Studies on Influence of Process methodologies on Mechanical properties of Al-Si-Mg alloys

Studies on Influence of Process methodologies on Mechanical properties of Al-Si-Mg alloys

Name of Research Scholar : N.H. Siddalingaswamy (P 05 0008 ME)

Title : Studies on Influence of Process methodologies on Mechanical properties of Al-Si-Mg alloys

Research Supervisors : Dr. M.A. Joseph, Asst. Professor, MED Dr. N.M. Nagarajan, Professor (Rtd.), MED

Date, Day and Time : First Presentation : 30th April 2009, Thursday, 4:15 pm

Second Presentation: 11th May 2009, Monday, 4:15 pm

Venue : MED Seminar Hall
National Institute of Technology Calicut
Department of Mechanical Engineering
Ph.D. Open Seminar Announcement

ABSTRACT

Aluminium-Silicon-Magnesium cast alloys have become potential engineering materials for
automotive, aerospace, defence and other general engineering applications due to their high specific
strength and specific modulus, low coefficient of thermal expansion (CTE), good thermal
conductivity, good resistance to corrosion and oxidation. Properties of Al-Si-Mg cast alloys mainly
depend on the shape, size and size distribution of second phase particles, as well as the
morphologies and amounts of intermetallic phases present in the matrix, which in turn depend on
the composition of alloy and solidifying conditions. Many researchers and material scientists have
been endeavouring over the past four decades for effective process variations towards achieving
improved mechanical properties of this alloy system. Significant efforts have been directed during
the last decade towards determining the effect of modification and heat treatment for obtaining
improved mechanical properties of Al-Si-Mg alloys. However, the combined effect of modification,
vibration during solidification and heat treatment on the properties has not been closely examined.
The present work deals with the following:

i. Modification of the alloy,
ii. Grain refinement by vibration during solidification,
iii. Heat treatment,
iv. Evaluation of Mechanical properties,
v. Wear behaviour and
vi. Machinability of Al-Si-Mg and Al-Si-Mg-Cu cast alloys.

The specific objectives of the present work are:
· To study the mechanical properties such as Tensile strength, Hardness and
Toughness, Wear behaviour and Machinability of hypoeutectic, eutectic and
hypereutectic Al-Si-Mg cast alloys with and without copper (minor) addition.
· To study the effect of mould material (sand and graphite) on the mechanical,
wear and machinability properties of cast alloys.
· To study the individual and combined effect of modification, vibration during
solidification and heat treatment on the mechanical, wear, and machinability
properties of the cast alloys.
· To perform simulation studies (using widely used commercial code) and
validate the same with experimental cooling curve studies of the
solidification of the cast alloys.

The test castings were prepared using sand and graphite moulds. Test specimens for the
characterisation studies were prepared from the castings made in different conditions. Tests have
been carried out to find the strength, wear, and machinability properties as per ASTM standards.
The metallurgical characterization has been made using Optical and Scanning Electron Microscope
(SEM).
Test results revealed that Al-Si-Mg alloys without modification or vibration during
solidification show low tensile strength, hardness and toughness, while considerable increase in
the above properties are observed in the alloys with combined effect of modification, vibration and
heat treatment. The enhancement in the properties is more pronounced with minor addition of
copper.

The results of wear studies with different conditions exhibit the dependence of silicon
morphology (eutectic), shape and size distribution of both 􀄮-Al grains (hypoeutectic) and primary
silicon (hypereutectic), apart from inter metallic compounds of Al-Si-Mg cast alloys. The reduction
in wear rates of cast alloys can be attributed to the presence of hard silicon particles in high silicon
content alloys, which restrict the dislodging of the mating surface. Wear rate is high in un-modified
specimens and least in modified-vibrated and heat treated specimens.
The machinability characteristics of Al-Si-Mg cast alloys were evaluated using lathe under
dry environment with constant feed rate, cutting speed and depth of cut using un-coated, PVD and
CVD diamond coated turning inserts. Surface roughness of machined alloys under different
conditions was quantified using Ra 􀀋􀈝m) and Rz 􀀋􀈝m) parameters with the help of surface roughness
tester. Improved machinability and good surface finish are obtained in the case of modified-vibrated
and heat treated cast alloys. Also, results revealed that the cutting forces are high for un-coated and
PVD insert where as substantially low for CVD diamond insert. Such low cutting forces for CVD
diamond insert could be due to the poor affinity of diamond for non-ferrous materials like Al-Si-Mg
cast alloys. The improvements in properties observed in the present studies are mainly due to the
variations in microstructure brought in by the effect of modification, vibration and heat treatment
apart from alloy composition.

The cooling curve studies and simulation studies were conducted to have a better
understanding of the solidification phenomenon on the properties of the alloys under unmodified,
modified and modified-vibrated conditions.