Application of the cryogenic cooling during the turning operation is shown in Figure 3. Liquid nitrogen was delivered to the cutting region through 4.
One was placed over the rake face of cutting tool, while other was placed at the back of the tool holder to deliver liquid nitrogen towards to cutting tool tip from the rake face of the cutting tool . Figure 4 shows the surface topography of samples machined under dry and cryogenic cooling conditions at the cutting speed of Feed marks on the surface of dry and cryogenically machined samples are visible. There is no big difference on the topography of the dry and cryogenically machined samples except dry machined sample has slightly wider valley of perforated feed marks.
While feed marks are visible on the surface of dry machined sample, much smooth surface was obtained with cryogenic machining.
Shape Memory Alloys
The smoother surface in cryogenic machining at high cutting speed can be attributed to reduced tool-wear and reduced thermal distortion. Cryogenic machining results in decreased surface roughness due to the smoother surface and reduced peaks and valleys on the machined surface, and hence, improved the surface quality, as shown in Fig. It is a well-known fact that failure or crack generally starts on the surface or subsurface where these deep feed marks could result in stress concentration at the surface and initiate the fracture and failure. Considering the relationship between surface quality and performance of machined products, particularly its contribution to the risk of failure of functional products, in general, a smoother and high quality surface is desired from machining processes.
Cryogenic machining meets this demand better than the dry machining. Surface and subsurface microstructure of dry and cryogenically machined NiTi alloys are shown in Figure 7. Grain refinement was not clearly observed from these optical microcopy images. Both samples were etched homogeneously by immersing samples into etchant. In other words, grain boundaries in that region are not as visible as grain boundaries in the bulk of the material.
Although other analysis such as DSC and X-Ray diffraction XRD of these layers confirms the occurring of high dislocation density in this layer, further investigation utilizing transmission electron microscopy TEM and scanning electron microscopy SEM are required to determine the depth of affected layer and grain refinement on the surface and subsurface of machined NiTi samples. It has to be noted that the obtained images from the samples machined at low cutting speed under dry and cryogenic cooling conditions did not show considerable difference.
Our previous studies in machining of room-temperature austenitic NiTi alloys revealed that machining process alters the phase transformation temperatures of the surface and subsurface of machined specimens .
A DSC measurement of machined surface and subsurface is a reliable and repeatable approach to quantitatively and qualitatively identify the machining-induced phase transformation temperatures of NiTi alloy. It also helps to understand whether the examined specimen was subject to any stresses, defects or dislocation density coming from processing by comparing it with as received specimen.
Figure 8 shows the DSC response of cryogenically machined sample. The increase in transformation temperatures can be attributed to formation of the residual stress and high dislocation. After first thermal cycle, peak broadening gets smaller and hence both temperatures As and Af were reduced and reach close to the as-received temperature. Figure 9 shows the DSC response of dry machined sample.
These results indicate that residual stress and dislocation density on the surface and subsurface of dry machined samples was much smaller than that of cryogenically machined sample. This study provides evidence that machined surface and subsurface have different phase transformation responses than the as received material. In this study, cryogenic and dry machining-induced surface integrity parameters surface quality, topography, surface roughness, microstructure and phase transformation temperature were investigated in machining of NiTi alloy.
Cryogenic machining process helped to improve the surface quality of machined components more than the dry machining. Although no clear machining-induced layer was observed on both dry and cryogenically machined specimens from the optical microscopy, further examination is required to determine the exact depth of affected layer.
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Martensite to austenite transformation temperatures are higher and transformation peak is broader in cryogenically machined sample than the dry machined sample which indicates that the cryogenic machining has more severe effects on the surface integrity characteristics of NiTi alloys by introducing high dislocation density and residual stresses on their surfaces and subsurface. The comparison of as received, dry machined, and cryogenically machined samples during the first cycle are shown in Figure The largest peak broadening, associated with increased temperature requirement to transform from martensite to austenite phase state, due to martensite stabilization or introduced high dislocation density and consequently reduced amount of transformed material, was observed with cryogenically machined sample.
The comparison of the DSC responses of as received, dry machined and cryogenically machined specimens. New York, Springer. Structural vibration control by shape memory alloy damper, Earthquake Eng Struc 32, p. An overview of NiTi shape memory alloy: Corrosion resistance and antibacterial inhibition for dental application, Journal of Alloys and Compounds , p. Corrosion resistance tests on NiTi shape memory alloy, Biomaterials 17, p. On the nature of the biocompatibility and on medical applications of NiTi shape memory and superelastic alloys, Bio-medical materials and engineering 6, p.
Tool-wear analysis in cryogenic machining of NiTi shape memory alloys: A comparison of tool-wear performance with dry and MQL machining, Wear , p. Surface integrity in material removal processes: Recent advances, Cirp Annals-Manufacturing Technology 60, p. A review of surface integrity in machining and its impact on functional performance and life of machined products.
International Journal of Sustainable Manufacturing 1, p. The effects of machining on microstructure and transformation behavior of NiTi alloy. Scripta Materialia 74, p. E, Jawahir, I. Machinability and surface integrity of Nitinol shape memory alloy. Journal of the Mechanics and Physics of Solids, accepted for publication. Study of surface quality in high speed turning of Inconel with uncoated and coated CBN tools.
Abstract of research paper on Materials engineering, author of scientific article — Y. Smarter vehicles. Smart structures and materials industrial and commercial applications of smart structures technologies Google Scholar. Elahinia Mohammad H. Suzuki Y.
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