Burr formation in orthogonal machining of A356 aluminum as cast and T6
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https://www.eduzhai.net American Journal of M aterials Science 2013, 3(5): 162-168 DOI: 10.5923/j.materials.20130305.08 Burr Formation in Orthogonal Machining of A356 Aluminum in an As-Cast and T6 State Andy Simoneau Department M echanical Engineering, University of New Brunswick, Fredericton, E3B 5A3, Canada Abstract Machining of alu minu malloys and specifically A 356 alu minu m co mponents is common across many industrial sectors. A persistent phenomenon when machin ing this material is the format ion of burrs as the cutting tool exits the workp iece. The microstructure of A356 alu minu m is known to influence cutting mechanics and the chip format ion process. In order to start to understand what role the microstructure of this common material might play in the burr formation phenomenon, a series of interrupted orthogonal cutting tests were used to generate burrs under various cutting conditions. Co mparing the resulting negative burr fo rmation that occurs when mach ining A356 in an as-cast (A356-F) and more co mmon heat-treated state (A356-T6), it is shown that variations between the microstructure of the two workp iece materials has an impact on the size of the negative burrs that form. For A 356-F, increases in cutting speeds reduced the horizontal length of the negative burrs at smaller feedrates, but the depth of the negative burr was found to be insensitive to cutting speed. For A356-T6 the overall size of the negative burr was found to be insensitive to cutting speed, but at larger feedrates there is a dramat ic increase in the depth of the negative burr. The resulting burrs from the cutting tests are presented and the results d is cus sed . Keywords A356 Alu minu m, Burr format ion, Machining, Negative Bu rr, Orthogonal Cutting 1. Introduction An increased use of materials with improved strength-to-weight ratios has lead to the wide spread adoption of aluminu m and alu minu m alloys in place o f steel for several machined co mponents. Both wrought and cast alu minu m alloys are increasingly being used in the automotive industry. One of the more co mmon cast alu minu m alloys used by the automotive industry is A356 alu minu m (SA E 323) which can be found in several automotive co mponents. Machining of this alloy is commonly performed in industry, however little work has been published specifically in this area. Instead, many studies of the material have focused on fatigue life and processing characteristics of the raw material for specific components manufactured fro m A356 such as wheels, scroll parts, and brake components[1-3]. To contribute to a better understanding of the behaviour an d ch aract eris t ics o f A 356 d u rin g mach in in g , an experimental orthogonal machining study has been carried ou t to examin e t he imp act microst ruct u re an d cutt ing parameters on the formation of burrs as the cutting edge exits the workpiece. Most machin ing operations will create a burr * Corresponding author: firstname.lastname@example.org (Andy Simoneau) Published online at https://www.eduzhai.net Copyright © 2013 Scientific & Academic Publishing. All Rights Reserved which usually requires a secondary operation for burr removal. These secondary operations usually co me at some great cost, accounting for as much as 30% of the total manufacturing cost of a part according to Chern and Dornfeld . The formation of burrs in mach ining is known to be linked to the chip formation mechanism and general cutting mechanics. Using A356 alu minu m in its as-cast state (A356-F) and its most common heat-treated state, A356-T6, a series of interrupted orthogonal cutting tests are used to study the influence of cutting para meters on the overall burr formation process. It is anticipated that research on the role of material microstructure and cutting conditions on cutting mechanics, surface quality, and burr formation could contribute to more efficient and h igher quality part production. Despite the common use of A356 alu minum in industry for cast and machined components, only a small amount of published research work is available on the topic. As a soft, easily mach ined metal, alu minu m has a tendency to stick and adhere to the tool rake face and cutting edge to form a built-up-edge (BUE) at low cutting speeds. Therefore, common practice when machining alu minu m alloys is to mach ine them at h igh speeds. The high thermal conductivity of the material causes it to thermally soften during higher speed cutting, creating a more fluid flow-zone along the tool-chip interface and preventing the formation of a BUE. It has also been demonstrated that the higher cutting speeds reduce thermal effects seen in parts, provide an imp roved American Journal of M aterials Science 2013, 3(5): 162-168 163 chip formation mechanis m, and a superior surface fin ish. Feedrates used when machining alu minu m are dependent on the desired surface finish. Fin ishing feedrates can be on the order of 0.05 to 0.15 mm/rev as opposed to roughing feedrates that could be as high as 4 mm depending on the grade of alu minu m and post machining requirements. Aluminu m's ability to experience significant plastic deformation prior to failure leads to tough continuous chips during machining wh ich can be d ifficu lt to handle and lead to interruptions in the cutting process, but adding a primary alloying element such as silicon, the strength and mach inability of the material can be dramatically improved. Examination of the effects of metallurg ical parameters on the mach inability of A356 alu minum has demonstrated that the formation of a BUE is common during drilling of as-cast A356 however, after heat treatment to a T6 state, the formation of a BUE is eliminated. Given the extensive tool wear associated with machin ing A356 alu minu m, the most prevalent machining studies of the material have been done in the area of a minimu m quantity lubricant (MQL) in high-speed milling and drilling. Cutting speeds as high as 5000 m/ min have been used when milling A356 alu minum in an as-cast state to study the effect of coolant strategies on the cutting process, and tool wear. 2. Review Burrs in metal cutting have been given considerable attention in the literature as evidenced by a recent, extensive review o f burrs and burr formation by Aurich et al.. This review goes into great detail on all types of burrs, their formation, control and removal, and some case studies. As a result, only the more co mmon ly observed burrs in metal cutting is presented. A burr is an undesirable end result of the metal cutting process. Gillespie and Blotter were some of the first researchers to classify the common burrs encountered in mach ining as one of four types: poisson burrs, rollover burrs, tear burrs, and cutoff burrs. These general burr descriptions were re-examined and classified by Nakayama and Arai based on the cutting edge and the direction and mode of burr formation. As an examp le, a forward burr will form as a cutting tool pushes forward and exits a wo rkp iece. At the exit a small amount of material will be pushed forward and depending upon conditions lead to a positive burr (excess material sticks out at the exit surface) or a negative burr (material fractures on a negative plane before the tool exits the workpiece). In his later examinat ion of turning operations Gillespie found that most burrs are side burrs and that during interrupted cutting rollover burrs are most commonly encountered. Hashimu ra et al. divided the burr format ion mechanis m into an 8-step process. The first five steps are common regardless of material properties, with the final common step being the formation of a negative shear zone that develops between the base of the shear zone and the plastic deformat ion on the free edge of the workpiece. This last common step is followed by three final steps depending on the workpiece material. These final steps involve crack initiat ion and growth, and the formation of a positive or negative burr. In h ighly ductile metals, this fracture is found to occur along the line of cutting. This leaves the cut surface as desired but leaves a burr extended out over the unsupported edge of the workpiece. In more brittle materials and at higher depths of cut, ductile fracture occurs along the line of negative shear deformat ion. This produces a negative burr or b reak out in the workpiece. It was Hashimura et al.'s assertion that materia l ductility dictated whether a positive or negative burr was formed. Experimental investigations of burr fo rmation have primarily focused on aluminu m alloys such as Olvera and Barrow's examination of shoulder milling of alu minum alloys. In their work they demonstrated that in some cases, down milling could be used effectively to reduce and eliminate the formation of burrs. Using aluminum, Toropov et al. performed an experimental study of the burr formation mechanism in the feed direction in turning. They found that three burr format ion mechanisms could be discerned, the formation of a sideward burr, bending of the excess material, and final shearing of the excess material. This work was further extended and using slip-line field theory a new model to predict the fu ll formation of a burr in orthogonal cutting was developed for cutting ductile mater ia ls . Two active researchers in the area of burr format ion has been Chern and Dornfeld, with much of their work based on using aluminum alloys. Chern and Dorn feld created a model to predict the size and geometry of this type of burr being formed in a simp le orthogonal cut and used SEM images of the burr format ion process to validate the results for positive burr formation. Using orthogonal cutting to produce a negative burr, Chern used SEM imagery to identify p lastic bending and shearing as the dominant mechanis ms in burr formation and examined the fracture process and surfaces during edge breakout (negative burr formation). Ko and Dornfeld examined the fracture and burr formation process during oblique cutting. Both Chern and Dornfeld were also both involved in the previously cited work by Hashimura et al.. 3. Experimental A series of interrupted orthogonal machining experiments were conducted using A356-F and A 356-T6 alu minu m. The interruptions in the workp iece material were the result of a set of grooves machined into the workpiece. The interruptions were used to recreate the orthogonal e xit of the cutting tool fro m the workp iece wh ich resulted in a series of burrs being generated. The feedrate was set to 0.1, 0.2, and 0.4 mm and the cutting speeds were set to 400, 600, and 800 m/ min. In all cases the width of cut was set to 4 mm. For each cut, a new cutting edge was used in order to maintain a 164 Andy Simoneau: Burr Formation in Orthogonal M achining of A356 Aluminum in an As-Cast and T6 State sharp cutting edge for the analysis. The cutting insert had a rake angle of 0o and a clearance angle of 11o. All cutting was done dry without the use of coolant or lubricant. Fo r the current study, the interruptions were oriented normal to the cutting direction such that the cutting tool always exited at o 90 . 3.1. A356 Al uminum Work piece Samples A356 alu minu m is a cast alu minu m alloy co mmon ly used in several industrial sectors. In its as-cast state, A356-F is characterised by the silicon rich dendrite structure in an alu minu m matrix as shown in Figure 1a. Fro m an application standpoint, A356 alu minu m is most often used in a T6 state. Fro m Figure 1b, the heat treat ment and ageing of the material alters the microstructure significantly. While still retaining the dendritic pattern of the A356-F material, the silicon rich dendrites are replaced with spherodized silicon highlighted in Figure 1c. (a) (b) (c) Figure 1. Microstructure of cast A356 aluminum. The as-cast state a) A356-F has the characteristic silicon rich dendrite structure (dark areas) in an aluminum matrix. Heat treatment to the T6 state in b) and c) shows the spherodized silicon in the aluminum matrix Changes in material microstructure translate into changes in the mechanical response and specifically the plastic deformation and fracture process when comparing A356-F and A356-T6[20, 21]. The irregular microstructure of as-cast A356-F means that wide variations in ductility between the silicon rich dendrites and the alu minu m matrix leads to a highly irregular deformat ion and ductile fracture. The ductile fracture process in A356-T6 is aided by the reorganization of the microstructure with spherodized silicon in an alu minum matrix. The p resence of these particles act as stress risers in the material. Part icle cracking, fracture, and debonding with the surrounding alu minum are all crit ical elements of the ductile fracture process in this material. Fracture and debonding of the spherical particles leads to void formation and growth. These voids will coalesce forming a ductile fracture plane. The particles can be used as an indicator of stress state when partic le c racking can be identified as tensile stresses are always orthogonal to the plane of fracture of the particles wh ich could prove useful when examin ing the burr formation process. 4. Results and Discussion Prior to considering the burrs that formed during the mach ining tests, the cut chips and machined surfaces were e xa mined. For the A356-F materia l continuous chips formed in all cases. At cutting speeds of 400 and 600 m/ min a BUE formed at the tool-chip interface. The BUE could be seen adhering at the tool-chip interface on the cut chip and prows could be seen along the machined surface indicat ing the formation of a BUE during cutting. The BUE was common for mach ining at 400 m/ min and appeared intermittently at 600 m/ min. When the cutting speed increased to 800 m/ min a BUE was no longer observed. Given that a BUE can influence the mechanics of the cutting process it is suspected that this would have some impact on the burr formation process. Similar to findings by Tash et al., for A356-T6 material a continuous chip formed, but there was no evidence of a BUE at any cutting speeds irrespective of the feedrates. This was expected g iven the structure of the material. The small spheroidal silicon particles would have continuously disrupted the format ion of any BUE as they passed over the tool rake face. A smooth machined surface formed in all cases and no prows were observed on the machined surface. For all machin ing tests a negative burr formed as the cutting tool exited the workpiece. An examp le of the type of burr observed in the cutting tests is shown in Figure 2. Fro m the side view, the negative burr has a characteristic downward sloping orientation extending along a plane fro m the machined surface to the workpiece free exit surface. A top view of the burr shows that in the case of orthogonal cutting and exit of the cutting tool, there is d istinct line where the negative burr plane initiates fro m the machined surface. There is a slight curvature of the negative plane similar to that predicted by previous researchers using slip-line field theory. This curvature begins at the point where the chip formation process stops, and the chip root and foot of the burr begin to move forward and rotates leading to the fracture along a negative plane as previously described by Hashimu ra et al.. (a) (b) Figure 2. Side (images on the left side) and top down (images on the right side) viewof a negative burr machining A356-F aluminum for a depth of cut of 0.2 mm and a cutting speed of a) 400 m/min and b) 800 m/min Examination of the negative burr that forms when mach ining A356-F at a depth of cut of 0.2 mm highlights the impact of machining alu minu m at lo wer cutting speeds such as 400 m/ min. The negative burr that formed at 400 m/ min in Figure 2a is characterised by several small surface cracks, American Journal of M aterials Science 2013, 3(5): 162-168 165 fractures, and prows along the machined surface prior to the actual burr fracture plane. These defects along the machined surface are expected due to the format ion of a BUE, however the frequency of these defects increased markedly as the cutting tool approached the initiation of the negative burr fracture p lane. Fro m the top down view of the machined surface at the burr initiat ion point in Figure 2a, several of these surface cracks can be observed. As a result of this, the true burr initiat ion plane or line along the machined surface is difficult to ascertain due to the uncertainty in the mechanics of the cutting process. The horizontal length of the burr was found to be approximately 700 µm and the vertical depth was 400 µm, ma king an angle of near o 30 from the machined surface which has not been previously observed in the literature. Similar results were observed when machin ing with the same feedrate and a cutting speed of 600 m/ min. For the same feedrate, but a higher cutting speed of 800 m/ min, a BUE will not form during machin ing. The resulting negative burr in Figure 2b is different than the burr that forms at 400 m/ min. Both the side and top down view in Figure 2b highlight that there are no noticeable surface defects prior to the initiation of the burr. The horizontal length of the burr is appro ximately 500 µm, and the depth is 280 µm wh ich makes an angle of o 29.2 . Higher cutting speeds lead to a better machined surface fin ish, a cleaner burr initiat ion point, and an overall smaller negative burr for A 356-F. Both burrs have a curvature associated with their fracture surface. At the lower cutting speed, this curvature does not begin until well below the mach ined surface at approximately 200 µm. For the higher cutting speed the curvature begins approximately 75 µm below the mach ined surface. The curvature that occurs is very similar, but they occur at different depths beneath the mach ined surface. The reason for this result is not currently known. Overall the dimensions of the curvature are very similar and most likely linked to the cutting and burr formation process and material properties. The init iation point of the negative burr fracture plane is examined in Figure 3 wh ich are close up images of the same burrs from Figures 2a and 2b. The in itiation points of the negative burrs show different features depending upon cutting speed. In Figure 3a, small fractures along the machined surface formed at a cutting speed of 400 m/ min near the init iation point of the negative burr which are not present at higher cutting speeds as shown in Figure 3b. There is deformation of the grains along the machined surface for all cutting speeds. However, at higher cutting speeds all of the grains along the machined surface are orientated and deformed in the direction of cutting. For the higher cutting speed, as soon as fracture occurs along the machined surface, the final stages of negative burr formation occur - ch ip format ion or chip flow along the tool-chip interface stops, and ductile fracture occurs along a negative plane. At the lower cutting speeds of 400 and 600 m/ min the formation and release of a BUE causes interruptions to the chip format ion process and flow o f material along the tool rake face wh ich will also cause interruptions to the burr formation process. These interruptions would be most noticeable at the initiation of the negative burr plane. The presence of a BUE at the burr init iation point along the mach ined surface may actually prevent the negative burr formation process until the BUE beco mes unstable and releases from the tool-chip interface. (a) (b) Figure 3. Initiation of negative burr. At a cutting speed of a) 400 m/min tearing of the machined surface ahead of the burr fracture surface. At b) 800 m/min no pre-tearing of the machined surface, aluminum grains extend outward in direction of burr formation When the feedrate was increased to 0.4 mm, depth of the negative burr increased only slightly as compared to the length of the burr which increased. For a cutting speed of 400 m/ min, the negative burr was found to be 950 µm long and had a depth of 300 µm. The angle of the burr breakout fro m o the horizontal was 17.5 . At a cutting speed of 800 m/ min, the length of the negative burr was found to be approximately 900 µm and the depth was approximately 280 µm and again the breakout angle was 17.2o. An increased feedrate leads to a majo r increase in burr length and a minor increase in burr depth for A356-F. Changes in feedrate and cutting speed change the burr shape and size wh ich is in-line with findings fro m Nakayama et al. where the mode of chip separation from the workp iece was found to have a strong effect on the negative burr size and shape. With A356-F, changes in feedrate or cutting speed will change the mode of ch ip separation fro m the workpiece material. At the larger feedrate of 0.4 mm, the impact of cutting speed is less pronounced with only a very minor reduction in burr length, and some reduction in the burr depth. Overall there is a reduction in burr size, but the benefits of a higher cutting speed that were observed at a lower feedrate of 0.1 and 0.2 mm is no longer present for a heavier cut such as 0.4 mm. Potential benefits of increasing cutting speed when mach ining A356-F are o ffset by the influence of increasing the depth of cut or feedrate. The resulting burrs fro m the machin ing trials with A356-T6 were similar to the results fro m A356-F in that a negative burr formed irrespective of cutting speed and feedrates as shown in Figure 4. Unlike the mach ining tests with A356-F, there were no identified surface defects on any of the machined specimens near the burr init iation point on the mach ined surfaces. This is expected given the change in the overall material microstructure and the fact that the formation of a BUE or prows was not observed for any of the cutting tests of this material fro m examination of the cut chips and machined surfaces. For s maller feedrates of 0.1 and 0.2 mm the negative burr sizes and pattern are consistent with each other and only a 166 Andy Simoneau: Burr Formation in Orthogonal M achining of A356 Aluminum in an As-Cast and T6 State minor increase in burr length and depth as the feedrate increased from 0.1 to 0.2 mm. The influence of cutting speed was slight with a minor decrease in burr size as cutting speed increased. At the smaller feedrates, there is a slight curvature of the fracture surface, however at the higher cutting speed this surface begins below the machined surface. At a larger feedrate of 0.4 mm the horizontal length and depth of the negative burr increased to 950 µm and 550 µm, respectively. Unlike the case for A356-F, an increased feedrate has a dramat ic impact on the burr size and profile. does reduce burr length for the as-cast A356-F wh ich is attributed to the elimination of interruptions in the cutting process that result fro m the formation of a BUE. Th is would also explain why the cutting speed has litt le impact on the A356-T6 since a BUE does not form when cutting that materia l at the cutting speeds used in this study. Figure 5. Side (image on the left) and top down (image on the right) view of a breakout burr for A356-T6 aluminum alloy. Feedrate is 0.4 mm and the cutting speed is 800 m/min (a) (b) Figure 4. Side (images on the left side) and top down (images on the right side) view of a negative burr machining A356-T6 aluminum for a depth of cut of 0.2 mm and a cutting speed of a) 400 m/min and b) 800 m/min A356-F material is heat-treated and artificially aged to a T6 state in order to imp rove its strength and machinability. This improvement is a result of the change of the microstructure of the material. These changes in microstructure that result in improved machinability of A356 would appear to be detrimental to the burr formation process. During negative burr format ion there is a negative shear zone similar to the primary shear deformation zone that occurs during continuous chip format ion. For A356-F, this zone crosses several structures that will all contribute to the deformation and fracture process simultaneously. The impact of stress and strain localizations due to physical microstructure is minimal. In the case of A356-T6 the presence of small spheroidal silicon particles act as stress risers. As the negative shear zone crosses these particles, particle fracture and debonding of the particles with the surrounding alu minum mat rix creates mult iple crack initiat ion points leading to the irregular fracture path as the chip formation process stops and the burr foot is fractured fro m the workp iece material. Cracked silicon part icles and evidence of debonding were found under the negative burr fracture p lane. It is also worth noting that the angle of the negative burr fracture plane with respect to the cutting plane is the reverse of the observations from A356-F which again is a direct result of the change in microstructure. A comparison of burr length for A356-F and A356-T6 in Figure 6 highlights that burr length increases with feedrate for both the A356-F and A356-T6 alu minum. Cutting speed Figure 6. Horizontal length of the breakout burr for A356-F and A356-T6 at different cutting speeds and feedrates Figure 7. Vertical depth of the breakout burr for A356-F and A356-T6 at different cutting speeds and feedrates The depth of the negative burrs was also found to increase with feedrate. Fro m Figure 7, the depth of the negative burrs increases with feedrate, but this effect is slight in the case of A356-F. Fo r both material states, increasing cutting speed had only a slight impact on reducing the depth of the negative burr. The substantial increase in the depth of the negative burr as feedrate increases for the A356-T6 material is lin ked to the material microstructure. At a larger feedrate or depth of cut, a distinct negative shear plane interacts with the microstructure of the A356-T6. Th is distinct, negative shear plane is probably poorly defined at the lower feedrates and thus any interaction with the structures of the workpiece American Journal of M aterials Science 2013, 3(5): 162-168 167 material is also poorly defined. A negative shear plane also occurs with A356-F, however, as is the case of the primary shear deformation zone when machin ing A356-F, this deformation zone crosses and interacts with several structures in the workpiece material simu ltaneously. As a result, the response of the material in this zone is fairly homogeneous without the presence of stress risers, and crack propagation points as is the case with A356-T6. 5. Conclusions REFERENCES  P. Li, D. M . M aijer, T. C. Lindley, and P. D. Lee, “A though process model of the impact of in-service loading, residual stress, and microstructure on the final fatigue life of an A356 automotive wheel,” M aterials Science and Engineering: A, vol. 460-461, pp. 20-30, 2007.  C. Kang and H. Jung, “A study on solutions for avoiding liquid segregation phenomena in thixoforming process: Part II. Net shape manufacturing of automotive scroll component.” M etallurgical and M aterials Transactions, vol. 32, pp. 129-136, 2001. The microstructure of A356 alu minu m p lays a role in burr formation as the cutting tool e xits the workpiece materia l. In an as-cast state, A356-F needs to be machined at high speed as it is susceptible to the formation of a BUE wh ich appears to translate into surface cracking along the mach ined surface just prior to the edge of the negative burr that forms as the cutting tool e xits the workpiece materia l and slightly longer horizontal length of the negative burr at lo wer cutting speeds. Cutting speed was found to have an impact the negative bur that formed only when it could lead to the formation of a BUE. As a result, the burrs that formed when mach ining A356-T6 were found to be insensitive to cutting speed since a BUE does not usually form when machin ing A356-T6. When considering feedrates, burr sizes increase with feedrate and this is most evident when machining A 356-T6. The large increase in burr size that occurs when machining A356-T6 at larger feedrates is a result of the formation of a negative shear plane as the cutting tool approaches the exit surface of the workpiece material. Th is negative or downward sloping plane interacts with the microstructure of the A356-T6 and specifically the spherodized silicon particles which fracture, debond, and lead to crack propagation throughout the material to form a negative burr plane. As part of a potential burr reduction strategy, decisions on mach ining in an as-cast versus heat-treated state may be important to consider in conjunction with the role of cutter path and cutting parameters. The impact on burr sizes resulting of roughing versus finishing feedrates and cutting speeds and depending on the state of the workp iece material should be considered into any burr reduction strategy for A356 alu minum. 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