vickers hardness was calculated by the area of indentation and the test pressure [ 36 ], the higher density of boundaries provided by the full reaction between A-B and the matrix contributed more to the hardness of the samples than that of the incomplete reaction of C-B.
Figure 9 a shows the compressive properties of the C-B addition samples prepared at different sintering temperatures. The compressive strength rose as the sintering temperature increased. This was because a high sintering temperature promotes the interface reaction between the element and the matrix. The same phenomenon occurred in Figure 9 b, which shows the compressive strength curves of the A-B addition samples. The results show that the compressive strength of the sample with the enhanced phase is significantly higher than that of the matrix material, and the yield strength was increased by 8.
Huang et al. When true strains reached 1. An equiaxed microstructure was obtained after the complete recrystallization of the alloy, and a good effect on the comprehensive performance of the material was reported [ 35 ]. The compression strength curves of different groups of samples, a with C-B addition; b with A-B addition; c comparison between TMCs and the matrix material; d specific node value. With the increase of the sintering temperature, A-B reacts with the matrix to form TiB whiskers. Review from Ringgold Inc.
Their topics include turning investigations on machining of titanium alloy Ti64 with different cutting tool inserts, the multi-objective optimization of drilling titanium alloy Ti64A4V, the laser assisted machining of titanium alloys, the influence of tool geometry and machining parameters on the surface quality and the effect of surface quality on the compressive strength of plastic reinforced with carbon fiber, and challenges in drilling multi-materials.
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