WHY MAGNESIUM IN ALUMINUM-SILICON ALLOYS

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Why copper in aluminum-silicon alloys

Copper as an alloying element increase the strength, hardness, fatigue, creep resistance and machinability in an aluminum-silicon alloy. Strength and ductility are depending on how copper is distributed in the alloy. Copper is found dissolved in the dendrite matrix or as aluminum-copper rich phases. Alloys with dissolved copper in the matrix shows the most increase of strength and retains ductility. Continues network of copper at the grain boundaries increases the strength to appreciable levels but the ductility decreases [7]. To increase the content of copper in the alloy a higher hardness is achieved and porosity formation increases [4]. Aluminum-silicon alloys that contains 1.5 % copper has the optimal mechanical properties comparing to alloys having lower or higher content of copper [9].

Why magnesium in aluminum-silicon alloys

coefficient of thermal expansion and its electrical resistively increases a little. Aluminum-magnesium alloys have high strength, good ductility and excellent corrosion resistance. Aluminum-magnesium alloys respond well on heat treatment and a higher ultimate tensile strength and yield strength is achieved. The purpose of magnesium in aluminum-silicon alloys are to precipitate Mg2Si particles but a disadvantage is that big intermetallic compounds can appear; those phases reduce the ductility. In alloys that have an amount of magnesium between 0.05 % to 0.3 % seems to decrease the amount of porosity [8, 10].

Why Strontium in Aluminum-silicon Alloys

Strontium is added to refine the structure of the silicon eutectic and in an attempt to increase ductility it disturbs the planar growth of the silicon eutectic. The silicon eutectic becomes smaller and more compact. It is complicated to add strontium to aluminum alloys because of the powerful effects of the oxide film. It is also an expensive way to improve ductility. It is hard to add strontium without increase the porosity. One theory about strontium is that the enhanced rate of the strontium means that any moisture in the environment is fast converted to the surface oxide and hydrogen is released in the melt. If strontium is added to an open furnace an increase of hydrogen porosity is usually achieved [11]. Addition of pure strontium is recommended because it has a faster dissolution rate with the melt; it has a lower content of iron than aluminum-strontium master alloy. Mechanical properties are positively affected by this modification, the elongation is increased up to 85 % without changes in the tensile or yield strength [12].

Casting processes

Casting processes are divided into two major categories, expendable mould and permanent moulds. Expendable moulds are sand casting, last wax method, vacuum molding and shell molding. Permanent moulds are high pressure die casting, gravity die casting, centrifugal casting and squeeze casting [13].

Sand casting

A new mould has to be created by a pattern each casting but the sand in the mould could be recycled. The sand has poor heat transfer and the surfaces of the metal will solidify and shrink and a gap will be formed which decrease the heat transport. The design of the gating system is important for a good delivery of the metal into the cavity of the moulds [13].

1 Introduction 
1.1 BACKGROUND
1.2 PURPOSE AND GOALS
1.3 LIMITATIONS
1.4 DISPOSITION
2 Theoretical background 
2.1 HISTORY
2.2 ALUMINUM
2.3 ALUMINUM ALLOYS .
2.4 SECONDARY DENDRITE ARM SPACING
2.5 WHY SILICON IN ALUMINUM ALLOYS.
2.6 WHY COPPER IN ALUMINUM-SILICON ALLOYS
2.7 WHY MAGNESIUM IN ALUMINUM-SILICON ALLOYS
2.8 WHY STRONTIUM IN ALUMINUM-SILICON ALLOYS
2.9 CASTING PROCESSES
2.10 MICROSTRUCTURE .
2.11 DEFECTS IN LIGHT ALLOYS CASTING
2.12 STRENGTH HARDENING PROCESSES
3 Experimental techniques 
3.1 MELT PREPARATION
3.2 CASTING PROCEDURES .
3.3 TENSILE TESTING .
3.4 MICROSCOPIC ANALYSIS .
4 Results and discussion
4.1 AS-CAST MICROSTRUCTURE
4.2 HEAT TREATED MICROSTRUCTURE
4.3 OTHER REMARKABLE DISCOVERIES
4.4 SCANNING ELECTRON MICROSCOPY
4.5 FRACTURE SURFACE
4.6 MECHANICAL PROPERTIES
5 Conclusions
6 Future work 
7 Acknowledgments 
8 References 
9 Index

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The influence of copper on an Al-Si-Mg alloy (A356) – Microstructure and mechanical properties

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