Machinability of Engineering Metals_Surreddi_02Dec2013

K. B. SURREDDI MTT105

Materials and Manufacturing Technology

Machinability of engineering metals

• Machinability • Machinability of engineering metals

– Iron and Steels – Cast iron – Nickel and Nickel alloys – Titanium and Titanium alloys – Magnesium – Aluminum and Aluminum alloys – Copper, Brass and other Copper alloys



Machinability • Ease or difficulty with which a given material can be machined to the required

specifications

• A resultant 'property' of the machining system which is affected, directly or indirectly, by the work and tool materials, machine tool, part, fixture, cutting fluid, and cutting conditions



Factors affecting machinability



Causes for poor/good machinability



Cutting conditions (cutting speed, feed, and depth of cut) Tool geometry (rake angle, inclination angle, cutting edge angle, etc.) Tool material including coating Chip-groove geometry Cutting fluid application Rigidity of the machine tool-cutting tool system (including work

holding devices) Nature of engagement (continuous, intermittent, etc.)

Machining variables affecting the machinability



Work material properties affecting machinability Hardness Tensile strength Chemical composition Microstructure Degree of cold work and strain hardenability Work material’s heat treatment level Shape and dimensions of the work piece Work piece rigidity



(a) Tool-life (b) Cutting force/Power (c) Surface quality (e.g., surface roughness) (d) Part accuracy (e) Chip-form/Chip breakability

Machinability Criteria testing of cutting tools

Methods of Machinability Testing (a) Tool-wear/Tool-life tests (b) Surface finish test (c) Cutting force test (d) Cutting temperature test (e) Power consumption test (f) Cuttability test (e.g., rate of penetration of a drill under constant feed pressure) (g) Simulated production tests (for optimum cutting conditions)

Machinability Test Standards - ISO Tests ISO 3685:1993 (E) Tool-life testing with single-point turning tools ISO 8688:1989 Tool life testing in milling - Part 1: Face milling; Part 2: End milling



Machinability ratings • Machinability ratings are generally expressed in terms of a reference index • Concept of a machinability rating a number assigned to indicate the ease or difficulty

of machining for materials • Machinability ratings for different materials can be developed by using one type of

cutting tool at a constant set of cutting conditions



Machinability Ratings The most commonly used relative measure is the machinability rating (MR) or index (Im) defined by where (VcT)mat is the cutting speed at which the material being rated yields the defined tool life (T = 90-, 60-, 20- or 15-minute tool life) for a specified feed rate, depth of cut, tool material, and tool geometry (VcT)ref is the cutting speed at which the reference material with a machinability rating of 100 yields the defined tool life under the same conditions.

Comparison of the machinability ratings of some popular materials



two broad categories of machinability tests: (i) does not require an actual machining test (ii) require a machining test.

a parallel subdivision of tests: • a relative machinability test known as ranking test to compare the machinability of two or

more work-tool material combinations, • an absolute test to indicate the merit of a given combination of work-tool pair

In 1958, Volvo (Sweden) developed an innovative machinability test that primarily compares the machinability of a given bar material to that of a standard freecutting steel in a 22-min tool-life results of the test provide a “B index,” which defines the machinability of a given material possible to compare and rank materials by their machinability machining is continued until a maximum flank wear (VB,max) of 0.7 mm is obtained on

the cutting tool The index indicates the ratio in percent of the machinability of a free-cutting steel, whose

machinability has been given the index 100

Assessment of Machinability



Polar Machinability diagram

5 different material properties are used Each property is graded in 10 levels; grade 5 is

average for the respective property A reference material is needed to compare the

materials Appropriate tool properties and cutting data can be

chosen based on the diagram

Polar machinability diagram (J.-E. Ståhl et al., 2007) is the method to describe and classify the machinability of different materials with material properties.

Some examples from literature*

the reference material

poor thermal conductivity & strain hardening

excellent thermal conductivity &

high wear due to free silicon crystals



Machinability Data Systems Machinability data is generally gathered from production experience and summarizes machining conditions which yield acceptable tool life and part quality under specified operating conditions Machinability database (MDB) systems to assist numerically controlled (NC) machines not only in selecting the recommended cutting speed, feed rate and depth of cut, but also the tool material, appropriate tool geometry from available inventory, tool-holder and other elements of automatic modular quick-change tooling system, sequence of operations, and optimum cutting conditions based on economic or productivity requirements

Machinability database system consisting of three basic modules



Turning tool bits Cutting units Holding systems

Parting & Grooving Threading

Tool holding Milling

SANDVIK Coromant

Boring http://www.youtube.com/watch?v=SqHLLCeSQwI

http://www.youtube.com/watch?v=SqHLLCeSQwI



Tool materials: • Carbon and low alloy steels • High speed steels (HSS) • Cemented carbides • Ceramic or oxide tools • Diamond like structure



Hardmetals: • Uncoated hardmetal containing primarily tungsten carbide (WC) • Uncoated hardmetal, also called cermet, containing primarily titanium carbides

(TIC) or titanium nitrides (TIN) or both • Hardmetals as above, but coated Ceramics: • Oxide ceramics containing primarily aluminium oxide (Al2O3) • Mixed ceramics containing primarily aluminium oxide (Al2O3) but containing

components other than oxides • Nitride ceramics containing primarily silicon nitride (Si3N4) • Ceramics as above, but coated Diamond: Polycrystalline diamond 1) Boron nitride: Cubic boron nitride 1)

Tool materials



Medium to coarse WC grain size: superior combination of high hot hardness and toughness; used in combination with CVD or PVD coatings in grades for all areas. Fine or submicron WC grain size: used for sharp cutting edges with a PVD coating to further improve the strength of the sharp edge; superior resistance to thermal and mechanical cyclic loads.

Cemented carbide powdery metallurgical material; a composite of tungsten carbide (WC) particles and a binder rich in metallic cobalt (Co) consist of more than 80% of hard phase WC WC grain size -adjusting the hardness/toughness The amount and composition of the Co-rich binder controls the grade’s toughness and resistance to plastic deformation Cubic carbonitrides, also referred to as γ-phase, are generally added to increase hot hardness and to form gradients. Gradients are used to combine improved plastic deformation resistance with edge toughness.



Tool coatings • Changing the tool surface properties. • improve the performance of both high speed steel and cemented carbide tool materials increased materials removal rates

• Coating a very thin layer of TiC or TiN over the WC-Co tool reduces the effects of adhesion and diffusion and reduces the crater wear

• Chemical vapour deposition and physical vapour deposition (CVD, PVD) depositing thin carbide layers onto tool materials

Coating – PVD Coating – CVD

• formed at 400-600°C • extensively used for

finishing applications • PVD-TiN, Ti(C,N),

(Ti,Al)N, oxide

• formed at 700-1050°C • high wear resistance and

excellent adhesion to cemented carbide

• CVD-TiC, Al2O3 and TiN coatings

• modern titanium carbonitride coatings (MT-Ti(C,N) or MT-TiCN



Ceramics or oxide tools Oxide ceramics: aluminium oxide based (Al2O3), with added zirconia (ZrO2) for crack inhibition; chemically very stable, but which lacks thermal shock resistance (1) Mixed ceramics are particle reinforced TiC, Ti(C,N); improves toughness and thermal conductivity (2) Whisker-reinforced ceramics: SiC whiskers; increase toughness and enable the use of coolant; ideal for machining Ni-based alloys (3) Silicon nitride ceramics : Si3N4; successful in grey cast iron, but a lack of chemical stability Sialon (SiAlON): combine the strength of a self-reinforced silicon nitride network with enhanced chemical stability; ideal for machining heat resistant super alloys (HRSA) • machining cast irons at high speeds • better wear resistance • cutting speed at 2-3 times > cemented carbides in uninterrupted cuts • required rigid tool mounts and rigid machine tools • inherent unreliability



Polycrystalline cubic boron nitride: CBN • excellent hot hardness • very high cutting speeds • good toughness and thermal shock resistance

Polycrystalline diamond • a composite of diamond particles sintered together with a metallic binder • good wear resistance but it lacks chemical stability at high temperatures and

dissolves easily in iron • limited to non-ferrous materials, such as high-silicon aluminium, metal matrix

composites (MMC) and carbon fibre reinforced plastics (CFRP) • PCD with flood coolant can also be used in titanium super-finishing

applications



• Machine time - elapsed time of operation of machine tool • Actual cutting time (most common definition of tool-life) • Volume of metal removed • Number of pieces machined • Equivalent cutting speed (Taylor speed) • Relative cutting speed

Specification of Tool-Life

Tool Failure Types and Failure Criteria • Flank wear development of a wear land on the tool due to abrasive rubbing between the

tool flank and the newly generated surface • Crater wear formation of crater in the rake face of the tool, as a result of diffusion wear • Built-up edge, thermal cracking, or deformation • Various combinations of the above

Properties of cutting tool materials: • Hardness, particularly at high temperature • Toughness to resist failure or chipping • Chemical inertness with respect to the work piece • Thermal shock resistance • Wear resistance, to maximize the lifetime of the tool

Tool materials and tool life



Criteria for tool failure: • Complete failure • Preliminary failure (e.g., appearance on the surface finish) • Flank failure • Finish failure • Size (dimension) failure • Cutting force (power), or thrust force, or feed force failure

Major Tool-Wear Mechanisms • Abrasion result of hard particles on the underside of the chip abrading the tool face by

mechanical action as the chip passes over the rake face • Adhesion the tool and the chip weld together at local asperities, and wear occurs by the

fracture of the welded junctions • Diffusion Wear from solid-state diffusion from the tool materials to the work piece at

high temperature due to intimate contact at the interface between the chip and the rake face • Chemical reaction • Oxidation



1 2 3

4 5 6

7

Crater wear Thermal cracks Edge chipping Flank wear Built-up edge (BUE) Notch wear Plastic deformation

1. Flank wear 2. Crater wear 3. Built-up edge (BUE) 4. Notch wear 5. Plastic deformation 6. Thermal cracks 7. Edge chipping

Types of wear



• The tool removes material near the surface of the work piece by shearing it to form the chip. • Material with thickness t1 is sheared and travels as a chip of thickness t2 along the rake face of the tool. • The chip thickness ratio (cutting ratio) r = t1 / t2

Chip formation



Type of machining chips – continuous chips, chips with BUE, discontinuous chips Continuous chips cutting ductile materials under steady stage conditions long continuous chips required chip breaker Discontinuous chips in brittle materials Ex: cast iron and cast brass, may occur in ductile materials machined at very low speeds and high feed Chips with BUE when the friction between the chip and the rake face of the tool is high, the chip may weld to the tool face • The accumulation of the chip material a built-up edge (BUE) • The formation of BUE is due to work hardening in the secondary shear zone at low speed • The BUE act as a substitute cutting edge (blunt tool with a low rake angle)

Ernst's classification of chip types.



Chip forms



Materials and Machining



• Commercially pure iron - poor machinability - it has relatively low shear strength but high ductility – tool forces are much higher than for copper – the forces decrease rapidly as the speed is raised - large contact area on the rake face of

the tool, and thick chips – No built-up edge is observed at any speed

Iron

Carbon steels Compositions and microstructures of steels

• Low-carbon steel with less than 0.3% C • Medium-carbon steel with 0.3 to 0.6% C • High-carbon steel with more than 0.6% C



Machining of Carbon and alloy steels • Machinability of carbon and alloy steels influenced by its carbon and alloy content,

microstructure, amount of soft or free machining particles, oxide inclusions, hardness, and work-hardening rate.

• free machining additives (sulfur, lead, bismuth, tellurium and selenium often in combination with sulfur)

• The reference steel, UNS Gl2120, has a 1-h cutting tool life at 50 m/min and was assigned an index of 100.

• The machinability index does, however, facilitate steel selection by ranking the relative machinability of a steel grade

• With regards to machinability of low carbon steels, the low hardness and high ductility is often a negative factor with considerable tendency for smearing and BUE which reduce tool life and give poor surface texture

• Carbon steels are most commonly turned, bored and milled with grooved carbide tools coated with single (TiN, TiC) or multilayer films to reduce cratering due to diffusion

• Carbon steels can be finished by turning with cermets, turned and bored with alumina-based ceramic tools, and also turned, bored and milled with PCBN tools



• The heat generated in cutting becomes a controlling factor • Constraints on the rate of metal removal and the tool performance machining costs

• Influence of alloying elements on the forces and stresses on the tool

• Cutting force is reduced by the addition of alloying elements however, the created contact area is much less the average compressive stress on the tool is always higher by adding the alloying elements

• When steels are heat treated to high strength and hardness the compressive stress on tools during cutting high enough to deform the cutting edge and destroy the tool

• When using high speed tools, the machining of steels with hardness higher than 300 HV becomes very difficult, Cemented carbide tools can be used to cut steels with higher hardness

• For cutting fully hardened steel, the tool materials must retain their yield strength to higher temperatures Ceramic tools can be used to machine steel hardened to 600 to 650 HV higher rates of metal removal and longer tool life with CBN tools

• Relative machinability of steel

• Generally high speed steel or cemented carbide tools are used • Nominal cutting speeds and feeds for machining different steels are often proposed by

tool manufacturers and in books and papers

Steels: Alloy steels and heat-treatments



Conditions of deformation of cemented carbide tools when cutting steels of different hardness

Steels: Alloy steels and heat-treatments Maximum interface temp vs cutting speed

‘Machining chart’ for steel cutting grade of carbide used for cutting Ni-Cr-Mo steel (Hardness = 258 HV)



• Combined influence of alloying, heat treatment, stress and temperature • To permit higher metal removal rates, steel work-materials are often heat treated to

reduce the hardness to a minimum • Annealing just below the transformation temperature (about 700°C) “spheroidizes”

the cementite - least strengthening effect • Low carbon steel containing a lot of pro-eutectoid ferrite, slow cooling from the

annealing temperature is essential • The yield stress of the steel work material, and its rate of strain hardening, are the

main factors in its machinability • The highest temperature, 850-900°C occurs at more than 1 mm from the tool edge.

The temperature at the tool edge is 600-650°C

• Built-up edge effects • Steels containing more than about 0.08% carbon, have an appreciable amount of

pearlite at lower cutting speeds, a built-up edge is formed • At a critical speed, the shape of the chip changes and the surface finish improves as

the built-up edge disappears

Steels: Alloy steels and heat-treatments



• Economics higher rates of metal removal and longer tool life high sulfur content

• The role of MnS additions in “free cutting” steels

– MnS - improves machinability

– all the sulfur is present in the form of manganese sulfide (MnS)

– Steel makers pay attention on MnS amount and also to the distribution

– particle shape, size and distribution control is achieved during the steel making

– extensively for mass production of parts on CNC and automatic machine tools

– permit the use of higher cutting speeds, give longer tool life, good surface finish, lower tool forces and power consumption, and produce chips which can be more readily handled

• MnS behavior on the steel-cutting grades of carbide

– layers of MnS are often found covering parts of the tool surface

– sulfide is found covering the contact area on the tool rake face

– at high speed, MnS seems to act as a lubricating layer - interposed between tool and work material on the rake face

Free cutting steels



• The role of lead additions in “free cutting” steels – Lead permits even higher cutting speeds – both better surface finish and better control of chips – the particles of lead in steel are plastically deformed during machining

• Variable machinability of non free-cutting steel – Sulfur content

• The rates of flank wear increase with decreasing sulfur content • Steels which have been cleaned of nonmetallic inclusions by electro-slag remelting

(ESR) are reported to be more difficult to machine – Variability between batches

• rates of metal removal, are found to vary widely for different batches of steel conforming to the same standard specification

• caused by variations in the amount of interstitial carbon and nitrogen in ferrite – Damaging aspects of Al2O3 and hard inclusions – Beneficial aspects of inclusions that naturally coat the tool Steel-cutting grades of

carbide tools are often covered with layers of oxide - aluminum, silicon and calcium strongly bonded to the worn rake face and, sometimes, the flank surfaces much longer tool life and permit higher cutting speeds

Free cutting steels



Stainless steels • Four categories depending on their primary constituent of the matrix:

• ferritic, martensitic, austenitic and duplex (combined ferritic/austenitic) stainless steels

• Stainless steels are considered to be difficult to machine due to their high tensile strength, high ductility, high work hardening rate, low thermal conductivity and abrasive character results in high cutting forces and temperatures, and tool wear rates, as well as a susceptibility to notch wear, difficulties with chip breakability, BUE formation and poor surface finish

• Ferritic steels are generally more machinable and their machinability decreases with increasing chromium content

• Martensitic stainless steels higher content of carbon and nickel reduces machinability • Duplex alloys are generally harder to machine due to their high annealed strength

• General guidelines for machining stainless steels include: • use of lower cutting speeds than for carbon steels, • use of rigid tooling and fixture to avoid chatter, • maintain feed above a minimum level to avoid poor surface integrity, • maintain depth of cut above the depth of work-hardened layer to avoid excessive notch

wear, • use of high sharp cutting edges to avoid BUE and reduce work-hardening effect, • use of proper cutting fluids with sufficient flow rates for heat removal



Austenitic stainless steels • More difficult to machine than carbon or low alloy steels • Chips often remain stuck to the tool after cutting

fragment of the tool with cemented carbides, giving poor, erratic tool performance

• The forces are similar to normalized medium carbon steel • Temperature pattern imposed on the tool is similar as when

cutting other steels, with a cool region at the cutting edge • The carbon content is very low, the higher temperature due

to nickel and chromium raises the temperature in the flow-zone

• BUE is formed in a cutting speed range somewhat lower than with carbon steels

• Cratering of high speed steel tools occurs in the high temperature region on the rake face by diffusion and by superficial shear

• Cratering by the hot shearing mechanism occurs at speeds lower than those for medium carbon steels

• Flank wear is characteristically very smooth true for both high speed steel and carbide tools increases regularly as cutting speed is raised.

Cutting tools: recommended for machining austenitic stainless steels • TiCN-TiN (4 µm) PVD coated M15

fine-grain carbide for finishing operations

• TiCNAl203-TiN (total 5.5 µm) CVD coated M15 carbide for finishing and semi-finishing operations

• the latter coated grade but with M25 substrate, and TiAlN-TiN (total 4 µm) PVD coated M25 carbide for medium and roughing operations

• the last two grades can also be used for machining duplex stainless steels



Cast Iron

Relative properties and micrographs of FGI, CGI and SGI (left column) with deep etched scanning electron images of FGI, CGI and SGI (right column)



Cast Iron

Relationship between the matrix microstructure of cast iron and tool life. (Milling)



• good machinability: low rates of tool wear, high rates of metal removal, relatively low tool forces and low power consumption

• flake graphite cast irons are considered to have very good machining qualities • dirty and dusty operation, throwing a fine spray of graphite into the air protection

required • fracture on the shear plane occurs at very frequent intervals, initiated by the graphite flakes • the length of contact on the rake face is very short so the chips are thin- a few mm in length -

readily be cleared • use of ceramic tools has increased greatly in recent years, mainly in mass production

turning, boring and milling operations • cutting with carbide or high speed steel tools, a BUE is formed which persists to higher

cutting speeds than with steels • the wear is by attrition and the longest tool life is achieved with fine grain WC-Co tools • at higher rates of metal removal the BUE disappears. Then, to resist cratering and diffusion

wear on the flank, a fine grained steel-cutting grade of carbide tool should be used. • Spheroidal graphite (SG) irons have better mechanical properties than flake graphite irons -

graphite is present as small spheres instead of flakes • SG chips are formed in rather longer segments, but these are weak, easily broken

Cast Iron



CGI machining • The best tool material for continuous machining of CGI is cemented carbide, while for

interrupted machining is PCBN could be used. • Wet cutting is more suitable for CGI turning, since dry condition will make the cutting

edge easier to break up. In contrast, both wet and dry conditions are viable in milling with carbides.

• At the same level of UTS, turning FGI requires more force than CGI. • PCD tools with coarse grain size and high bond content work similar to carbide tools at

low cutting speed (150 – 275 m/min) in turning. At higher cutting speeds PCD are not working for Continuous machining of CGI

• PCD tool with the assistance of dry CO2 coolant gains an extraordinary performance at a cutting speed of 210 m/min.

• To improve the machinability of CGI, the improvement could be done in insert materials grade, improvement in the material (reduction of carbides, titanium % etc.) and by improvement of machining methods (Cutting fluids, Coolant and Lubricant, method etc.)

• For CGI machinability ultimate tensile strength is a good indicator. The pearlite% effect in milling has different opinions and results among some researchers. However, increase in pearlite% is always decreasing the machinability of CGI.



Commercially pure nickel • Nickel has a lower melting point (1,452ºC) than iron (1,535ºC) • Nickel is a very ductile metal with a face-centered cubic structure • Commercially pure nickel has poor machinability • Nickel and its alloys are, in general, more difficult to machine than iron and steel • Tool life tends to be short and the maximum permissible rate of metal removal is low • The tools fail by rapid flank wear plus deformation of the cutting edge, at relatively low cutting

speeds • With high speed steel tools, a recommended turning speed is 50 m/min at a feed rate of 0.4 mm/rev • Tool forces are higher than when cutting commercially pure iron • The contact area on the rake face is very large, with a small shear plane angle and very thick chips. • no built-up edge is formed, and the tool forces decrease steadily as the cutting speed is raised • high temperatures generated in the flow-zone lead to high rates of

tool wear • commercially pure nickel, there is a characteristic adverse distribution

of temperature in the tools

Nickel and Nickel alloys

Rake face of tool used to cut nickel and temperature contour



Lightly alloyed nickel work materials • when the alloying additions result in considerable strengthening of the nickel, the

cutting forces are often reduced contact length on the rake face is smaller, the shear plane angle is larger and the chip thinner

• the temperatures with nickel alloys are higher at any given speed, as compared with iron and steel

• the temperature distribution in tools temperature pattern similar in character to that in tools used to cut steel and unlike that when cutting commercially pure nickel

Nickel and Nickel alloys

Maximum interface temp vs cutting speed

Temperature distribution in high speed steel tool used to cut Ni-Cr-Ti alloy at 23 m/min



Nickel and Nickel alloys Highly creep-resistant nickel alloys used in aerospace industries • The highly creep-resistant alloys used in the aerospace industry, are some of the most difficult

materials to machine • These alloys are strengthened by a finely dispersed second phase, as well as by solid solution

hardening • A BUE is formed when cutting these two-phased alloys at low cutting speeds • As the speed is raised, the built-up edge disappears but very high temperatures are generated even at

relatively low speeds in the flow-zone at the tool/work interface • The temperatures are often high enough to take into solution the dispersed second phase in the

nickel alloy, and may be well over 1,000°C • These alloys retain high strength at elevated temperatures stresses in the flow-zone are very high • Cemented carbides, usually WC-Co alloys of medium to fine grain size, are used for turning, facing,

boring and sometimes in milling operations and for drilling large holes • Carbide tools are more efficient because of the higher speeds and longer tool life • Steel-cutting grades of carbide are usually worn more rapidly than the WC-Co grades • The cost of machining the nickel-based aerospace alloys is very high • Using both sialon and A1203/SiC whisker ceramics, cutting speeds up to 250 m/min are now

employed for the machining of nickel-based gas turbine discs.





• Titanium and its alloys - poor machinability

Commercially pure titanium – 1668ºC – ductile hcp at RT and bcc at 882ºC

• Tool forces and power consumption are considerably lower than when cutting iron, nickel or even copper

• The rates of metal removal for a reasonable tool life are much lower than when cutting iron

• Segmented chip formation in commercially pure titanium

• Low tool forces are associated with a much smaller contact area on the rake face of the tool the compressive stresses on the delicate cutting edge are very high - tool life is short - it is terminated by flank wear and/or deformation of the tool

• Because of the small contact area, the shear plane angle is large and the chips are thin, often not much thicker than the feed

Commercially pure titanium



• Titanium chips are continuous but are typically segmented • Narrow bands of intensely sheared metal being separated by broader zones of lightly sheared • “segmentation cycle” each period of thermoplastic shear is very short-lived and relieves the stress • Compressive strain continues by dislocation movement until the next thermoplastic shear band is

initiated • At the tool surface, the flow-zone is continuous and bonded very strongly to high-speed steel or

carbide tools • No built-up edge is formed • During normal disengagement of the tool, the chip frequently remains attached • Permissible rates of metal removal are low • The high temperatures and unfavorable temperature distribution in tools temperatures in the flow-

zone are higher than when cutting iron at the same speed • The temperature distribution is like that when cutting iron, but the cool zone close to the edge is very

narrow, and the high temperature region is much closer to the tool edge. • When machining CP titanium, although the tool forces are low, the stress on the rake face is high

highly stressed region near the tool edge is at a very high temperature leads to deformation of the tool edge and rapid failure, with the formation of a new heat source on the deformed and worn flank

• Failure is initiated at the nose radius of the tool

Segmented chip formation in commercially pure titanium



Titanium and titanium alloys • The flow zone in titanium alloys is initiated at a temperature close to 0.5 of MP of titanium • With titanium and its alloys, the flow-zone is very thin - usually less than 12 μm thick • The temperature gradients in tools used to cut titanium alloys are similar to CP Ti • The alloying additions raise the temperature for any set of cutting conditions reduces the

permissible cutting speed • With alloys containing a second phase, the temperature increase for any cutting speed is much more

marked • Machining of Ti-6Al-4V – tools rake face - over 900 °C at a cutting speed of 19 m/min • Cemented carbide tools longer tool life is achieved with the use of the WC-Co alloys than with the

steel-cutting grades containing TiC and TaC • The introduction of TiC into the cutting tool adverse effect in relation to diffusion wear the

cubic carbide grains containing tic are lost more rapidly by diffusion into titanium flowing over the tool surface than are the WC grains

• Apart from deformation, diffusion wear seems to be the main process, responsible for the wear both of high speed steel and of carbide tools when cutting titanium alloys

• Resistance to diffusion wear and resistance to deformation at high temperatures make the WC-Co grades of carbide useful for cutting titanium alloys still, cutting speeds - low, e.g., 30 m/min

• The strong adhesion of the chip to the tool may also cause problems when the machining operation involves interrupted cuts

• Breaking away of the adherent chip, removing fragments of the tool edge, and causing inconsistent tool life

• Ceramic tools based on alumina are worn more rapidly by processes of attrition



• Hcp structure, low ductility, low melting point • The magnesium alloys die casting alloy AZ91B (AZ91D); gravity casting alloys

AZ91C, (AZ91E), and AZ92A; and wrought alloys AZ31B, AZ61A, AZ80A, ZK21A, and ZK60A

• Easiest to machine • Rates of tool wear is low • Turning speeds may be up to 1350 m/min • Tool forces are low • Steel and carbide tools • Short contact length on the tool rake face • drawback-small chips can ignite and BUE formation • Safety is highly important due to chances of fire accidents • flank BUE due to adhesion - machining magnesium dry at a cutting speed of 900 m/min

with uncoated and Ti coated carbide tools • PCD tools are not preferred due to danger of chip ignition in dry machining

Magnesium and Mg alloys



Magnesium and Mg alloys Material Considerations Chip Formation depends on the composition, form, and temper of the alloy and rate of feed Single point tools short and well broken for heavy feeds, short and partially broken for medium feeds, and long and curled for light feeds Distortion due to high specific heat and good thermal conductivity -temperatures developed when cutting magnesium are usually low however considerable heating of the part can occur if a large amount of metal is being removed Thermal Expansion The mean coefficient of thermal expansion in the temperature range of 20 to 200°C extends from 26.6 to 27.4 µm/m/°C, depending on the magnesium alloy care to be taken for precision parts Cold Working Distortion of magnesium parts due to cold working during machining seldom occurs poor machining practice can induce extremely high stresses in the surface to a depth of as much as 0.5 mm Stress relief should be used only as a last resort to relieve machining-induced stresses Clamping on heavier sections not to be high shims should be used for thin parts Tools: For high-production jobs, carbide-tip tools are usually preferred because they last longer and are more accurate than high-speed steel tools For maximum tool life, back rake angles can range up to 20° very important rule is that tools must be kept as sharp and as smooth as possible Cutting Fluids frequently machined without a cutting fluid



• High machinability index • Good tool life with high speeds • Machinability of Al-Si alloys

– Hyper eutectic alloy – large grains (70µm) – due to large Si grains- high stress and temperature – increases wear rate – Can cause rapid attrition wear – Polycrystalline diamond tools for hypereutectic Al-Si alloys

• Al alloys Vs CP Al – Most Al alloys are easier to machine than CP Al – Tool forces are low for Al alloys as compared to CP Al – Area of contact on the rake face of the tool is very large for CP Al – i.e., high feed force, low shear plane angle, very thick chips high cutting

force and high power consumption – No BUE in CP Al

Aluminum and Al alloys



• Fcc structure, ductile so long, thick and continuous chips • Entangle the tooling, clogging interruption cost increase • Modified design of tools required chip breakers or curlers • Free machining Al alloys

– Addition of lead or tin or antimony up to 0.5% • Complete automatic production can be possible

Aluminum alloys



Tool forces vs cutting speed - magnesium and aluminum



• Fcc structure, ductile, high melting point (1083oC) • Both HSS and cemented carbide tools can be employed • Good tool life, flank wear or cratering or both • Mass production of electrical and water fittings with high speed automatic machines • High conductivity copper

• No BUE • Flow zone at tool and work interface over a wide range of cutting speed • tool forces are very high • the most difficult materials to machine • poor surface finish • At higher speeds, cutting forces are lower and surface finish improves, but tangled

coils of continuous chips are difficult to clear • Better after cold working and improved by alloying

• Additions (0.3% sulfur) are made also to high conductivity copper to improve its machinability - forms plastic non-metallic inclusions of Cu2S - to reduce greatly the tool forces, particularly at low speeds; Cu2S as a free-machining phase are: i) its plasticity during deformation in cutting, and ii) its strong adhesion to the tool surface, which prevents it from being swept away

Copper and Cu alloys



• Brasses – Tool forces are lower for single phase 70/30 brasses,

even lower for the two-phased 60/40 brasses – thin chips and small areas of contact on the tool rake

face – forces are lowest and power consumptions is low in

alloys of high zinc content with high beta phase high machinability

– Temperatures were higher for 70/30 brass than for 60/40 brass

– with copper and brass, very high speeds can be used when cutting with high speed steel tools

– only with 70/30 brass that high temperatures, and consequent high wear rates, limit the rate of metal removal with high speed steel tools

– Continuous brass chips can be broken into short lengths by forming grooves parallel with the cutting edge on the rake face of tools

Copper alloys



Copper alloys Free machining brass • addition of lead in the proportion of 2 to 3% precipitating particles usually between 1 and 10 µm

in diameter, which should be uniformly dispersed to achieve good machinability • environmental issues now prevent the use of leaded brass - a serious health hazard • thin chips are produced, not much thicker than the feed & fragmented into very short lengths which

are readily disposable • addition of lead greatly reduces tool forces and reduces the tool wear rate in comparison with other

types of brass • many small parts are economically made due to low machining cost, in spite of the high price of Cu • no definite flow-zone is formed on the rake face of the tool • Absence of a flow-zone, the greatly reduced strain in chip formation

Gun metal • Gun metals are similar to zinc brasses but contain a solution - for example, 5% Zn, 5% Sn, 1-5% Pb

with the balance copper • castings with lead added to improve machinability • cutting forces are low and are nearly constant over the whole cutting speed range, particularly the

feed force • some continuity on the under side but no flow-zone - tool interface temperatures are low • at high cutting speeds, where the interfacial lead is melted during cutting, the rate of wear is

increased compared with that at lower speeds where the interfacial lead is solid



Aluminum bronze • 10% Al, 5% Fe and 5% Ni • 6% Al, 2% Si and 0.5% Fe • numerous fine precipitated particles containing Fe, Si and Al dispersed in the matrix • similar machining behavior for both, very different from that of brasses • at low speeds chips are completely segmented and discontinuous • at high speeds chips are segmented, but the segments are joined together by a continuous

flow-zone at the tool/work interface • only slight plastic deformation takes place in the body of the chip before sudden fracture occurs

along the line of the shear plane - leads to large fluctuations of cutting force • synchronized with the periodic fracture, and a characteristic high noise level during cutting • 10% Al alloy generates much higher temperatures, and the 6% Al alloy can be cut at much

higher speeds for this reason • high speed steel tools are generally used, but cemented carbide (WC-Co alloys) can be used with

much longer tool life and at higher cutting speed

Copper alloys



Cupro-nickel alloys • highly ductile, single-phase alloys forming a continuous series of solid solutions • machining investigations on two alloys with 10% and 31% nickel - direct opposite to that of

aluminum bronzes • continuous chips are formed with no segmentation over the whole cutting speed range • work material is strongly bonded to high speed steel or carbide tools over the whole contact area • low cutting speed, the cutting and feed forces were very high and the chips very thick with a

very small shear plane angle • as cutting speed increased - chips became thinner and cutting and feed forces dropped rapidly • no built-up edge was formed at low speed

Copper alloys



Tool steels • More difficult to machine than the lower-carbon and the lower-alloy constructional

steels • Alloying elements readily form carbides - adverse effects in machining - markedly

influenced by the size, shape, and distribution of the carbide particles in the matrix of the steel

• Tools made from high-speed tool steel, carbide, and cast Co-Cr-W alloy are all used for turning tool steels

• Indexable coated carbide, cubic boron nitride (CBN), and ceramic tooling inserts have also been used for turning because of their higher resistance to wear and hence longer tool life



Documents

Machinability of Engineering Metals_Surreddi_02Dec2013