Chapter of Al6061 alloy in wt.% Alloy Si

Chapter
3

 

MATERIALS
AND EXPERIMENTAL METHODS

 

 

1.        

1.1.      
MATERIALS

1.1.1.          
Basealloy

 

The Al6061 alloy is chosen as the base
alloy. The chemical composition of the Al6061 is given in Table 3.1.Alloy 6061
is one of the most widely used alloys in the 6000 Series. This standard
structural alloy, one of the most versatile of the heat- treatable alloys, is
popular for medium to high strength requirements and has good toughness
characteristics. Al6061 mainly used for manufacturing of Aircraft fittings,
camera lens mounts, couplings, marine’s fittings and hardware, electrical
fittings and connectors, decorative or misc. hardware, hinge pins, magneto
parts, brake pistons, hydraulic pistons, appliance fittings, valves and valve
parts; bike frames(ASM
International, ASM International. Handbook Committee, 1990).

Table3.1 Chemical composition of Al6061 alloy in
wt.%

Alloy

Si

Cu

Mg

Cr

Fe

Mn

Al

Al-6061

0.6

0.35

0.11

0.08

0.7

0.15

Bal

 

1.1.2.          
Master Alloys

 

Al50%Cu master alloy-Aluminum Copper master alloy is the alloy of Aluminium and copper which is used
as additive for copper in aluminium alloys as hardener. Al50Cu master alloy
contains (48-52wt.%)Cu.

Al50%Si master alloy-Aluminum Siliconmaster alloy is the alloy of Aluminium and Silicon which
contains 50wt.%Si.

Al20%Ni master alloy- Aluminum Nickelmaster alloy is the alloy of Aluminium and Nickel which contains
20wt.%Ni.

Al10%Sr master alloy-Aluminum Strontiummaster alloy is the alloy of Aluminium and Strontium which
contains 10wt.%Sr used as modifier.

1.2.      
Materials preparation

1.2.1.          
Gravity Die Casting

 

Gravity die casting is a manufacturing
process for producing accurately dimensioned, sharply defined, smooth or
textured-surface metal parts. It is accomplished by gently pouring molten metal
into reusable metal dies under the force of gravity. The term “die
casting” is also used to describe the finished part(“Aluminium
Castings | Gravity Die Castings | Arrow Butler Castings,” 2018). The piston alloys were prepared by
gravity die casting technique using a diesel fired tilting furnace. The as
received materials such as commercial Al-6061 alloy, Al-50% Si, Al-50% Cu and
Al-20% Ni master alloys were used for the preparation of the alloys. Al10%Sr
master alloy was used for modification of piston alloys. The preheated ingots
were charged in to the furnace when the crucible attains a temperature of
700°c. The melting temperature was maintained at 750±5°C. The molten melt was
continuously degassed by bubbling Ar gas into the melt. The molten metal was
poured at temperature of 720-730°C in to the open preheated metal mold and
after solidification, mold allows to water quenching.

1.2.2.          
Heat Treatment Process

 

The T6 heat treatment process were
carried out by using electrical muffle furnace. Solutionizing of the specimens
were done at a temperature of 450±5°C for six hours, followed by water
quenching at a temperature of 20°C. The ageing or precipitation treatment is
carried out at a temperature of 180°C for 10hrs and the specimens were removed
from the furnace and followed by air cooling (Zeren,
2007).

1.3.      
MICROSTRUCTURAL STUDIES

 

The structural features of the alloy have
been characterized usingOptical emission spectroscopy(OES), Optical microscopy
(OM), Scanning Electron Microscopy (SEM) and EnergydispersiveX-ray spectroscopy
(EDS).

1.3.1.
Optical emission spectroscopy

 

Optical emission spectroscopy using arc
and spark excitation (Arc Spark OES) is the preferred method for trace metal
analysis to determine the chemical composition of metallic samples. This
process is widely used in the metal making industries, including primary
producers, foundries, die casters and manufacturing. Due to its rapid analysis
time and inherent accuracy, arc spark optical emission spectroscopy systems are
most effective in controlling the processing of alloy(Smelting,
1998).The chemical compositions of the piston
alloys were measured by SPECTROMAXx Optical Emission Spectrometer using arc
spark excitations.

1.3.2.
Optical Microscopy

 

The
metallographic specimens are cut from the castings using band saw with
carborandum wheels. The specimens are polished using silicon carbide emery
paper of sizes varying form 80, 200, 400, 600, 800 and 1200 grit sizes. The
specimens are washed thoroughly using liquid soap and water while going to next
paper size and the orientation of polishing surfaces was changed by 90°. After
completion of the paper polishing the specimens was polished by disc polisher
using 400 mesh Al2O3 powder with water.Then the specimens
were polished using “Silvo” and final polishing was done using 0.25?m
diamond paste in a rotating wheel (around 500 rpm) with a gentle applied
pressure. The specimens were washed very well with liquid soap solution. The
specimens have been observed and analysed in etched (using Keller’s reagent
(2.5% HNO3+ 1.5% HCl + 1%HF + 95%H2O) solution) condition. The microstructural
features of alloy specimens were observed using Olympus CX21i Microscopeat different locations with varying
magnifications.

1.3.3.
Scanning Electron Microscopy (SEM)

 

The scanning electron microscope (SEM)
uses a focused beam of high-energy electrons to generate a variety of signals
at the surface of solid specimens. The signals that derive from electron-sample
interactions reveal
information about the sample including external morphology (texture), chemical
composition, and crystalline structure and orientation of materials making up
the sample. In most applications, data are collected over a selected area of
the surface of the sample, and a 2D image is generated that displays spatial
variations in these properties(Swapp,
2006).Scanning electron microscopy was
performed in heat treatedpiston alloy specimens. The piston alloys sampleswere deep etched using hydrofluoric acid. A ‘JEOL JSM 6460’ scanning electron microscopewasused for the analysis.

1.3.4.
Energy-dispersive X-ray spectroscopy (EDS)

 

Energy Dispersive X-Ray Spectroscopy (EDS or EDX) is a
chemical microanalysis technique used in conjunction with scanning electron
microscopy (SEM). The EDS technique detects x-rays emitted from the sample
during bombardment by an electron beam to characterize the elemental
composition of the analyzed volume(Materials Evaluation and Engineering, 2014).The EDS was used for chemical analysis of different
phases and intermetallic compoundsappeared in the piston alloys. Only selectedpiston
alloysamples weresubjected to EDS. The ‘Oxford INCA Energy EDS’ attached to SEM
were used for the studies.

1.4.      
MECHANICAL CHARACTRISTICS

1.4.1.
Microhardness

 

The Vickers method is based on an
optical measurement system. The Microhardness test procedure, ASTM E-384,
specifies a range of light loads using a diamond
indenter to make an
indentation which was measured and converted to a hardness value. It is very
useful for testing on a wide type of materials, but test samples must be highly
polished to enable measuring the size of the impressions. A square base pyramid
shaped diamond is used for testing in the Vickers scale. Microhardness
measurements of piston alloys were determined by using Vickers Microhardness
testing machine.Vickers Microhardness number (VHN) measured by applying a load
of 5kg for 15 s.An average of at least six readings on the surface of the
specimen was taken to obtain a microhardness value.

1.4.2.
Brinell Hardness

 

Brinell hardness measurements of piston alloys were determined
using Brinell hardness testing machine. Brinell hardness number (BHN) was calculated
using the formula,

Where P
– applied load (kg)
D – diameter of the indenter ball (mm)

d – diameter of the impression (mm)

Brinell hardness testing was
conducted using a 10mm diameter indenterball and 500 kg load applied for 30 s
loading time. The results of the hardness were averaged from three
determinations.The surface of the piston alloy specimens on which the
impression was made have been machined and polished to 400 grit size. The
distance between the centers of indentation from the edge of specimen or edge
of another indentation is maintained at least half the diameter of the
indentation. By knowing the indentation diameter, the hardness values are
obtained from a standard table for a particular applied load and indentation
ball diameter.

1.4.3.
Ultimate Tensile Strength

 

The tensile test of heat treated piston
alloy samples were performed by using INSTRON tensile testing machine in
accordance with the ASTM E8M standard and each tensile test data was an average
of three tensile specimens of accuracy.The tensile specimens are fabricated
according to the ASTM standards B557M.The dimensions of tensile specimen’sasper
ASTM E8Mstandardare given in Fig. 3.1. The results of the hardness value were
averaged from three determinations.

Fig.3.1Standard
tensile specimen

1.5.      
Wear and friction characteristics

1.5.1.
Reciprocating pin-on-plate Tribometer

 

An indigenously developed
pin-on-reciprocating plate wear test rig conforming to modified ASTM G133-05
standard was used to investigate the dry reciprocating mode wear and friction characteristics
of piston alloys as shown in Fig. 3.2(b) (Rajeev,
Dwivedi, & Jain, 2009).The wear tests were not in full
compliance with the provisions of Test Method G133, Procedure A, because the
load in the tests was 15 – 75 N instead of 25 N, the stroke length was 100 mm
instead of 10 mm, reciprocating velocity was 0.2 to 1m/s instead of 0.1 m/s,
the sliding distance was 500 – 1500 m instead of 100m as prescribed by the
standard. In addition, the wear characteristics were reported in terms of
weight loss instead of wear volume. The modifications included the use of flat
pins instead of balls and adjusted wear test conditions. All other provisions
of Test Method G133 have been followed(ASTM
International, 2011a). In each revolution of motor crank the
slider plate moves a total wear track length of 200 mm which equals two times
of the stroke length as shown in Fig.3.2a. The upper pin specimen was mounted
on a pin holder which was attached to the lever arm. The frictional force was
measured by using a calibrated strain gauge ring attached to the pin holder and
data acquisition system (DAQ) (Agilent U2352A). The U2300A series DAQ device
can generate sampling rate up to 3MSa/s for a single channel.

 

Fig.3.2.
(a) Schematic test rig specimen configuration for reciprocating pin-on-plate
tribometer, (b) reciprocating pin-on-plate tribometer, (c) schematic test rig
specimen configuration for pin-on-disc tribometer (d) pin-on-disc tribometer.

 

1.5.2.
Pin-on-disc Tribometer

 

A
pin-on-disc wear test rig conforming to ASTM G99-05 standards was used to
investigate the dry continuous mode wear characteristics of piston alloys as
shown in Fig. 3.2d(ASTM International, 2011b). The diameter
of the wear track, d was taken as 63.5 mm as shown in Fig. 3.2c.In each
revolution the disc attains an average periphery of the wear track length equal
to ?d =? ×63.5?200mm. The piston alloy pin specimens were held against the
counter face of a rotating disc (EN31 steel disc) with wear track diameter 63.5
mm. The pin was loaded against the disc through a dead weight loading system.
The frictional force was measured by using a calibrated load cell attached to
the pin holder and DAQ system (Agilent U2352A).

A cylindrical pin of 6
mm diameter and 30mm length cut from the heat treated samples was used as the
wearing material for the tests in both modes. High wear resistant commercially
available hard EN 31 steel (65HRC) was used as the counter surface tribo
material for the reciprocating plate and rotating disc in the pin-on-plate and
pin-on-disc tribometers respectively. Both the pin and counter body wearing
surface were polished with 600 grit SiC abrasive paper to a surface roughness
of 0.35 and 0.27µm respectively. Each wear test data was take as an average of three
experimental data. The difference in the weight loss of pin specimen before and
after the test was used as the measure of sliding wear loss using ZHIMADZU
weighing balance of resolution 0.01mg. The relative humidity of the laboratory
atmosphere during the wear testing was measured to vary between 60-65%. The
coefficient of friction characteristics varies with respect to the sliding mode.
Fig.3.3 shows the coefficient of friction characteristics of Al-Si piston alloy
in reciprocating and continuous mode.Standard deviation was taken as the
measure of coefficient of friction in reciprocating mode.Average value was
taken as the measure of coefficient of friction in continuous mode.

 

Fig.3.3 Coefficient of friction
characteristics of Al-Si piston alloy a) reciprocating mode, b) continuous mode,
under a load of 75N, reciprocating velocity/sliding speed 0.6m/s slid through
1000m

 

 

 

Aluminium Castings |
Gravity Die Castings | Arrow Butler Castings. (2018). Retrieved January 13,
2018, from http://www.arrowbutlercastings.co.uk/gravitydie.html

ASM International, ASM International. Handbook Committee,
& A. I. A. P. D. C. (1990). Metals Handbook: Properties and selection
(Vol. 2). Asm International.

ASTM International. (2011a). Standard Test Method for
Linearly Reciprocating Ball-on-Flat Sliding Wear. G133 ? 05, 5(Reapproved
2010), 1–10. http://doi.org/10.1520/G0133-05R10.2

ASTM International. (2011b). Standard Test Method for Wear
Testing with a Pin-on-Disk Apparatus. G99 ? 05, 5(Reapproved
2010), 1–5. http://doi.org/10.1520/G0099-05R10.2

Materials Evaluation and Engineering. (2014). Energy
Dispersive X-Ray Spectroscopy | EDS Failure Analysis | EDS Material Analysis |
EDX Failure Analysis | EDX Material Analysis. Retrieved January 13, 2018, from
http://www.mee-inc.com/hamm/energy-dispersive-x-ray-spectroscopyeds

Rajeev, V. R., Dwivedi, D. K., & Jain, S. C. (2009).
Effect of Experimental Parameters on Reciprocating Wear Behavior of Al-Si-SiC p
Composites under Dry Condition. Japanese Society of Tribologists, 4(5),
115–126. http://doi.org/4.2474/trol.4.115

Smelting, C. (1998). Optical Emission Spectroscopy. Retrieved
January 13, 2018, from
http://www.spectro.com/products/optical-emission-spectroscopy

Swapp, S. (2006). Scanning Electron Microscopy (SEM).
Retrieved January 13, 2018, from
http://serc.carleton.edu/research_education/geochemsheets/techniques/SEM.html

Zeren, M. (2007). The effect of heat-treatment on
aluminum-based piston alloys. Materials and Design, 28(9),
2511–2517. http://doi.org/10.1016/j.matdes.2006.09.010