Inconel 718, an efficient superalloy for energy and aerospace applications, is currently machined with cemented carbide tools at low speed (vc≈60 m/min) due to its unfavorable mechanical and thermal properties. The article presents results of superalloy machinability study with uncoated and coated PCBN tools aiming on increased speed and efficiency. Aspects of tool life, tool wear and surface integrity were studied. It was found that protective function of the coating, increasing tool life up to 20%, is limited to low cutting speed range. EDX and AFM analyses suggested dominance of chemical and abrasive wear mechanisms. Residual stress analysis has shown advantageous compressive surface stresses.49010
1. Introduction
Creation and use of efficient and sustainable equipment for energy sector depends on the materials selected for their design and on the production costs of components manufactured from them. Nickel-iron based superalloy Inconel 718 possesses advantageous service mechanical and thermal properties: high strength at elevated temperatures, high oxidation and corrosion resistance and low thermal conductivity [1]. Yet these same properties reduce its machinability. Currently low production efficiency, with cutting speed around 60 m/min for traditionally used cemented carbide tools, requires increased speeds through application of new tool materials. Attempts on application of PCBN tools, which are the first choice for high speed machining, has been made in 1990-th. Tools available in that period had high cBN content (90-95%) with metallic or ceramic (AlN, AlB2) binder [2]. Application of such tools was limited to the cutting speed of 90-120 m/min [2].
Machinability of other Ni-based superalloys with PCBN tools has also been the object of study [3, 4]. It was found that content and type of alloying elements plays a crucial role on tool life of PCBN tools. Chemical reactions of cBN with alloying elements (Fe, Cr, Ni, Ti, etc.) stemmed from high cutting temperature were found to govern the wear rate of the tools [3]. Recent developments in the PCBN materials related to optimization of cBN content and a type of binder has led to extension of cutting speed range to 200-300 m/min [5, 6]. Application of ceramic binder of TiN, TiC, Ti(C,N), etc., reduces the amount and exposure of cBN particles to the chemical wear. Another approach for increase in efficiency of machining Inconel 718 is through application of coatings. When machining with cemented carbides, application of coatings allows an increase in cutting speed from ~30 m/min up to ~60-100 m/min [7]. Issues of surface integrity in machining Inconel 718 with PCBN tools were given very limited attention. Arunachalam et al. [8] have conducted an assessment of residual stresses generated on the
V. Bushlya et al. / Procedia CIRP 3 (2012) 370 – 375 371
machined surface when facing aged Inconel 718. The results has shown that machining with PCBN tools results in the generation of advantageous compressive stresses, but the distribution of the residual stresses into the subsurface was not studied.
The aim of the presented study is to assess the machinability of aged Inconel 718 when high speed turning with uncoated and coated PCBN tools. The following issues are addressed: cutting forces, tool life, tool wear mechanisms and surface integrity.
2. Experimental details
Continuous longitudinal turning was selected as the machining operation. All tests were conducted on a modern CNC lathe. Workpiece material was the heat-resistant superalloy Inconel 718 in solution annealed and aged state ( 45 HRC). A bar of 70 mm in diameter and 250 mm in length was machined with polycrystalline cubic boron nitride (PCBN) tools. PCBN tools were selected in uncoated and titanium nitride (TiN) coated state. PCBN grade with low content of cBN (approx. 50%) was selected for the tests following the tool manufacturer recommendations. Grade with ceramic TiC-based binder and cBN with grain size of 0.5-2 m was selected (see Figure 1.a). PCBN刀具高速切削加工英文文献和中文翻译:http://www.751com.cn/fanyi/lunwen_51806.html