Till now titanium and its alloys used in di/uniFB00erent industrial sectors. Unique material characteris- tics make it as desirable raw material for the automotive, aerospace, petroleum, chemical, marine and biomedical industries. It requires deformation and fabrication process as di/uniFB03cult-to-cut material. There are several challenges hidden under the processes. Therefore, advanced machining process performance investigation in titanium and its titanium alloys machining has taken part of the research concern. A number of research works has been done in every year to show the research direction. However, most of them are speci/uniFB01cally in one machining process. It’s important to have a clear picture of a research area for further research consideration. Therefore, this review aim to study recent articles of non-traditional machining process of titanium and its alloys. The focus of this review was on the contribution for solv- ing existing problems by using non-traditional machining processes, most e/uniFB03cient process and general overview. At the end it also provided a summary of sustainable issue of non-traditional machining pro- cesses. /one.pnum Introduction Unique material properties makes titanium alloys to a widely used alloys in the automotive, aerospace, petroleum, chemical, marine and biomedical industries. It’s commonly used in aeronautic industry due to the characteristics like low density, strength and high temperature resistance. However, it also considered as an expensive material because of deformation and fabrication process. In many cases, it’s required ma- chining works where titanium alloys categorized as di/uniFB03cult-to-cut materials. There are several materials, mechanical and machining properties that in/uniFB02uence to the titanium alloys machining process Gupta and Laubscher (/two.pnum/zero.pnum/one.pnum/six.pnum); Niknam et al. (/two.pnum/zero.pnum/one.pnum/four.pnum); RAHMAN et al. (/two.pnum/zero.pnum/zero.pnum/three.pnum) such as low thermal conductivity which leads heat concentration on the cutting edge; high chemical reactivity causes rapid wear, chipping or cutting tool failure; continuously high temperature and stress generation in between chip and cutting edge (within /zero.pnum./five.pnummm); low elastic modulus. To solve these problems, research studies is being carried out into new tool developments considering tool design and tool materials; optimization of cutting parameters(cutting force, speed, depth of cut, feed rate etc.) Niknam et al. (/two.pnum/zero.pnum/one.pnum/four.pnum); and development of advanced material removing processes e.g. Ultrasonic Vibration Assisted Machining(UVAM)Muhammad et al. (/two.pnum/zero.pnum/one.pnum/four.pnum), Laser Assisted Machining(LAM)Dandekar et al. (/two.pnum/zero.pnum/one.pnum/zero.pnum), Electric Discharge Machining(EDM)Hasçalık and Çaydaş (/two.pnum/zero.pnum/zero.pnum/seven.pnum). A number of advanced machining process performance investigation in hard-to-machine materials includ- ing titanium alloys machining has taken part of the research concern due to the capability for overcom- ing existing problems. The /one.pnum shows the research works on non-traditional machining of titanium alloys. In this case, ultrasonic machining, electrochemical machining (ECM), electrical discharge machining, laser assisted machining, abrasive water jet machining (AWJM) are considered as non-traditional machining pro- cess. The graph represents the number of research articles has published in major journals in the year /two.pnum/zero.pnum/zero.pnum/nine.pnum to present. It is clearly indicates that increasing of research in the /uniFB01eld of titanium machining. The growth in USM, ECM and EDM signi/uniFB01cantly higher than LAM and AWJM. However, number of research works is uplift- ing for the LAM and AWJM. In addition, research issues in non-traditional machining processes can be seen as machinability in term of comparison between conventional and non-traditional machining; modelling and optimization; in/uniFB02uence of process parameters; performance such as surface quality after machining process completed, temperature during machining process etc. (see /two.pnum). For machining process like USM, 1 Figure /one.pnum: Number of articles on non-traditional machining of titanium alloys extracted from di/uniFB00erent jour- nals database. ECM and LAM, data shows that research works are carried out to investigate in/uniFB02uence of process param- eters and machining performance such as surface quality of machined workpiece much more than the investigation of machinability and modelling and optimization. Moreover, for EDM the graph indicates the highest research works done in machining performance. For AWJM, surprisingly research on modelling and optimization has taken place the top among the others. Overall, the graph reveals the research focus in non-traditional machining process. Figure /two.pnum: Percentages of studies categorized by types of non-traditional machining extracted and summa- rized from di/uniFB00erent journals database. There are several research works has been done in every year that shows research direction or trends. However, most of them are speci/uniFB01c in one machining process. Sometime that create complexities for new /two.pnum research direction. In addition, it’s important to have a clear picture of a research area for further research consideration. Therefore, this study aim to review last ten years articles in non-traditional machining pro- cess of titanium and its alloys. The focus of this review will be on the contribution for solving existing problems by using non-traditional machining processes mentioned earlier. This article organized with the start of introduction, secondly, recent works of non-traditional machining and thirdly, improvement com- pared to conventional machining process. Then, /uniFB01nally concluded with the suggestions of most e/uniFB03cient machining process for titanium alloys machining. In this study, most of the articles are collected from major journals. /two.pnum Ultrasonic Machining (USM) Ultrasonic machining or ultrasonic vibration assisted machining process remove materials from workpiece through high frequency and low amplitude vibrations, piezoelectric actuator used as the source of vibra- tion. The frequency is about /two.pnum/zero.pnum to /four.pnum/zero.pnum kHz. This machining process used as an alternative option of EDM and good choice for hard materials machining. USM considered as one of the high e/uniFB03cient machining process for titanium alloysXu and Zhang (/two.pnum/zero.pnum/one.pnum/five.pnum). Result of Maurotto et al. (/two.pnum/zero.pnum/one.pnum/two.pnum) shown that at least /seven.pnum/five.pnum% of cutting can be reduced compare to conventional machining of titanium alloys while using USM. He also provides a comparison of surface quality which revealed a clear statement on advantage of USM in term of surface quality compare to conventional machining. In the study of Koshimizu (/two.pnum/zero.pnum/zero.pnum/eight.pnum), shows a reduction of /three.pnum/three.pnum–/five.pnum/zero.pnum% cutting force. At the same time, the study states that semi-dry cutting condition is suitable for ultrasonic assisted turning. Similar improvement results found in di/uniFB00erent studies using USM in term of surface quality, cutting force and cutting temperature Maurotto et al. (/two.pnum/zero.pnum/one.pnum/three.pnum); Muhammad et al. (/two.pnum/zero.pnum/one.pnum/four.pnum); Maurotto et al. (/two.pnum/zero.pnum/one.pnum/two.pnum); Patil et al. (/two.pnum/zero.pnum/one.pnum/four.pnum); Wu et al. (/two.pnum/zero.pnum/one.pnum/three.pnum). A summary of recent works and improvement from conventional machining has shown in the Table /one.pnum. Author(s) USM Improvements compared with conventional machining Type Mode Chen et al. (/two.pnum/zero.pnum/one.pnum/eight.pnum) UVM /one.pnumDReduction of /two.pnum/two.pnum./eight.pnum% in cutting temperature near the cut- ting zone Sofuoğlu et al. (/two.pnum/zero.pnum/one.pnum/eight.pnum) UVT /one.pnumDReduction of cutting force /seven.pnum/zero.pnum%; Maximum e/uniFB00ective stress at least /two.pnum/five.pnum% Ni et al. (/two.pnum/zero.pnum/one.pnum/eight.pnum) UVM /one.pnumDCutting force components Fx and Fy reduced by /two.pnum/one.pnum./five.pnum- /three.pnum/seven.pnum./two.pnum/four.pnum% and /three.pnum/one.pnum./zero.pnum/two.pnum-/four.pnum/six.pnum./three.pnum/zero.pnum%; Surface roughness Ra /two.pnum/five.pnum./nine.pnum% and /four.pnum/eight.pnum./three.pnum% Llanos et al. (/two.pnum/zero.pnum/one.pnum/eight.pnum) UVT /one.pnumDReduction of /four.pnum./seven.pnum/six.pnum% for tangential force, /eight.pnum./six.pnum/two.pnum% for axial force /nine.pnum./zero.pnum/four.pnum% for radial force Wang et al. (/two.pnum/zero.pnum/one.pnum/nine.pnum) UVM /one.pnumDRadial force, axial force, and tangential force decreased by /six.pnum/zero.pnum%, /two.pnum/seven.pnum./seven.pnum%, and /three.pnum/three.pnum% Tan et al. (/two.pnum/zero.pnum/one.pnum/nine.pnum) UVT /two.pnumD Surface roughness value reduced about /three.pnum/one.pnum% Puga et al. (/two.pnum/zero.pnum/one.pnum/nine.pnum) UVT /one.pnumDCutting force in feed direction decreased by /two.pnum/four.pnum./eight.pnum% and in the width of cut direction decreased by /two.pnum/nine.pnum./nine.pnum% Zheng et al. (/two.pnum/zero.pnum/one.pnum/eight.pnum) UVM /one.pnumDAverage friction coe/uniFB03cient wear volume decrease /three.pnum/five.pnum% and /six.pnum/seven.pnum%, in oil lubrication Hu et al. (/two.pnum/zero.pnum/one.pnum/nine.pnum) UVT /one.pnumD /four.pnum/two.pnum% of cutting force decreased Bai et al. (/two.pnum/zero.pnum/one.pnum/seven.pnum) UVM /one.pnumDIn CT, grain size beneath machined surface was larger at lower depth and smaller at higher depth; grain size in shear band was bigger in case of UAT. Vibration amplitude had insigni/uniFB01cant e/uniFB00ect on grain size Chen et al. (/two.pnum/zero.pnum/one.pnum/nine.pnum) UVM /one.pnumDcutting forces reduced by /six.pnum/four.pnum%, the surface roughness re- duced, compressive residual stress increased by /eight.pnum/five.pnum% *UVM-Ultrasonic vibration assisted milling; UVT- Ultrasonic vibration assisted turning. Table /one.pnum: Performance of USM in titanium alloys It can be seen from the previous section that most of the research output concluded about the comparison and improvement. Among them, surface quality, cutting force, cutting temperature was the main concern in their studies. However, some other issues such as in/uniFB02uence of cutting /uniFB02uid, ultrasonic parameters also has been considered. Dixit et al. (/two.pnum/zero.pnum/one.pnum/nine.pnum) concluded in his review article that it is important to have research /three.pnum focus on in/uniFB02uence of friction in ultrasonic vibration assisted machining process. He also found less stud- ies from economic and power consumption perspectives. So, the further study can be to emphasize on sustainable manufacturing for titanium alloys machining process. /three.pnum Electrochemical Machining (ECM) The electrochemical machining (ECM) is a modern nontraditional method of machining based on the elec- trochemical dissolution of metal aimed at the production of parts of prescribed shape, dimensions, and surface /uniFB01nish McGeough (/one.pnum/nine.pnum/seven.pnum/four.pnum); Rumyantsev and Davydov (/one.pnum/nine.pnum/eight.pnum/nine.pnum); Wilson (/one.pnum/nine.pnum/seven.pnum/one.pnum). There are several ap- plication of it in titanium part preparations such as gas turbine engine parts, engine blades etc. Figure /three.pnum gives a scheme of one type of ECM, the formation of a hole of circular cross-section using a tubular tool- electrode (TE). A hole is formed by deepening TE into the workpiece at a constant rate equal to the rate of metal dissolution at the bottom of the hole. Thereby, the inter electrode distance (the machining gap SF) is held constant. To enhance the accuracy of replication of TE onto the workpiece (WP), the inter electrode distance should be as small as possible, frequently, less than /one.pnum/zero.pnum/zero.pnum /uni00B5m. To remove the products of electro- chemical and possible chemical reactions from the working zone and refresh the electrolyte composition in the working zone, an intense electrolyte /uniFB02ow is organized. Many speci/uniFB01c features of ECM depend on the type of workpiece, type of required operation, and the tech- nology applied. For example, the machining of large-sized parts and complex-shaped parts on special ma- chines, the machining with the removal of large amounts of metal and application of powerful sources and large volumes of electrolyte di/uniFB00er signi/uniFB01cantly from the micro- and nanomachining with simple-shaped TE in the form of a needle or plate on small machines with the dissolution of small amounts of metal Davydov et al. (/two.pnum/zero.pnum/one.pnum/seven.pnum). In addition, ECM does not cause heat-a/uniFB00ected zones and internal stresses on the machining surface Xu et al. (/two.pnum/zero.pnum/one.pnum/six.pnum), therefore it can produced parts with high accuracy and better surface quality for hard-to-cut material like titanium. Figure /three.pnum: Scheme of ECM of workpiece (/one.pnum) by tubular tool-electrode (/two.pnum) with insulated side surface (/three.pnum). The arrow at VTE shows the direction of TE feed, the arrows from Ve show the direction of electrolyte /uniFB02ow, SF is the end face inter electrode gap, Ss is the side inter electrode gap, I denotes the machining zone (working zone), II denotes area adjacent to the working zone, and III denotes the zone of passive titanium Davydov et al. (/two.pnum/zero.pnum/one.pnum/seven.pnum). There are di/uniFB00erent types of ECM, however, micro-ECM is become very popular in recent years due to the high accuracy, small amount of material removal with a low current which better than conventional ECM. Unfortunately, this method can’t be applied for the titanium as highly prone to passivation metal when /four.pnum coated with thick passive resistive oxide /uniFB01lms, because the oxide /uniFB01lms exhibit signi/uniFB01cantly higher resis- tance than the solution layer in the inter electrode gap. In addition, it is important to choose an appro- priate electrolyte and relevant current parameters. Xu et al. (/two.pnum/zero.pnum/one.pnum/six.pnum) concerned that self-passivation is one of the problem in ECM process, which inhibits the dissolution process. Therefore, he suggested to select corresponding process parameters. In/uniFB02uence of process parameters Anasane and Bhattacharyya (/two.pnum/zero.pnum/one.pnum/seven.pnum) is given below: /one.pnum. Electrolyte: Electrolyte plays an important role in term of surface quality, texture, accuracy, mate- rial removal rate etc. for example methanol electrolyte is better for shape and surface; Solution of methanol sulfuric acid has e/uniFB00ective in electro polishing of titanium. /two.pnum. Machining Voltage: voltage between cathode and anode is also critical for controlling anodic disso- lution. Di/uniFB00erent amount of voltage has in/uniFB02uence to remove material in di/uniFB00erent rate. /three.pnum. Pulse Duty Ratio: Duty ratio represents the percentage of time for which pulse remains on i.e. per- centage of time available for both faradic and non-faradic current. Increase in duty ratio in turn increases time available for faradic current. Hence, the amount of faradic e/uniFB00ect and current density increases results in more material removal. /four.pnum Electric Discharge Machining (EDM) Electric discharge machining (EDM) is one of non-traditional material removing process, which is success- fully used in di/uniFB03cult-to-machine materials like titanium. Many industry used the process due to the high accuracy, ease of cutting complex shape and better surface roughness. EDM process introduce a residual stress free material because of the tool (electrode) and workpiece material untouched condition Qudeiri et al. (/two.pnum/zero.pnum/one.pnum/eight.pnum). The principle of the EDM technique is to use thermo-electric energy to erode a workpiece by rapidly recurring electrical discharges (sparks) between the untouched electrode and workpiece. The small distance between tool (electrode) and workpiece named discharge gap, which /uniFB01lled with dielectric /uniFB02uid. Flushing the dielectric /uniFB02uid during the machining process carries away debris (removed solid particles) and restores the sparking condition in the gap. As no cutting forces are used in between tool and the workpiece results an untouched condition which helps to eliminates vibration or stresses during machining. Figure /four.pnum: Classi/uniFB01cation of Electric Discharge Machining Process In EDM process there are several process parameters such as peak current, pulse o/uniFB00 time, discharge volt- age, pulse on time, polarity, pulse wave form, discharge gap, /uniFB02ushing of the dielectric /uniFB02uid, and workpiece rotation; which are a/uniFB00ected the EDM performance e.g., surface quality, material removal rate and elec- trode wear rate(EWR). MRR is one of the concerning issue in EDM due to the low material removal rate compare with the other non-traditional machining process Qudeiri et al. (/two.pnum/zero.pnum/one.pnum/eight.pnum). EDM forms in di/uniFB00erent name with basic principles in various process. Qudeiri et al. classi/uniFB01ed EDM process in /five.pnum category shown in /uniFB01gure /four.pnum. Many research has been carried out to investigate the e/uniFB00ect of process parameters on performance of tita- nium alloys in EDM process. Moreover, several methods has been introduced to improve the performance such as surface quality, machining accuracy and material removal rate. Bhaumik and Maity investigated e/uniFB00ect of electrode material in di/uniFB00erent EDM aspect of titanium alloys. They concluded that copper and brass electrode provides good surface /uniFB01nish Bhaumik and Maity (/two.pnum/zero.pnum/one.pnum/eight.pnum). In addition, zing electrode re- sulted higher defects. Some of recent studies of titanium alloys in EDM process is shown in the Table /two.pnum. In the EDM process, several research objective reported in recent works namely modelling and optimiza- tion; surface quality; dielectric /uniFB02uid research; electrode materials used in EDM process; Machinability and process performance. /five.pnum Author(s)EDM TypeObjective Remarks Pramanik et al. (/two.pnum/zero.pnum/one.pnum/nine.pnum)W OptimizationInvestigates geometrical errors and trend found that in/uniFB02uence of tension in wire on the /uniFB02exibility of wire, the dependence of heat generation and dissipation on pulse on time, and ability of the /uniFB02ushing pressure to control the cooling, as well as debris removal from the machining zone. Sahu and Mahap- atra (/two.pnum/zero.pnum/one.pnum/eight.pnum)D-S OptimizationMaterial removal rate (MRR), tool wear rate (TWR) and average surface roughness (Ra) are used to ana- lyze optimized by GRA method combined with Fire/uniFB02y algorithm Tonday and Tigga (/two.pnum/zero.pnum/one.pnum/nine.pnum)WCharacterization of Surface In- tegritycutting factors of wire EDM signi/uniFB01cantly in/uniFB02uences the surface characteristics of machined samples Ahmed et al. (/two.pnum/zero.pnum/one.pnum/eight.pnum)D-SMachinability and performanceAl electrode: the lowest surface roughness Graphite electrode: maximum material removal rate Negative tool polarity: rough machining Positive tool polarity: /uniFB01ne machining. Kumar et al. (/two.pnum/zero.pnum/one.pnum/eight.pnum)WProcess perfor- manceEDM performance is assessed in terms of MRR and TWR. Surface integrity of the machined specimen is evaluated in purview of surface morphology and topographical features including surface roughness, surface crack density, white layer thickness, mate- rial migration, phase transformation, residual stress, and micro indentation hardness Ramamurthy and Muthu- ramalingam (/two.pnum/zero.pnum/one.pnum/eight.pnum)WMachinability and performanceConventional brass wire electrode has produced higher material removal rate. The zinc di/uniFB00used brass wire electrode can produce lower surface rough- ness. Shabgard and Khosrozadeh (/two.pnum/zero.pnum/one.pnum/seven.pnum)D-SMachining performanceSurface micro cracks reduced in carbon nanotubes (CNT) particles in dielectric. Addition of CNTs into dielectric exhibited advantages in reducing MRR and TWR Kumar et al. (/two.pnum/zero.pnum/one.pnum/seven.pnum)D-SElectrode Mate- rialE/uniFB00ect of deep cryogenic treatment (DCT) on tool wear rate during electric discharge machining of Ti- /five.pnumAl-/two.pnum./five.pnumSn titanium alloy process parameters namely cryogenic treatment of electrode material, peak cur- rent, pulse-on o/uniFB00 time and /uniFB02ushing pressure. Chen et al. (/two.pnum/zero.pnum/one.pnum/four.pnum) /uni00B5Performance and Dielectric /uniFB02uidAdding concentration of Ti powder into dielectric solvent prevent the formation of surface cracks and micro-cracks, raise the wettability on the surfaces of Ti Kolli and Kumar (/two.pnum/zero.pnum/one.pnum/four.pnum)P-MPerformance and Dielectric /uniFB02uidAddition of B/four.pnumC powder in various concentrations has in/uniFB02uenced the material removal rate, surface roughness and tool wear rate. Jabbaripour et al. (/two.pnum/zero.pnum/one.pnum/three.pnum)P-MPerformance and Dielectric /uniFB02uidDi/uniFB00erent powders such as aluminum, chrome, sili- con carbide, graphite and iron is performed to in- vestigate the output characteristics of surface rough- ness and topography, material removal rate (MRR), electrochemical corrosion resistance of machined samples and also the machined surfaces are inves- tigated by means of EDS and XRD analyses Table /two.pnum: Studies of titanium alloys in EDM process. /six.pnum /five.pnum Laser Assisted Machining (LAM) The idea of laser assisted machining (LAM) has come with the concept of localized heat for reducing cutting force during machining process, and ultimately it results decrease in yield strength Dandekar et al. (/two.pnum/zero.pnum/one.pnum/zero.pnum). In this process, workpiece material heated by laser (e.g., CO/two.pnum and Nd:YAG) and material removed with traditional cutting tools Venkatesan et al. (/two.pnum/zero.pnum/one.pnum/four.pnum). For titanium alloys treatment Nd:YAG and CO/two.pnum usually used, characteristics of this laser type shown in table /three.pnum. Most of the research is concerning advantages of LAM and challenges in traditional machining process. However, the performance of LAM depends on various parameters such as laser parameters and machining process parameters. Laser power and spot diameter of laser beam are main parameters of LAM. In term of in/uniFB02uence of process parameters and it performance, many studies has been conducted. As a hybrid machining process, investigation of LAM provides various results. Table /four.pnum, shows a summary of LAM machining process and performance of titanium and its alloys. Properties Nd:YAG CO/two.pnum Wavelength ( /uni00B5m) /one.pnum./zero.pnum/six.pnum /one.pnum/zero.pnum./six.pnum Overall e/uniFB03ciency (%) /one.pnum-/three.pnum /five.pnum-/one.pnum/zero.pnum Output power in CW mode =/one.pnum/six.pnum kW =/two.pnum/zero.pnum kW Focused power density (W/cm/two.pnum) /one.pnum/zero.pnum/five.pnum-/seven.pnum(/uniFB02ash lamp), /one.pnum/zero.pnum/six.pnum-/nine.pnum(diode) /one.pnum/zero.pnum/six.pnum-/eight.pnum Pulse duration (sec) /one.pnum/zero.pnum-/eight.pnum-/one.pnum/zero.pnum-/three.pnum /one.pnum/zero.pnum-/four.pnum Fiber coupling - x Table /three.pnum: Characteristics of laser in titanium alloys machining process Lee et al. (/two.pnum/zero.pnum/one.pnum/six.pnum). Author(s) Objective Remarks Rashid et al. (/two.pnum/zero.pnum/one.pnum/two.pnum) E/uniFB00ect of laser powerHigher laser power(/one.pnum./two.pnum-/one.pnum./six.pnumkW) e/uniFB00ective for re- ducing cutting force Ayed et al. (/two.pnum/zero.pnum/one.pnum/four.pnum)Physical properties dur- ing MachiningDistance between the axis of the laser beam and the tool rake face is the most important parameter that can reduce /five.pnum/zero.pnum% of the cutting force Oh et al. (/two.pnum/zero.pnum/one.pnum/eight.pnum)Machining characteristic and energy e/uniFB03ciencySpeci/uniFB01c cutting energy reduced /six.pnum% than con- ventional milling Abdollahi et al. (/two.pnum/zero.pnum/one.pnum/nine.pnum) Process optimizationIncrease of vibration amplitude, laser power, pulse frequency and gas causes MRR Chatterjee et al. (/two.pnum/zero.pnum/one.pnum/eight.pnum)Performance Character- isticsLaser energy and pulse width signi/uniFB01cantly in/uniFB02u- ence on circularity, taper and spatter area Bermingham et al. (/two.pnum/zero.pnum/one.pnum/five.pnum)Performance of Tool wearAt low cutting speeds, LAM o/uniFB00ered a marginal improvement in tool life compared to room temperature dry machining Habrat (/two.pnum/zero.pnum/one.pnum/seven.pnum) MachinabilityTangential cutting force reduced /six.pnum/zero.pnum%. No im- provement in chips forming Hedberg et al. (/two.pnum/zero.pnum/one.pnum/five.pnum) Machining performanceCutting force reduced by /three.pnum/zero.pnum-/five.pnum/zero.pnum%; Cutting speed can be increased /three.pnum/zero.pnum% without reducing tool life; Compressive residual stress reduced by /one.pnum/zero.pnum%; A /three.pnum/three.pnum% cost can be reduced using LAM Table /four.pnum: Studies of LAM on titanium alloys. Among the types of machining process, some important issues, challenges and opportunities exist. For hybrid conventional laser assisted machining process it can be categorized by machining cost, tool life, and /uniFB02ow stress. At the same time the challenges are to overcome by solving the problem of complex control system and application to material with various shapes. /six.pnum Abrasive Water Jet Machining (AWJM) Abrasive water jet machining (AWJM) is one of the /uniFB02exible material removal method that become popular among other machining process. The reason behind this is elimination of thermal e/uniFB00ect and minimum /seven.pnum stress in workpiece Kartal (/two.pnum/zero.pnum/one.pnum/six.pnum). In this machining process, mechanical energy of water with the abrasive particle used to cut the material by erosion Li and Wang (/two.pnum/zero.pnum/one.pnum/five.pnum). The water pumped to high pressure (/four.pnum/zero.pnum/zero.pnum- /six.pnum/zero.pnum/zero.pnum MPa) and exit through the nozzle (ranging from /zero.pnum./one.pnum to /one.pnum./zero.pnum mm) Shukla (/two.pnum/zero.pnum/one.pnum/three.pnum). The velocity of water is about /one.pnum/zero.pnum/zero.pnum/zero.pnumm/s which used to cut materials like metals, ceramics, glass etc. To improve the machining performance for abrasive water jet process, many research works has been conducted. As hard-to-cut material and thermal characteristics of titanium alloys, number of research has been done. Most of the research focus on the in/uniFB02uence of machining process parameters, machining performance such as surface quality and the modelling and simulations. Table /five.pnum, shows the summary of the recent work on titanium and its alloys using abrasive water jet machining. Author(s)Process Parame- tersObjective Remarks Pahuja and M. (/two.pnum/zero.pnum/one.pnum/nine.pnum)Pressure, Speed ModellingPower-to-speed ratio is e/uniFB00ective lumped control parameter which physically represented the en- ergy deposited per unit linear distance moved by the jet. Kumar and Shukla (/two.pnum/zero.pnum/one.pnum/two.pnum)particle impact angle, velocity, depth, erosion rateModeling Simula- tionSigni/uniFB01cant variation of crater geometry for impact of up to the /uniFB01rst /one.pnum/seven.pnum particles, depending on impact- ing velocity and angle. Vasanth et al. (/two.pnum/zero.pnum/one.pnum/six.pnum)Water pressure, abrasive /uniFB02ow rate, feed rate, stando/uniFB00 distanceE/uniFB00ect of process parame- tersAbrasive /uniFB02ow rate and stando/uniFB00 distance has the most signi/uniFB01cant role on determining surface qual- ity. Higher abrasive /uniFB02ow rate with higher stando/uniFB00 distance produces higher surface roughness ow- ing to larger and random energy distribution. Bui et al. (/two.pnum/zero.pnum/one.pnum/seven.pnum)Gaussian curve, feed rateModellingThe model rapid and e/uniFB00ective as the accuracy of the models obtained over a number of con/uniFB01gura- tions was to within the order of /five.pnum% Hlaváč et al. (/two.pnum/zero.pnum/one.pnum/five.pnum)Traverse speed, limit depth of cut, limit anglePerformanceThe theoretical model and the experiment show a very good correlation. Perec (/two.pnum/zero.pnum/one.pnum/eight.pnum)Traverse speed, concentration of abrasiveAbrasive materialThe garnet abrasive material gave the greatest cutting depth, Olivine caused the greatest abra- sive wear of the focusing tube, Crushed glass abra- sive caused the smallest value of wear coe/uniFB03cient. Alberdi et al. (/two.pnum/zero.pnum/one.pnum/five.pnum)stack con/uniFB01gura- tion, the traverse feed rate, the cutting tool and the pressure over the kerf pro/uniFB01leMachinability perfor- manceThe most signi/uniFB01cant factors for the taper angle on both materials are the pressure and the stack con- /uniFB01guration, followed by the traverse feed rate. To minimize the taper angle and the roughness, it is recommended to use high-pressure, low traverse feed rates. Li and Wang (/two.pnum/zero.pnum/one.pnum/five.pnum)Water pressure drilling timeE/uniFB00ect of process parame- tersIt was found that both the hole depth and diam- eter increased as drilling time increased but in a decreasing rate. An increase in water pressure in- creased both the hole depth and the hole diame- ter. Table /five.pnum: Summary of recent works on titanium alloys. Some other researchers also focus on surface quality and classifying the surface of workpiece material. Pal and Choudhury (/two.pnum/zero.pnum/one.pnum/four.pnum) generated pocket on titanium surface with adjusting parameters namely pressure, stando/uniFB00 distance and abrasive size. Surface waviness, roughness and crater has been measured. Results shown that small abrasive size provides good surface quality. In another work, Kerf geometry, surface roughness (Ra), microstructural analyses and material removal rate (MRR) of Ti-/six.pnumAl-/four.pnumv were investigated using Abrasive water jet machining Gnanavelbabu et al. (/two.pnum/zero.pnum/one.pnum/eight.pnum). He mentioned surface roughness is a/uniFB00ected by high pressure of water jet. Results of Babu and Muthukrishnan (/two.pnum/zero.pnum/one.pnum/four.pnum) also provides conclusion for surface quality. /eight.pnum /seven.pnum Sustainable issues in Non-Traditional Machining Sustainability is one of the important issue in modern machining process. For some complex production, it is impossible to produce the product without non-traditional machining process. The concerning issues can be a/uniFB00ected health, environment or even economy. Table /six.pnum shows the most common issue regarding non-traditional machining process and the properties that are responsible for that. Process Properties Sustainable issue USM Hard Particles.Environmental problems; Health problems due to ECF, ul- trasound and slurry /uniFB02uids causing acoustic trauma, tinnitus, dermatitis. ECMHigh dissolution rate at very large current densi- ties.Corrosions risk; High energy consumption; Complex waste management; Electrolyte disposal issues; Liver, kidney and skin irritation; Handling and storage di/uniFB03culty. EDMVolatility, Odorless, /uniFB02ash point, environment friendly, viscosity.Harmful vapor, skin irritation, odor, risk of /uniFB01re and explo- sions, high oxidation rate, high rate of sludge accumulation. LAMGas, noise, magnetic /uniFB01eld.Health problems. AWJMChips, liquid, Noise, Vi- bration.High energy consumption, recycling issue, health issue. Table /six.pnum: Sustainable issue in non-traditional machining process Rajurkar et al. (/two.pnum/zero.pnum/one.pnum/seven.pnum); Saxena et al. (/two.pnum/zero.pnum/one.pnum/eight.pnum). /eight.pnum Conclusion Non-traditional machining process for titanium alloys due its capability to solve existing problems. Research works in the /uniFB01eld of non-traditional machining is still going on. This paper reviewed several articles on di/uniFB00erent types of non-traditional machining process. Therefore, this section can be arranged with some conclusions. /one.pnum. First section of this paper presented current trends on non-traditional machining process. Its shows EDM has the /uniFB01rst priority for the research work as the number of published articles in last /one.pnum/zero.pnum years. Moreover, the research focuses speci/uniFB01cally on investigation of machining process parameter and performance. /two.pnum. Ultrasonic machining has successfully reduced cutting force, temperature and improve surface qual- ity for titanium alloys. However, more research need to have in term of cutting mechanics and solving sustainable issues such as energy consumption and economic aspects. /three.pnum. For electric discharge machining, review shows that dielectric /uniFB02uid has signi/uniFB01cant e/uniFB00ect on machining performance. In addition, low discharge energy provides better surface with less defects. Integration of other hybrid process resulted better outcome. Future research can be focused on combining EDM with other process like laser assisted machining. /four.pnum. As the main problem of electrochemical machining on titanium is prone to passivation in the aque- ous electrolyte solutions. Which is required for high amount of dissolution of titanium. Therefore, electrolyte solutions can be the focused for the further research. In addition it’s important to have focus for sustainable issue as ECM machining process used much more chemical compared to other non-traditional machining process. /five.pnum. Laser assisted machining has already achieve great attention for the capability of overcome by reduc- ing cutting energy and improve surface roughness of titanium alloys. The review reveals that LAM signi/uniFB01cantly improve the product quality by decreasing cost. /six.pnum. In Abrasive water jet machining Garnet, silicon carbide dust, and aluminum oxide are most commonly used as abrasive materials, however more research work need to do for further development of this process. /nine.pnum /seven.pnum. Sustainability is most common issue in modern manufacturing process. Specially, for non-traditional machining process, there are not many studies has been carried out. Therefore, more research should be focused on environmental and economic issues.