A Review of Machining of Inconel 718 using CBN Inserts

Abstract

Nickel Based super alloys like Inconel 718 has wide application in Aircraft industries mainly because of

their thermal resistance and exceptionally good mechanical properties at high temperature. However

machining of these alloys are also extremely difficult due to their excessive work hardening properties, low

thermal conductivity which result in reduced tool life or detrimental effects on the machined surface. Carbide

tools were used earlier for machining these super alloys, however it had the trade-off of very low productivity.

Therefore Carbon Boron Nitride (CBN) Tools and Ceramic tools were introduced for High Speed Machining of

Inconel 718.In this paper, the cutting performance of various CBN tools are reviewed extensively under

different cutting conditions and stages. Focusing on both the productivity and the surface finish, developed

cutting forces, tool life, surface Integrity, developed residual stresses are taken as the index of performance.

Also reviewed separately is the performance in dry machining of Inconel 718 as compared to machining of the

same using the lubricants, keeping in mind the increasing attention of the environmental hazards and cost-

related issues of usage of lubricants in machining Inconel 718.

Based on the performance indexes mentioned above, machining of Inconel 718 is analysed mainly in

turning process, and to some extent, of milling process. Recent works and advancements of machining of

Inconel 718 are presented here, and are analysed and reviewed extensively.

Introduction

Nickel Based super alloys are widely used in aerospace industries. They are particularly used for the

hot sections of gas turbine engines, owing to their high temperature strength and high corrosion resistance.

Especially super alloys like Inconel 718 find their application in various aerospace industries all round the

world. However, the ability of Inconel 718 to maintain its mechanical properties at high temperature severely

hinders its machinability .They are known to be among the most difficult-to-cut materials [1].

The obstacles faced in machining Inconel 718 can be classified into two major problems: 1. Difficulty

in machining due to mechanical and chemical properties of the work-tool material and 2. The detrimental

effects on surface finish and surface integrity after machining.

The properties responsible for poor machinability of Inconel 718 are as follows [2]:

A major part of its strength is maintained during machining due to its high-temperature properties.

Inconel 718 is very easily work-hardened. Such high rated of work hardening leads to increase in its

strength, causing higher tool wear and shorter tool life.

Due to its poor thermal conductivity, high cutting temperatures up to 1200 ⁰C is generated at the

chip-tool interface during cutting operation.

The highly abrasive carbide particles contained in the microstructure cause abrasive wear of the tool.

Nickel based superalloys have high chemical affinity towards most of the tool materials, which causes

rapid diffusive wear of the tool.

Welding and adhesion of nickel alloys with the cutting tools often occur during machining, leading to

severe notching as well as detrimental alterations of the rake surface due to consequent pull out of

the tool material.

High rate of Built Up Edge (BUE) formation due to high temperature generated and the high

adhesiveness of the work-tool material.

On the other hand, detrimental effects of machining on superalloys like Inconel 718 are observed

universally mainly due to the high amount of heat generation and plastic deformation during cutting

operation. The heat generated alters the microstructure of the alloy, resulting in residual stresses. Residual

stresses are also produced as a result of plastic deformation. These residual stresses have harmful

consequences on the fatigue life of the workpiece. Excessive heat generation and deformation may also

generate cracks and microhardness variations.

Quality of machined surface plays a crucial role in possibility of premature failures as a result of creep,

fatigue and corrosion cracking. Though the quality of machined surface is usually assessed in terms of

geometrical parameters like surface roughness, dimensional inaccuracies, lay etc, for high-strength materials

such as Inconel 718, it is necessary to also assess the subsurface properties like residual stresses, micro-

hardness, resistance to creep, fatigue and corrosion cracking as well.

Therefore, extreme care must be taken while machining Inconel 718, and it must also be ensured that

proper tool material, along with efficient machining parameters are chosen. The requirements of any tool

material to be used for machining Nickel based alloys are [3]:

Good wear resistance

High strength and toughness

High hot hardness

Good thermal shock properties

Adequate chemical stability at high temperature

These required characteristics for the tool material have restricted the manufacturers into use of only a

few tool materials. Earlier High-speed steel, Carbide and Cemented Carbide tools were mostly used for

machining of these alloys. However, Carbide tools had the disadvantage of very low productivity as at high

speed, most of the carbide and cemented carbide tools show high level of BUE formation, flank wear etc. at

high speeds. As suggested in experiment by Liao and Shiue, efficient machining could not be achieved at

cutting speeds more than 30 m/min for most of the carbide based tools [4]. Some Cemented Carbide tools,

however (like SCLCR1616H-09 CCMT 120404-SM, PWLNR, WNMG 060404-MF1 CC PVD TiAlN) have shown

good cutting performance at speeds up to 50-70 m/min [5]6].

Therefore, in demand of higher cutting speed which will lead to more productivity, ceramic tools and

Carbon Boron Nitride (CBN) tools were introduced for machining Nickel-based super alloys. Mixed (Al₂O₃+TiC)

showed good performance in machining even at the cutting speed up to 240 m/min [7]. However tool life

under these conditions were severely limited due to excessive notching at the depth-of-cut region pull-out and

welding [8]. Reasonable tool life was achieved though, when machining Nickel super alloys using mixed

ceramic tools such as Al+Zr+W grade tools [9][10]. Besides, SiC whisker-reinforced alumina ceramic tools also

showed efficient cutting even up to cutting speed of 750 m/min and feed rate of .375 mm/rev[11][12].

However, rapid notch wear of the Ceramic tools persisted to be an issue in machining.

However notch wear decreased significantly when Carbon Boron Nitride (CBN) tools were used for

machining Nickel alloys such as Inconel 718. While flank wear showed little variation, notch wear decreased

reasonably as CBN contents of the tool material kept increasing.