Heattreatment analysisThis topic discuses about the variation ofmechanical properties of the alloys (Al-Si-Cu/Mg) with heat treatment. It isfound that properties such as hardness and tensile strength of the alloy isinfluenced by heat treatment and this is studied to achieve optimal mechanicalproperties of the alloy. Heat treatment improves the strength of aluminiumalloys through a process known as precipitation hardening which occurs duringthe heating and cooling of Al alloys in which precipitates are formed in thealuminium matrix. This improvement depends on the change in the solubility ofthe alloying constituents with respect to temperature. 21.1 SolidificationprocessThe solidification rate under a real-timecondition will be greatly influenced by the nucleation efficiency and the atomdiffusion between phases.
2 The solidification rate determines the coarsenessof the microstructure including the fraction, size and distribution ofintermetallic phases and the segregation profiles of solute in the ?-Al phase. Togain an optimum property of an alloy, the Dentrite arm spacing (DAS) must beminimized and distributed homogeneously. 2 The tablebelow identifies the sequence of phase precipitation in hypoeutectic Al-Sialloys. Al in the eutectic has the same crystallographic features, as theprimary ?-Al dendrites in unmodified alloys. 2 Thefigure shows different sequence of phase precipitation in Al-Si alloys. Table 1 Copperforms an intermetallic phase with Al that precipitates during solidificationeither asblockyCuAl2 or as alternating lamellae of ?-Al + CuAl2.
In the presence of nucleationsites or high cooling rates during solidification can result in fine Cu-Al2 particles.2 Thesolidification of industrial alloys generally proceeds through the formation ofsingle phase primary crystals such as dentrites or polly phased structures suchas eutectics. Because eutectics alloys have a low melting point and excellentcasting behavior they are often used for casting. Directional solidification ofeutectic alloys is a self-assembling procedure that allows fabrication frommelt of fine homogenous microstructures controlled by temperature gradient andgrowth rate. 5 The directional solidification technique by bridgeman- type equipmentis used for achieving crystal growth, which minimizes casting defects onaluminium and its alloys this is known as controlled solidification technique.
When eutectic alloys are direc’tionally solidified using this technique, two orphases that are aligned in the growth direction are formed and thismicrostructural morphology effects the mechanical and thermophysicalproperties. The value of micro hardness and tensile strength for the quaternaryAl-Cu-Si-Mg eutectic alloy increases with increase in growth rate and this isdue to the existence of inter metallic phase in these alloys which is not thesame with ternary and binary eutectic alloys. 5 Thematerial properties of cast aluminium alloys have not been adequate even whenhigh qualities Al alloys have been created, this is because the mechanicalproperties of Al alloys are still low compared with those of conventional castiron materials. Twin role continuous casting is one of the casting technologiesto make cast components with high solidification speed resulting in finemicrostructure near the surface of the cast components.
3 Temperature vs cooling time curvesofof the SGC and HMC processes 3 Figure.2Al2024alloy in Al-Cu-Mg alloy system is developed to use in aerospace applicationsbecause it has low density and good damage tolerance. The high strength of thisalloy is primarily due to the precipitation and redistribution of fine Al2CuMgparticles. In the journal of M.H.Ghoncheh, S.
G.Shabestariand M.H.Abbasi they concluded with the following observations:5 Ø Solidification characteristics are influencedby the cooling rate. The temperature of various phases reactions is shifted toshorter time intervals with an increasing the cooling rate. Ø Increasing the cooling rate decreasessignificantly both nucleation temperature and finish point of solidification.
Also, solidification time is decreased, and the range of solidificationtemperature is increased by cooling rate enhancement. Ø By increasing the cooling rate, both liquidusundercooling temperature and recalescence undercooling temperature initiallyincrease and after reaching the summit, go downward. Ø The plot of Dentrite arm spacing(DAS) as afunction of the cooling rate shows an exponential relationship. Increasing the coolingrate from 0.4 to 17.5 C s-1 decreases DAS about 89 %.
A numerical correlationbetween the cooling rate and DAS is calculated for Al2024 alloy. 2. Effect onmechanical properties with respect to temperature. With heated mold continuous casting(HMC) themicrostructure of ADC12 alloys is oriented uniformly resulting in high strengthand ductility. With heating process the ductility is further increased. Thetensile strength decreases with increase of heating temperature to 400degreeCelsius in advance and then increases when heated further to around 520degreeCelsius and when heated beyond 540degree Celsius tensile strength drops. Thehardness and fatigue strength of these alloys are similarly altered. because ofthe residual stress and precipitation characteristics of these alloys theductility increases non-linearly with increase in heating temperature.
TheHMC-520degree Celsius samples may display excellent mechanical properties suchas high strength and ductility, in the present approach. 3The aluminium alloys are heated continuouslyfor one hour immediately following heated mold continuous casting (HMC) andsand gravity crafting (SGC) and the result is graphically represented. 3 Vickers hardness for HMC and SGC ADC12 with andwithout heating process. 3 Figure.3Tensile properties of HMC and SGCADC12 with and without heating process. 3 Figure.4 Themechanical properties of as-cast AL-Cu-Si alloys like the yield strength(YS),Ultimate tensile strength and the elongation at 300degree Celsius reach 96 MPa,117Mpa and 10.3% respectively.
The alloy, with a chemical composition ofAl-27%Cu-5%Si and minor additions of Ni, was produced through a rapid solidificationcasting method. The various phases during the solidification process of thedeveloped alloys was investigated with Thermo-Calc software. The results fromthe thermodynamic calculations revealed that ?-Al2Cu intermetallic phase firstprecipitates out of the Al-27%Cu-5%Si base alloy, followed by a binary eutecticreaction and with the last melt solidifying in a ternary eutectic structure.
Itis found that the addition of Ni to the base alloy alters its solidification profile,and a Ni-rich phase (Al7Cu4Ni) is produced in the alloy which forms ahead ofthe eutectic front at a temperature greater than the eutectic temperature. SEM-EDSanalysis of the cast alloys shows a bimodal eutectic composite microstructurein the Al-27%Cu-5%Si base alloy, which consists of binary a eutectic cellsurrounded by a ternary eutectic matrix. The SEM characterization of the alloyscontaining Ni addition reveals that Al7Cu4Ni intermetallic was formed in thealloys in addition to the composite eutectic structure. Thepresence of the Al7Cu4Ni inter-metallic contributes significantly to the elevated-temperaturetensile properties of the alloy. A significant increase in high-temperaturetensile strength of 272 MPa was achieved in cast Al-Cu-Si(-1.5%Ni) eutecticalloy at 300 ?C compared to 91 MPa attained in the reference alloy A319 at thesame temperature. The yield strength and the ultimate tensile strength of thealloy containing 1.5% Ni at 400 ?C were observed to be 220% and 309% higher,respectively, than for conventional A319 reference alloy.
7 It is found that the mechanical properties ofas-cast alloys at elevated temperatures with and without Ni additions is foundto have improved. The bar graph given below summarizes the result of theobservations. Bar graph showing comparison of yield strengthof the present and the conventional A319-Al alloys at elevated temperatures.
Figure.5 7 Bar graph showing comparison of tensilestrength of the present and the conventional A319-Al alloys at elevatedtemperatures. Figure.
6 7 Bar graph showing comparison of elongation ofthe present and the conventional A319-Al alloys at elevated temperatures.Figure.7 7 3. Structuralchanges with respect to hot compression tests. Hotcompression tests of 7150 aluminum alloy were per-formed on Gleeble-1500 systemin the temperature range from 300 °C to 450 °C and at strain rate range from0.01 s?1 to 10 s?1, and the following structural changes are studied by analyzingthe observations of metallographic and TEM. To find that the true stress–truestrain curves exhibit a peak stress at a critical strain, after which the flowstresses decrease monotonically until high strains, showing a dynamic flowsoftening. It is noticed that the peak stress level decreases with an increasein the deformation temperature and decreasing strain rate, which can be representedby a Zener–Hollomon parameter in the hyperbolic sine equation with the hotdeformation activation energy of 229.
75 kJ/mol. Anotherobservation was that the deformed structures exhibit elongated grains withserrations developed in the grain boundaries. A decreasing Z value leads tomore extensive dynamic re-crystallization (DRX) and coarser recrystallizedgrains. It is found that as the Z value is increased thesubgrains is found to exhibit high-angle sub-boundaries with large number ofdynamic precipitation in subgrain interiors and a certain amount ofdislocations. The dynamic recovery and recrystallization are the main reasonsfor the flow softening at low Z value,4 but the dynamic precipitation andsuccessive dynamic particles coarsening have been assumed to be responsible forthe flow softening at high Z value. 4 In hot deformation of metallic materials, therelationship between peak stress or steady state stress, strain rate andtemperature can be expressed as.
11-13 where A1, A2, n1 and ? are constants, Z is theZener?Hollomon parameter, Q is an activation energy for hot deformation and R isthe gas constant. Automotive cast Al-A319 alloys have been increasingly used in themanufacture of engine blocks due to a combination of good fluidity propertiesand mechanical strength. Since engineblocks operate over a wide range of temperatures and stress conditions,alloying elements such as Cu and Mg are often added to improve the room andhigh temperature strength of these alloys. The tensile properties of A319-Alalloys are affected by the increasing tendency to develop porosity partly as aresult of Cu and Sr additions.The tensile properties of an as-cast Al A319 were investigated as afunction of temperature and found that alloy Al-A319 is inherently brittle asthe alloy fractured prior to reaching the maximum defined by the Considerecriterion. In particular, the ?* = n condition was not reached and alloy brittlenesswas found to be dominant in the temperature range of ?90 ?C < T < 270?C..
Microstructural observations of regions in the vicinity of the fracturesurfaces, as well as on the fracture surfaces indicated that at temperaturesbelow 270 ?C the dominant mode of failure was controlled by continuous crackingof intermetallic particles including Si and also at temperatures above 270 ?Cthe mode of failure becomes ductile and it manifests by typical dimplefracture. In this case, the Considere criterion is satisfied and the ` ?* = ncondition is met.