Abrasive Size Research Articles

Selective laser melting of oxide dispersion strengthened MA956 alloy and its surface finishing by magnetic field assisted finishing

Selective laser melting (SLM) is a prominent metal additive manufacturing (AM) capable of processing a myriad of engineering materials with high precision and design freedom. However, similar to other AM processes, poor surface finishing has been an omnipresent problem in SLM technology. In this study, magnetic field assisted finishing (MAF) was used to finish SLM fabricated oxide dispersion strengthened (ODS) MA956, an iron‑chromium‑aluminum alloy. The effect of laser processing parameters on part density and surface roughness was first studied. Using a multi-objective optimization technique, the optimal parameters to obtain the highest density and lowest surface roughness were determined. Finally, MAF was applied to the parts built with the obtained optimal SLM parameters, but without significant improvement on the surfaces of as-printed samples. Hence, the surfaces of as-printed samples were post-processed (ground) to yield better initial surface conditions prior to MAF. The effect of initial roughness, iron particles size, and abrasive size on MAF performance was studied. Initial roughness had the most dominant effect followed by abrasive size. The underlying mechanism behind the dependency on initial roughness on final surface quality was analyzed by studying the change on the surface profiles with different starting initial roughness. The initial roughness required for MAF to be effective was determined. Using the optimal processing conditions, MAF was applied to the post-processed samples to attain the final average surface roughness (Ra) as little as 0.36 μm starting from initial average roughness (Ra) of 1.53 μm.

Evaluating the efficacy of recycled garnet abrasives in enhancing hard rock cutting performance of abrasive waterjet systems

Abrasive waterjet (AWJ) systems represent an emerging technology for hard rock cutting in urban underground spaces, offering low vibration and precise cutting performance. However, the high consumption of abrasives, which accounts for approximately 60% of the total cutting cost, presents a practical limitation for AWJ systems. Recycling used abrasives is a potential approach to improve the economic feasibility of AWJ technology for commercial applications. This study investigates the effects of abrasive recycling on granite specimens, using cutting performance metrics such as depth and volume. Additionally, the optimal particle size for reusability was determined through abrasive particle size analysis. The results reveal that a 40%–60% recycled garnet mixture is the optimal abrasive mixing ratio for AWJ rock cutting, potentially enhancing the cost-effectiveness of AWJ projects.

Influences of process parameters on chemical mechanical polishing effect of ZnGeP2 crystal

To obtain the ultra-smooth surface of ZnGeP2 crystal (ZGP), the workpiece-abrasive-polishing pad contact model during polishing was established to calculate the load Fw and the cutting depth hw of a single abrasive. The influences of polishing methods, abrasive hardness, abrasive size, polishing pressure, polishing time and other factors on the polishing effect were studied, and the mechanism underpinning the formation of surface morphology after polishing was revealed, and the mathematical model between material removal rate (MRR) and surface roughness (Ra) was established. The results show that the critical cutting depth hc of ZnGeP2 crystal is 95.21 nm, when the polishing load is 5 N, the material removal under mechanical polishing and chemical mechanical polishing entails plastic removal. Compared with mechanical polishing, the MRR of chemical mechanical polishing is increased by 23.4% and Ra is decreased by 26.5%. The increases of abrasive hardness, abrasive size and polishing load will increase the cutting depth hw of the abrasive, resulting in the increases of MRR and Ra, but the influence forms of various factors are different. After optimizing the process parameters, the minimum Ra of ZnGeP2 crystal is 0.76 nm. MRR and Ra will decrease significantly with the increase of polishing time, and there is a significant correlation between them.

Environment-friendly, efficient process for mechanical recovery of waste lithium iron phosphate batteries.

Technology for recycling retired lithium batteries has become increasingly environment-friendly and efficient. In traditional recovery methods, pyrometallurgy or hydrometallurgy is often used as an auxiliary treatment method, which results in secondary pollution and increases the cost of harmless treatment. In this article, a new method for combined mechanical recycling of waste lithium iron phosphate (LFP) batteries is proposed to realize the classification and recycling of materials. Appearance inspections and performance tests were conducted on 1000 retired LFP batteries. After discharging and disassembling the defective batteries, the physical structure of the cathode binder was destroyed under ball-milling cycle stress, and the electrode material and metal foil were separated using ultrasonic cleaning technology. After treating the anode sheet with 100 W of ultrasonic power for 2 minutes, the anode material was completely stripped from the copper foil, and no cross-contamination between the copper foil and graphite was observed. After the cathode plate was ball-milled for 60 seconds with an abrasive particle size of 20 mm and then ultrasonically treated for 20 minutes with a power of 300 W, the stripping rate of the cathode material reached 99.0%, and the purities of the aluminium foil and LFP reached 100% and 98.1%, respectively.

Experimental Study on Shear Rheological Polishing of Si Surface of 4H-SiC Wafer

In this study, shear rheological polishing was used to polish the Si surface of six-inch 4H-SiC wafers to improve polishing efficiency. The surface roughness of the Si surface was the main evaluation index, and the material removal rate was the secondary evaluation index. An experiment was designed using the Taguchi method to analyze the effects of four critical parameters (abrasive particle size, abrasive particle concentration, polishing speed, and polishing pressure) on the Si surface polishing of SiC wafers. By evaluating the experimental results for the signal-to-noise ratio, the weight of each factor was calculated using the analysis of variance method. The optimal combination of the process parameters was obtained. Below are the weightings for the influence of each process on the polishing result. A higher value for the percentage means that the process has a greater influence on the polishing result. The wear particle size (85.98%) had the most significant influence on the surface roughness, followed by the polishing pressure (9.45%) and abrasive concentration (3.25%). The polishing speed had the least significant effect on the surface roughness (1.32%). Polishing was conducted under optimized process conditions of a 1.5 μm abrasive particle size, 3% abrasive particle concentration, 80 r/min polishing speed, and 20 kg polishing pressure. After polishing for 60 min, the surface roughness, Ra, decreased from 114.8 to 0.9 nm, with a change rate of 99.2%. After further polishing for 60 min, an ultrasmooth surface with an Ra of 0.5 nm and MRR of 20.83 nm/min was obtained. Machining the Si surface of 4H-SiC wafers under optimal polishing conditions can effectively remove scratches on the Si surface of 4H-SiC wafers and improve the surface quality.

Super hard and brittle material removal mechanism in fixed abrasive lapping: Theory and modeling

Currently, the problems of inefficient fixed abrasive lapping (FAL) for super hard and brittle material wafers (such as sapphire) with fixed abrasive pads (FAPs), restrict the development of the semiconductor industry. Most importantly, the mechanisms of FAL processes are still unclear, often analyzed based on the theoretical models of chemical mechanical polishing (CMP). However, the material removal models of CMP are specific to loose abrasives, do not effectively apply to the lapping processes for super hard and brittle material wafers with a FAP. In this paper, the spherical crown model modified by elastic-plastic mechanics and brittle-plastic transformation mechanism was applied to super hard and brittle material removal mechanism in FAL processes. In consideration of the influences of the yielding abrasives embedded in the FAP matrix and the matrix surface topography on the wafer-pad contact, a material removal model suitable for super hard and brittle material wafers lapped by FAPs was obtained. The loads applied on the fixed abrasives and the matrix asperities during the elastic-plastic-brittle deformation processes of wafers were analyzed, respectively. The model indicates that the back-off of the abrasives embedded in the FAP matrix intensifies along with the decrease of the matrix hardness and the increase of the abrasive size, respectively. The penetrating depth of the fixed abrasive can be raised by increasing matrix hardness, applied pressure, abrasive size, and decreasing abrasive number, as same as the elastic recovery of the super hard and brittle material slid by the abrasive after unloading. The pad matrix surface dressed by loose grains greatly affects the MRR of wafers. The predicted MRR of sapphire wafers based on the FAL model is consistent with experimental data. Finally, the keys to improving the processing efficiency of super hard and brittle material wafers are clarified in detail.

Polarized laser scattering detection of subsurface damage of quartz glass induced by grinding

Subsurface damage (SSD) is inevitably generated during the grinding process of quartz glass. It has a great impact on sustainability, lifetime and optical performance of quartz glass components and systems. This paper proposes a non-destructive detection method for detecting SSD in ground quartz glass based on polarized laser scattering (PLS). A PLS system was built to detect SSD in ground quartz glass samples prepared with different abrasive particle sizes. The PLS detection signal value had a positive correlation with the SSD depth. The finite-difference time-domain (FDTD) method was used to simulate the laser scattering process at the SSD. The distribution of electric field intensity could reflect the location of SSD. It is concluded that the PLS system can effectively detect SSD in the ground quartz glass.

The Impact of Abrasive Grit Size on Roughness of Sanded Beech Wood Surface

The study assessed the changing of Ra, Rp, Rv and Rz roughness parameters of the sanded beech surface (Fagus sylvatica L.) as a function of different grit sizes and different measurement direction. The milled samples were ground with a belt sander BS-75 E-set from Festool with belt grit P60, P100 and P150. The sanding belts were Rubin 2 from Festool. Roughness was evaluated using a Keyence VHX-7000 digital microscope. The evaluation length of the roughness measurement was 12.5 mm (λc = 2.5 mm and λs = 8 mm). The R-parameters were measured in accordance with the latest standards ISO 21920 (2022) in the direction parallel to the grain, and in the direction perpendicular to the grain (profile). The paper proves the theoretical assumptions about the reduction of R-parameter values. The measurements showed that the sanded surface was less rough in the direction perpendicular to the grain at P150 sanding belt grit and in the grain direction at P100 grit.

Influences of abrasive particles on tribological behaviours of rotary vane steering gear seals under oil

The wear particles often cause the abnormal wear processes to the rotary vane seals, which lead to seals failure and dropping hydraulic oil pressure, and eventually result in losing of the function of rotary vane steering gear. Clarifying the friction and wear mechanisms between the vane seals and the cylinders and extracting their key wear characteristics are the keys to extend the service life and realize intelligent fault diagnosis of the rotary vane steering gears. Different kinds of hydraulic oils mixed up with different sizes of iron particles were chosen to investigate the abrasive particles on the wear behaviours between the polyurethane polymer and carbon steel specimens. The wear characteristics such as coefficients of friction (COF), wear morphologies, frictional-induced vibration signals were examined and analyzed synthetically. The results showed that the coefficients of friction, wear morphologies, surface roughnesses and vibration signals increased as a function of the iron particle sizes, which indicated that the wear state was gradually worsening. The three-body friction and wear mechanisms were disclosed clearly. There was a certain mapping relationship between the wear characteristics and wear states, and sensing the tribological information between the rubbing pairs, including coefficient of friction, abrasive particle size in hydraulic oil, wear morphology, vibration signal and other characteristics, was a potential and effective way for evaluating or predicting the wear states or failure modes of rotary vane steering gear seals. This study provides a theoretical basis for revealing the friction and wear mechanisms of ship rotary vane steering gear seals, and is useful for accomplishing their intelligent monitoring and fault diagnosis.

Fabrication and abrasive wear property of high chromium cast iron with high vanadium and high nitrogen content (HCCI-VN)

High chromium cast irons (HCCIs) have excellent wear resistance because of high hardness chromium carbides (M7C3). However, the hardness of M7C3 is relatively low compared with MC-type carbides, such as VC, NbC and TiC. To improve wear resistance of HCCI, a new HCCI, named HCCI-VN, was fabricated by replacing wt.5.0% chromium with 5.0 wt% vanadium and adding 0.1 wt% nitrogen in traditional HCCI with 20 wt% Cr, and the abrasive wear properties of HCCI-VN were researched under different abrasive size and load conditions. Results show that the microstructure of HCCI-VN was characterized by many V(C, N) carbides besides chromium and molybdenum carbides (M7C3, M23C6, M2C), which were distributed in the matrix composed of martensite and austenite. The new HCCI-VN has excellent wear resistance. The relative wear resistance HCCI-VN is 2.2–9.1 times that of HCCI with 23 wt% Cr under different abrasive size and wear load. Among them, the reduction of abrasive size or the increase of wear load will bring better wear resistance. The excellent wear resistance of HCCI-VN is attributed to high hardness V(C, N), which cooperates with multi-scale and multi-type chromium and molybdenum carbides to resist abrasive scratch efficiently.

Study on the Wear Modes of PVD Films Using Different Concentrations of Al2O3 Abrasive Particles and Textured Rotating Balls

Abrasive wear is a wear mechanism that results in a loss of material from the interaction of a surface with hard particles. This type of wear is frequently found in the surface of machining tools. Microscale abrasion equipment is often used to characterize the resistance to abrasive wear of a surface. The different parameters able to control micro-abrasion wear tests, such as ball rotation, sliding distance between ball and surface sample, abrasive slurry concentration, normal load acting on the sample, and abrasive flow rate over the sample, have been widely studied. The combination of different variables, including sliding distance, concentration of abrasive particles, their hardness, and size of abrasive particles, promotes the transition between two-body, three-body, or mixed abrasive wear modes. However, the influence of the ball surface on the dragging of abrasive particles, which is reflected in the wear modes, is still poorly studied. One of the variables possible to control and less studied is the influence of the ball surface texture on the dragging of abrasive particles in micro-abrasion wear tests. This work intends to correlate the effect of different testing times (500, 1000, and 1500 cycles) and different concentrations of 3 μm Al2O3 abrasive slurry (25, 35, and 45 g/100 mL) on the micro abrasion resistance of a TiN thin coating film, using balls of AISI 52100 steel whose texture and roughness were prepared by 60 s chemical etching. The rotation speed of each test was 80 rpm, applying a normal load of 2 N. Subsequently, the craters were carefully analyzed using SEM to evaluate the transition of the wear mode as a function of the applied load, the abrasive particle concentration, and the sliding distance. The textured ball tracks were observed via SEM to assess the particle dynamics. The results showed that, contrarily to what is reported in the literature regarding wear modes where rolling is promoted with increasing abrasive concentration, in this work grooving took place instead. This is a result of the rough balls use in the experiments which, due to the embedment of abrasive particles in the ball grooves, promotes the abrasion mechanism. The higher the abrasive concentration, the higher the grooving mechanism, since more particles are available to scratch the surface.

A new theoretical model for surface roughness prediction in rotational abrasive finishing process

Rotational abrasive finishing (RAF) is a new nano-finishing technique in which the finishing forces are applied to the workpiece by the opposite rotations of a stirring-blades and the workpiece. The RAF process allows for finishing the inner and outer surfaces of workpieces, particularly complex ones with axial symmetry. The present study aims to propose a new theoretical model to obtain deeper insights into the material removal mechanism, surface roughness prediction, and forces in RAF process. Since the abrasive-workpiece interaction is random and complex in nature in the RAF, some initial assumptions were considered. To validate the repeatability of the experimental results, the experiments were designed based on the Response surface method (RSM). To validate the new proposed theoretical model, a number of influential parameters were investigated. It was found that the stirring-blade speed (S), working gap (W), and abrasive grain size (A) had significant effects on Ra. The minimum surface roughness (Ra) was obtained to be 46.87 nm at a rotational speed of 600 rpm, a working gap of 1 mm, and grain size of 18 μm. The experimental results were relatively in good agreement with the theoretical results so that the maximum error was about 24%. This can be assumed that the most important explanation for the difference between the theoretical and experimental results can be attributed to the initial theoretical assumptions.

Analytical grinding force prediction with random abrasive grains of grinding wheels

A reliable prediction of grinding forces and surface morphology is critically important to the design of a grinding process. However, due to the complex microstructure of a grinding wheel which contains randomly-sized and randomly-distributed abrasive grains, a practical prediction model has been unavailable. This paper aims to develop a novel stochastic model to take into account the random nature of the abrasive grain size and their distribution in a grinding wheel, and hence to predict grinding forces more accurately. In addition, the evolution of the ground surface morphology, the real-time undeformed chip thickness, the interactions between the abrasive grains and the grain-workpiece contact mechanics are integrated into the modelling. Nanoindentation and tribology experiments were conducted to determine the micromechanical properties of the workpiece and abrasive grain. Grinding experiments were then performed to validate the predictions of the model. It was found, both theoretically and experimentally, that the stochastic model thus established can reliably predict grinding forces. It was also demonstrated that the proposed model can be effectively used to reveal the underlying mechanisms of grinding processes.

Predictive modeling of tool wear in rotary tool micro-ultrasonic machining

In the current research endeavor, the predictive model of tool wear in rotary tool micro-ultrasonic machining (RTMUSM) process using artificial intelligence has been reported for the first time. An adaptive neuro-fuzzy inference system (ANFIS) and artificial neural network (ANN) approaches are adopted for modeling of total volumetric wear (TVW) in RTMUSM process. The models were developed after conducting the exhaustive experimentation. Six RTMUSM input process parameters (i.e. abrasive size, rotation speed, depth of channel, slurry concentration, feed rate, and power rating) were selected as input data with TVW of tool as output data. The developed model has been validated through experimental results. Further, statistical analysis by calculating variance account for (VAF), coefficient of correlation ( R-value), mean absolute percentage error (MAPE), and root mean square error (RMSE) was carried out to estimate the potential of the models. The results obtained by statistical analysis proclaimed that the performance of ANFIS model ( R-value = 0.9966, MAPE = 1.26%, RMSE = 1.11%, and VAF = 99.94) was superior as compared to ANN model and it can be used to measure the TVW of tool in RTMUSM. Furthermore, genetic algorithm based optimization was performed and required wear compensation of tool is recommended.

Research on Chemical Mechanical Polishing Technology for Zirconium-Based Amorphous Alloys

Crystallization often occurs in the processing of amorphous alloys, causing the materials lose their excellent properties. The study adopts chemical mechanical polishing of amorphous alloys, presenting the effect of the rotational speed of the polishing turntable, size of abrasive, polishing pressure, and oxidant concentration. The Taguchi method is used to find the best processing parameters, and AFM is used to characterize the machined material surface. At the same time, XPS is used to detect the change of oxide film composition with the addition of oxidant. The results indicate the optimum process parameters: rotational speed of the polishing turntable is 75 r/min, polishing pressure is 28.3 kPa, the size of abrasive is 0.5 μm, and the size of abrasive is a significant factor affecting surface roughness Sa. In addition, as the size of abrasive increases, the material removal rate increases while the surface roughness Sa increases. At pH 10, with an abrasive particle size of 0.5 μm, as the H2O2 concentration increases, the MRR first rapidly decreases at 0.21 wt.% H2O2, and then gradually increases, while the Sa decreases. Furthermore, with the addition of oxidant, the main composition of the surface oxide film changes from oxide to hydroxide, and the contents of Zr4+ and Cu0/Cu1+ elements increase. The findings can provide a feasible chemical mechanical polishing process for zirconium-based amorphous alloys to obtain a satisfactory polishing effect.

Surface generation modelling and form maintenance strategy in maskless fluid jet polishing of structured array surface

Structured array surfaces are extensively utilized in the fields of functional optical components and molds. However, high surface form accuracy of these surfaces is usually needed to implement superior performance, leading to great challenges in manufacturing. In this study, fluid jet polishing (FJP) was utilized to polish the cylindrical array surface. The effect of abrasive size and jet pressure on the form maintainability was elucidated by combining computational fluid dynamics (CFD) simulation. The abrasive trajectory and abrasive impact information were analyzed to reveal the material removal process. Tool influence functions (TIF) at different positions were simulated to model the surface generation process and provide a theoretical basis for the optimization of polishing parameters. A surface form maintenance strategy was proposed to achieve high form accuracy together with excellent surface quality by controlling the feed rate of the nozzle. A series of experiments were conducted to validate the effectiveness of the form maintenance strategy. The results revealed that the form error agreed well with the requirement after polishing with the designed feed rate. This study paves a new way for the polishing of structured array surfaces with a good surface form control.

Analyzing the Effect of Various Sizing Machine Settings on Abrasion Resistance and Size Pick-Up of Polyester/Cotton Blend Sized Yarn Using Box-Behnken Design

ABSTRACT The main aim of this study relies on analyzing multiple sizing machine setting on sized yarn pick-up and abrasion resistance using response surface methodology. To perform the experiment, three independent variables each at three levels (wet zone yarn tension (A) (340N, 380N, 420N), squeezing pressure (B) ((13N, 15N, 17N), and sizing machine speed (C) 30 m/min, 49 m/min and 68 m/min)) have been taken. Fifteen experimental runs have been performed using design expert 11 software (with Box-Behnken design). It has been observed from the Analysis of variance (NOVA) results of the experiment that wet zone yarn tension, squeezing pressure, and speed of the sizing machine have a significant effect on size pick-up% and abrasion resistance of the yarn. The interaction effect of BC with (p-value = .0053) and the interaction effect of AC with (p-value = .0143) have also a significant effect on abrasion resistance. Moreover, the interaction effects (AC and BC) have a significant effect on size pick-up. Regression analysis shows that wet-zone yarn tension and squeezing pressure have a negative relation with size pick-up and the speed of the machine has a positive relation. Whereas machine speed and wet-zone yarn tension have a negative relation with abrasion resistance and squeezing pressure has a positive relation.

Material Removal Characteristics of Spherical-Array-Focused Ultrasonic Abrasive Machining.

To improve the ultrasonic energy and realize far-field ultrasonic abrasive machining of complex surfaces, a spherical-array-focused ultrasonic abrasive machining system was established. By combining ultrasonic field simulation, detection and a single-factor experiment, the influences of the ultrasonic generator current, abrasive concentration, and particle size on the material removal properties and surface quality evolution of quartz glass were investigated. When the current was less than 0.4 A, the material removal showed plastic removal at the nanoscale. When the current was more than 0.5 A, the cavitation phenomenon formed micron-scale impact removal traces on the workpiece surface. The increase in abrasive concentration increased the impact density and material removal rate, while excessive abrasive concentration increased the impeding effect between abrasive particles and reduced the material removal rate. Moreover, the increase in abrasive particle concentration enhanced heterogeneous cavitation nucleation, promoted the removal of abrasive impact materials under the action of a cavitation jet, and inhibited the removal of direct surface cavitation. The abrasive particle size affects the heterogeneous cavitation nucleation and the acceleration of the cavitation jet on abrasive particles, which affects the material removal rate and surface quality. By controlling the energy of the focused ultrasound and abrasive parameters, the plastic or brittle domain removal of quartz glass can be achieved at the micro- and nanoscales.

Multi-response optimization of magnetic abrasive finishing process parameters on AISI H13 hot die steel using grey relational analysis

This paper presents a new approach for the optimization of machining parameters on magnetic abrasive finishing (MAF) of AISI H13 hot die steel with multiple responses based on the orthogonal array with grey relational analysis. The process has been explored widely for the finishing of plane surfaces on pillar drilling machine. Taguchi orthogonal arrays L27 Designs of experiment method was used for planning of experiments. Total of 27 Experiments were conducted on AISI H13 hot die steel to check the best factor level combination. Five process parameters such as abrasive size, cutting speed, finishing time, machining gap, and magnetic flux density are optimized using multiple responses such as surface roughness (Ra) and metal removal rate (MRR). The Grey Relational Analysis (GRA) was utilized to analyze the results to find the optimum condition and identify the significant parameters affecting the surface roughness and the metal removal rate on H13 hot die steel. Magnetic abrasive finishing machining parameters combination of experiment no. 14 has the highest grey relation grade (GRG) of 0.922. The ideal parameter combination to achieve optimum output are cutting speed of 118.24 m/min, abrasive mesh size of 600, finishing time of 10 min, Machining gap of 1 mm, and Magnetic flux density of 485 gauss. The surface roughness (Ra) value was improved from 0.753 µm to 0.452 µm.

Research Trends in Abrasive Water Jet Machining Using Numerical Simulation Tools: A Bibliometric Review

Background: Abrasive water jet (AWJ) machining process is one of the most sought-after machining technologies for the processing of advanced hard-to-cut materials. It involves parameters, such as abrasive size, shape, density, pressure, standoff distance (SOD), abrasive concentration, feed rate, etc., which govern the quality of machining. It is very crucial to understand the influence of these parameters on the quality attributes. Objective: Due to the complex nature of the process that involves complex process parameters, the accurate prediction of response by experimental methods is difficult. In this scenario, numerical methods are helpful in understanding the mechanisms of material removal. Thus, a comprehensive summary of these research trends is needed. Methods: In this article, a bibliometric analysis is carried out on scientific publications pertaining to AWJ machining (AWJM) using numerical simulation tools. Citation and bibliographic coupling analyses have been carried out to identify the current research trends, the important journals, authors, institutions, and countries engaged in research on AWJM using numerical simulation. Results: The analysis revealed Shandong University to have the maximum number of affiliated researchers working in this area. The International Journal of Machine Tools and Manufacture was the leading journal based on CiteScore, SJR rank and SNIP ranks, and the largest volume of articles was published by this journal. The critical topics of research and international collaborative opportunities were identified through the analysis of keywords, Sankey and network diagrams. Conclusion: The present article would be valuable for academia and industry, aiding them in updating their knowledge on the latest developments in AWJM.