[attachment=12407]
ABSTRACT
Laser assisted machining (LAM) can improve the machinability of materials by locally
heating the material prior to its removal. The work presented here is a study of the laser assisted machining of Inconel 718 (NiCr19FeNb at 46 HRc) with ceramic insert. The tests have shown a reduction in the cutting force, and have highlighted the impact of laser assistance on the integrity surface (roughness, appearance, residual stress) and the tool life.
Also , It not only efficiently reduces the cutting force during the manufacturing process but also improves the machining characteristics and accuracy with regard to difficult-to machine materials. The prediction of relative deformations between the cutting tool and workpiece is important to improve the accuracy of machined components. And in the paper , the deformation errors caused by thermal effects in the laser-assisted machine tool using finite element method are also discussed.
INTRODUCTION
INCONEL 718 -
INCONEL alloy 718 (UNS N07718/W. Nr. 2.4668) is a high-strength, corrosion resistant nickel chromium material. It is nickel – based super alloy. It have good balance
between corrosion and oxidation resistance. The Inconel provides good mechanical properties. The Inconel is suited for directional solidification casting but can also be used for conventional or single crystal casting techniques. It is well suited for the hot section components such as blades, vanes and ring segments for gas turbine engines. It can be used with various thermal barrier coatings. It also have a very good material strength at high temperatures. These properties permit their use in components for gas turbine engines where the retention of excellent mechanical properties at high temperatures is required. Hot section components include vanes, rotating blades and ring segments.
The ease and economy with which INCONEL alloy 718 can be fabricated, combined with good tensile, fatigue, creep, and rupture strength, have resulted in its use in a
wide range of applications. Examples of these are components for liquid fueled rockets, rings, casings and various formed sheet metal parts for aircraft and land-based gas turbine engines, and cryogenic tankage. It is also used for fasteners and instrumentation parts.
LASER ASSISTED MACHINING -
Laser Assisted Machining is a high temperature cutting process using a laser beam as the heat source . The principle of the process is to reduce the cutting force necessary to machine the material by increasing the temperature to the point where the strength of the material is reduced . Indeed at high temperature, the specific cutting energy is weak, which improves workability. General illustration is shown in the figure.
The main purpose of LAM is not to cut the material but to provide better and easy
environment for machining operation.
Working Principle -
Laser Assisted Machining (LAM) is a new and innovative way of machining hard-towear
materials, which are difficult to machine using conventional methods. LAM combines
laser technology with traditional machining methods such as turning and milling. The laser is used as a heat source with the beam focused on the unmachined section of the workpiece directly in front of the cutting tool . The addition of heat softens the surface layer of the material, so that ductile deformation rather than brittle deformation occurs during cutting.
REQUIRMENT OF LAM IN INCONEL 718 MACHINING -
Inconel is a difficult metal to shape and machine using traditional techniques due to rapid work hardening. After the first machining pass, work hardening tends to elastically
deform either the workpiece or the tool on subsequent passes. For this reason, age-hardened Inconel 718 is machined using an aggressive but slow cut with a hard tool, minimizing the number of passes required. In order to increase productivity certain types of ‘assistance’ can be used to facilitate the cut. It has already been shown that LAM makes it possible to machine high strength alloy like Inconel 718.
Inconel does not change its mechanical properties even in high temperature .Hence, conventional machining (CM) of Inconel is slow and inefficient because only slow cutting speeds can be used. In order to increase productivity certain types of ‘assistance’ can be used to facilitate the cut. It improves workability by decreasing the cutting forces and byincreasing the tool life. This process is currently the only process able to machine very hard materials.
USING LAM WITH CONVENTIONAL METHOD
Traditional methods of machining hard-to-wear materials include, grinding or diamond machining which accounts for 60%-90% of the final cost of the product Another method used is hard turning, where cutting speeds are increased greatly and a cubic boron nitride (CBN) tool is used. At these high speeds friction heat anneals the material directly in front of the cutting tool, which is then removed by the tool itself, leaving the machined surface unaffected. However CBN tools are expensive and it is estimated that tooling costs are very high.
Also it is seen that Direct laser machining has also been researched as a means of shaping hard materials. However it involves melting or evaporating the material, which can cause surface cracks and undesirable changes to the surface microstructure. Therefore, Laser Assisted Machining is very useful in case of Inconel 718.
Various machining parameters -
The following variables are considered while machining.
These parameters are considered as per recommendation of CeTIM ( Center for Technology & Innovation Management) for the insert used.
• Cutting speed – It is kept as Vc = 220 m/min.
• Feed – 0.18 mm/rev.
• Depth of cut – Ap = 1.5 mm.
• Material removal rate - 59.4 cm3.min-1
• Laser distance from cutting tool – Approximately 5 mm.
• Laser power - 0 Watts (for conventional machining), 1500 Watts
• Insert – CC670 Ceramic Insert (Sandvik) ref. RNGN 090300
The three components of the cutting force, the surface integrity (roughness, residual
stresses), and the tool life were measured for each test.
A new cutting edge was used for each test.
Cutting fluids -
Almost any cutting fluid, or none, can be used in machining Inconel. In many applications,Inconel respond well to ordinary sulfurized mineral oil; sulfur imparts
improved lubricity and anti weld properties. If the temperature of the oil and workpiece
becomes high enough during machining to cause brown sulfur staining of the work, the stain can be readily removed with a cleaning solution of the sodium cyanide or chromic-sulfuric acid type. This should be done before any thermal treatment, including welding, because during further exposure to high temperature the staining may cause inter granular surface attack. To avoid inter granular corrosion, the parts should be immersed in cleaning solution only long enough to remove the stain. High-speed machining operations that create high temperatures might preclude the use of a sulfurized oil because of sulfur embrittlement of carbide tools. (Many sintered carbides have a nickel or cobalt matrix that is sensitive to sulfur attack at high temperature.) However,flooding the cutting area with cutting fluid generally cools the tool enough to avoid breakdown of the carbide bond.
Water-base fluids are preferred in high-speed turning, milling, and grinding because of their greater cooling effect. These may be soluble oils or chemical solutions. Except for grinding, which depends almost entirely on cooling and flushing, some chemical activity is always desired and is generally provided by chlorine, amines, or other chemicals. For slower operations, such as drilling, boring, tapping, and broaching, heavy lubricants and very rich mixtures of chemical solutions are needed. Oils should be used when drilling nickel 200 and Inconel X-750. In the drilling and tapping of small-diameter holes and in other operations in which lubricant flow and chip flushing are restricted, solvents will improve performance. These less viscous fluids can be used alone or can be used for diluting mineral and lard oils. A cutting
fluid of the spray-mist type is adequate for simple turning operations on all alloys.