MACHINABILITY OF ALUMINUM ALLOYS

Group 36

CONTENTS

1. Introduction

2. Comparison of aluminum and steel

3. Properties of aluminum

4. Alloying elements and its effects

5. Cutting forces

6. Power

7. Effect of Si Content on Machinability

8. Conclusions

INTRODUCTION

• Machining Aluminum and Aluminum Alloys: Traditional machining operations such as turning, milling, boring, tapping, sawing etc. are easily performed on aluminum and its alloys. The machines that are used can be the same as for use with steel, however optimum machining conditions such as rotational speeds and feed rates can only be achieved on machines designed for machining aluminum alloys.

• Among the elements added to free-cutting aluminum (Al) alloys to replace Pb, viz., Si, Ni, Fe or Mn, Si has been found to be the most effective for chip breakability. Moreover, it has been reported that the fracture of primary Si which is located near the cutting edge during machining caused chip breaking and increased the tool wear in hypereutectic cast Al-Si alloys.

• But first • machinability may be seen in terms of • 1. tool wear rate

2. total power consumption 3. work piece material 4. attainable surface finish

• Machinability also is closely linked to physical and mechanical properities of work piece.

• Hard & brittle metals are more difficult to machine than soft .

• Also dependent on the type and the geometry of tool used .

COMPARISON OF ALUMINUM AND STEEL

• Properties of aluminum

• 1. Light metal its density (2700 kg/m3) which allows high speeds of rotation.

• 2. Modulus of elasticity is one third that of steel which require appropriate chucking and clamping arrangement .

• 3. Coefficient of linear expansion (twice of steel ) makes heating undesirable ,if criteria of dimensional stability are to be satisfied unlike steel , there is no need to provide heat treatment of the “stress-free annealing” type during machining.

• 4. Due to AL IS Soft material built up edge problem is faced at low cutting speeds as a result (heat treatments are applied on aluminium to increase hardness to decrease the tendency to sticking).

ALLOYING ELEMENTS AND ITS EFFECTS

• Series          Primary Alloying Element

• 1xxx             Aluminum - 99.00% or Greater

• 2xxx             Copper           

• 3xxx             Manganese

• 4xxx              Silicon

• 5xxx              Magnesium

• 6xxx              Magnesium and Silicon

• 7xxx              Zinc

• When we add alloying element it affects machinability.

LEAD (PB) AND BISMUTH (BI)

• Lead and bismuth are added to aluminium to assist in chip formation and improve machinability. These free machining alloys are often not weldable because the lead and bismuth produce low melting constituents and can produce poor mechanical properties and/or high crack sensitivity on solidification.

• Cutting forces • The cutting force required depends not only on the dimensions of the

chips but also on the cooling lubricants used and the tool construction:

• A cooling lubricant has opposing influences: The cooling action reduces the temperature at the shear zone thus tending to increase the cutting force required. At the same time, the lubricating action eases chip motion, thereby reducing the cutting force required.

• As far as the tool geometry is concerned, the cutting force can be influenced by the rake angle, A larger rake causes less chip compression resulting in a lower cutting force.

• The wear condition of the cutting edge has a relatively large influence. As a consequence, the cutting force increases with machining time

• For the same cutting force chip removal is three time higher with aluminum. High level of Mg increase the cutting forces while low percentage of CU decrease cutting forces.

• (6061) heat treatable with aging influences the forces only at low cutting speeds at high cutting speeds the influence is negligible.

CLASSIFICATION OF ALUMINIUM ALLOYS BASED ON MACHINING CHARACTERISTICS

Group Alloy type Alloy example Characteristic machining properties

Group 1:Wrought low strengthAl alloys

• Non-heat-treatable alloys in soft orpartially hardened state• Heat-treatable alloys in unaged condition

Pure aluminium, AlMn, AlMg1, AlMgMnAlMgSi0,5,AlMgSi1

Soft, ductile, homogeneous, lowstrength, no hard components. Sticking during machining Tendency for edge build-up, no virtual Chips

Group 2.1:Wrought Alalloys ofhigherstrength

• Non-heat-treatablealloys in strained condition• Heat-treatable alloys in aged and/or strainhardened state

AlMn, AlMg1 to AlMg5, AlMgMn,AlMg4,5MnAlCuMg1, AlZnMg1,AlZnMgCu0,5, AlZnMgCu1,5

Strength between 300 and 600 N/mm²with good elongation values, no hardcomponents (low tool wear), decreasing tendency for edge build-up with increasing strength, no virtualChips

Group 2.2:Free-machining alloys

• Heat-treatable wrought alloys withchip – breaking components

AlMgSiPb, AlCuBiPb, AlCuMgPb Short chips due to presence of chipbreakers, strength 280 to 380 N/mm²,low tendency for edge build-up, novirtual chips

Group 3.1:AlSi casting alloys with up to 10 % Si

• AlSiCu alloys• AlSiMg alloys

AlSi5Cu1, AlSi6Cu4, AlSi8Cu3, AlSiCu3AlSi5, AlSi7Mg, AlSi9Mg,AlSi10Mg

Strength up to 250 N/mm² Strength up to 360 N/mm², increased tool wear due to hard alloy components and inclusions, good chip breaking properties and smooth surfaces, tendency for edge build-up for more than than approximately 5 % silicon. Increasing virtual chips

Group 3.2:AlSi casting alloys of low Hardness

AlSi alloys with about12 % silicon

AlSi12 Low hardness of matrix material. Hardmetallic alloying components andeventually inclusions, high tendencyfor edge build-up and for virtual chips

Group 3.3:AlSi-casting alloys of high hardness

AlSi alloys with over12 % silicon

AlSi18CuMgNi,AlSi21CuNiMg,AlSi25CuMgNi,AlSi17Cu4FeMg

Medium strength, high hardness, verylow ductility. High wear caused byvery hard intermetallic constituentsand primary silicon; high tendency foredge build-up and for virtual chips

POWER

• WORKPIECE ENERGY CONSUMED :

HP/in3/min

• Magnesium Alloy 0.25

• Aluminum Alloy 0.40

• Free Cutting Brass 0.50

• Leaded Steel 0.70

• Copper 0.80

• Cast Iron 1.40

EFFECT OF SI CONTENT ON MACHINABILITY

• The aluminum alloys, having a silicon content of higher than 11,8% are called hypereutectic, and almost all of them have:• Good strength characteristics• Higher fatigue limits• Excellent wear resistance.

• One of the circumstances, making the machining difficult, is that aluminum is an easy-to-machine material, soft and ductile.

• But with increasing the Si-content, the abrasive effect of the alloys increases and the difficulties, arising during the machining, are on the increase. Because of the primary silicon crystalls, embedded in the aluminum matrix, the chips become easy-to-break

• However, the presence of these hard particles leads to the quick wear of the insert, due to their strong adhesion and chemical reactions as well as low abrasive resistance with Al-Si alloys.

• In case if the primary Si-particles contact the tool edge in the cutting zone then they wear it intensive, furthermore, due to their hardness they hinder the formation of surface of good quality.

• Therefore, the precondition of the favourable surface roughness is the even dispersion, small grain size and favourable shape of the primary Si-particles, otherwise the particles, adhered to the edge on the ‘‘adhesion way”, can ‘‘plough” the surface completely, to be machined.

• Eutectic Si or primary Si was served to improve the chip breakability were investigated. • To enhance chip breakability, eutectic Si made the chips thin, and cracks that

formed in primary Si during machining acted as nuclei for chip breaking.

• Eutectic Si had a stronger effect on surface roughness than primary Si.• Eutectic Si reduced the adhesion on the cutting edge. The decrease in adhesion

on the cutting edge led to a corresponding decrease in surface roughness. • The cracking of primary Si was responsible for the increase in surface roughness

in hypereutectic alloys.

• Tool wear increased with increasing amount of eutectic Si.• In hypereutectic alloys, tool wear

was accelerated by the contact between the tool and cracked primary Si.

CONCLUSIONS

(1) With increasing Si content, chip breakability and surface finish improved, while tool life worsened for the eutectic composition. For the hypereutectic composition, chip breakability improved further with increasing Si content, while surface finish and tool life became inferior.

(2) Eutectic Si made the chips thin, and cracked primary Si acted as nuclei of chip breaking for chip breakability. Since eutectic Si reduced the adhesion on the cutting tool, the effect of eutectic Si on surface roughness was larger than that of primary Si. On the other hand, the cracking of primary Si was responsible for the increase of surface roughness in hypereutectic alloys. Tool wear increased with increasing amount of eutectic Si. In hypereutectic alloys, tool wear was accelerated by the contact between the tool and fractured primary Si.