Maquinação de metal duro pré-sinterizado com diamante CVD
authors Pedro Gonçalo Pereira Duarte
supervisors Filipe José Alves de Oliveira
thesis type Mestrado
abstract Machining of pre-sintered parts is an important hardmetal processing step. It defines the quality of the pieces after sintering, overall product quality and the ability of keeping the production on schedule since some parts require over 15 h of machining time. A faster solution should simultaneously warrant the same quality levels and part integrity as well as process reliability. The aims of this work are the production and test of CVD diamond coated drill bits as an alternative to conventional tools with Ni bonded diamond particles. A large HFCVD reactor (50000 cm3) was used in this work. The coating conditions were optimized, starting with parameters used in smaller reactors. The number of filaments, current per filament, pressure, gas composition and substrate temperature were adjusted to produce nanocrystalline and microcrystalline diamond films. When coating a single drill bit the best parameters are, for NCD: P=20mbar, Ts=900 °C, Tf= 2075 °C and CH4/H2=0,0204; and for MCD: P=20mbar, Ts=900 °C, Tf= 2075 °C and CH4/H2=0,0152. For the industrial drilling experiments, 5 drill bits were coated simultaneously for both conditions with homogeneous diamond films that have a larger grain size than for a single drill bit. These tools were identified as coarse MCD and fine MCD coatings. Wear erosion tests were used to select two hardmetal grades for the drilling operations: the grade MD2NC, with high wear resistance and commercially important and MD4, with the lowest erosion wear resistance. Blocks of these two materials (100x80x40 mm3) were prepared at Durit under standard production methods. Two industrial milling machines were used for drilling the blocks, placed over a tri-axial dynamometer for real-time evaluation of the cutting force. The drilling strategy was substantially modified relatively to the standard procedure when using Ni bonded diamond tools. Due to the absence of a sharp cutting edge and the existence of exposed large diamond grains, the machining mode of these tools is close to that of a grinding wheel. So, a 35 mm deep drill can only be done by small stages of 1.5 to 2 mm while with the CVD Diamond coated tools, only two 17.5 mm stages are needed due to the easiness of cut chip/powder removal. The machining conditions were also altered relatively to the traditional tools, sweeping from cutting speeds about 4 times lower up to the milling machines maximum (10 m/min to 87 m/min) while the infeed rate (from 43 mm/min to 875 mm/min) was tested. Independently of the MCD tool used, coarse or fine, both hardmetal grades were machined at the maximum possible cutting speeds (65,5 m/min and 87,1 m/min) with feeds of 93 μm/rev and 117 μm/rev. With conventional tools feed values do not exceed 5 to 10 μm/rev. Under these conditions the axial cutting force reaches between 4 to 6 N for conventional drill bits while for the CVD coated drill bits the maximum value is below 3 N for the cutting condition that uses simultaneously the fastest cutting speed and infeed rate, and slightly smaller for the fine MCD. The very low axial and tangential components of the cutting force indicate that the integrity of the parts machined using the new technology should at least equal that of the traditionally machined pieces. The axial force increases with the feed, at higher rates for feeds above 150 μm/re, according to the law F = K f.mvn. Conversely, at the low feed regime the dependence of the cutting force with feed is small, the force increasing only slightly with the feed. Using the best cutting conditions, 48 holes were made on the blocks of both hardmetal grades, without any wear signs at the cutting tip. Finally, the MCD coatings allow machining times 20x faster when comparing with traditional drill bits, with a simultaneous increase in the edge quality of the holes.
year published 2013