In order to investigate the effects of brazing temperature, heating rate and cooling methods on shear strength, hardness, magnetic saturation and coercivity of the ultrafine cemented carbide, the ultrafine cemented ca...In order to investigate the effects of brazing temperature, heating rate and cooling methods on shear strength, hardness, magnetic saturation and coercivity of the ultrafine cemented carbide, the ultrafine cemented carbide was fabricated according to conventional powder metallurgical procedures, and then brazed to the stainless steel with silver-based filler alloy by supersonic frequency induction brazing. The microstructure was observed using scanning electron microscope （SEM）, energy dispersive spectroscopy （EDS） and the magnetic properties were tested utilizing coercimeter and cobalt magnetism instrument. The results show that no micro-crack is found in the cemented carbide after brazing because of silver-based sandwich compound used as filler alloy. In the melted silver layer, there is more carbon in the region adjacent to the cemented carbide. Varied shear strengths, hardnesses, magnetic saturations and coercivities are present under different brazing temperatures, heating rates and coolings. This phenomenon is correlated with some factors such as wettability and fluidity of filler alloy, brazing stress, oxidation of cemented carbide, and allotrope transition of cobalt. Shear strength reaches the maximum of 340 MPa and hardness of ultrafine cemented carbide remains 1879 HV at the brazing temperature of 730℃. The carbon increases with the of increase of the heating rate. What＇s more, and there is no r/phase found under this condition. content decreases with the increase of brazing temperature, and it the lowest magnetic saturation reaches 81.8% of the theoretic value,展开更多
Binderless nanotwinned cubic boron nitride(nt-cBN) synthesized from onion-structured BN precursors under high pressure and high temperature shows a very fine microstructure consisting of densely lamellar nanotwins(ave...Binderless nanotwinned cubic boron nitride(nt-cBN) synthesized from onion-structured BN precursors under high pressure and high temperature shows a very fine microstructure consisting of densely lamellar nanotwins(average thickness of 4 nm) within nanograins. The unique nanotwinned microstructure offers high hardness, wear resistance, fracture toughness, and thermal stability which are essential for advanced cBN tool materials. Thus, a circular micro tool of nt-cBN was fabricated using femtosecond laser contour machining followed by focused ion beam precision milling. Thereafter turning tests were performed on hardened steel using the studied micro tool. To evaluate the cutting performance, the machined surface quality and subsurface damage of the hardened steel were characterized. The wear mechanism of the nt-cBN micro tool was also investigated. It is found that the fabricated nt-cBN micro tool can generate high quality surface with surface roughness less than 7 nm and nanograin subsurface of about 500 nm deep. In addition, abrasive wear is found to be the dominant wear mechanism of the nt-cBN micro tool in turning hardened steel. These results indicate that nt-cBN has outstanding potential for ultra-precision cutting hardened steel.展开更多
基金Project(2013GZX0146)supported by the Science and Technology Projects of Sichuan Province,ChinaProject(11DXYB096JH-027)supported by Chengdu Science and technology Program,China
文摘In order to investigate the effects of brazing temperature, heating rate and cooling methods on shear strength, hardness, magnetic saturation and coercivity of the ultrafine cemented carbide, the ultrafine cemented carbide was fabricated according to conventional powder metallurgical procedures, and then brazed to the stainless steel with silver-based filler alloy by supersonic frequency induction brazing. The microstructure was observed using scanning electron microscope （SEM）, energy dispersive spectroscopy （EDS） and the magnetic properties were tested utilizing coercimeter and cobalt magnetism instrument. The results show that no micro-crack is found in the cemented carbide after brazing because of silver-based sandwich compound used as filler alloy. In the melted silver layer, there is more carbon in the region adjacent to the cemented carbide. Varied shear strengths, hardnesses, magnetic saturations and coercivities are present under different brazing temperatures, heating rates and coolings. This phenomenon is correlated with some factors such as wettability and fluidity of filler alloy, brazing stress, oxidation of cemented carbide, and allotrope transition of cobalt. Shear strength reaches the maximum of 340 MPa and hardness of ultrafine cemented carbide remains 1879 HV at the brazing temperature of 730℃. The carbon increases with the of increase of the heating rate. What＇s more, and there is no r/phase found under this condition. content decreases with the increase of brazing temperature, and it the lowest magnetic saturation reaches 81.8% of the theoretic value,
基金supported by the National Natural Science Foundation of China(Grant Nos.51205343,51332005&51421091)Hebei Provincial Science Foundation of China(Grant No.E2016203372)
文摘Binderless nanotwinned cubic boron nitride(nt-cBN) synthesized from onion-structured BN precursors under high pressure and high temperature shows a very fine microstructure consisting of densely lamellar nanotwins(average thickness of 4 nm) within nanograins. The unique nanotwinned microstructure offers high hardness, wear resistance, fracture toughness, and thermal stability which are essential for advanced cBN tool materials. Thus, a circular micro tool of nt-cBN was fabricated using femtosecond laser contour machining followed by focused ion beam precision milling. Thereafter turning tests were performed on hardened steel using the studied micro tool. To evaluate the cutting performance, the machined surface quality and subsurface damage of the hardened steel were characterized. The wear mechanism of the nt-cBN micro tool was also investigated. It is found that the fabricated nt-cBN micro tool can generate high quality surface with surface roughness less than 7 nm and nanograin subsurface of about 500 nm deep. In addition, abrasive wear is found to be the dominant wear mechanism of the nt-cBN micro tool in turning hardened steel. These results indicate that nt-cBN has outstanding potential for ultra-precision cutting hardened steel.
