Effect of preheating practices for unhomogenized 6063 alloy
Billet in billet microstructure and extruded properties
Prepared.by-malcolmJ.coupermarkA.cooksey,natalia V.danilova,comalco
Research ＆ technical support (CRTS),cominium Ltd.,Ji Yong yao, CRC for Cast metais manufacturing(CAST),The University of Queensland
ABSTRACT-The effect of extended preheating on the performance of unhomogenized 6063 billet has been studied.The work also provides an improved understanding of homogenization and its benefits.preheating trials were conducted by heating unhomogenized billet slices to temperatures of 450-600℃ for hold times up to 2 hours. The hardness and microstructural changes were examined .including composition of intermetallic phases present,and the segregation and matrix solution leveis of Mg and si, The observations were compared with caiculated equilibrium phase diagrans using Thermocalc software.The alloy sklvus temperature was experimentally determined to be about 490 ℃,and was found to be a critical parameter. A billet preheating temperature of 570℃ was found to obtain the calculated equillbrium αAl8Fe2Si phase.For selected preheat conditions,billets were extruded and the mechanical peoperties were determined for T1,T4,T5＆T6 heat'treatments.The use of unhomogenized billet leads leads to a decrease in T5 hardness,tensile properties and unnotched impact energy.However international minimun prooperties can still be met.As the billet preheating conditions approach those of conventional homogenization,the difference becomes negligible.The T5 notched impact energy and thd T6 tensileproperties are mot adversely affected by using unhomogenized billet.The effect on extrudability needs to be further quantified.
BACKGROUND
Homogenization is commonly used to improve the metallurgical quality and extrusion performance of billet(1).Soak time,soak tempersture and cooldown rate are all considered important parameters of hooomogenization.Batch homogenizing furnaces are giadually being reproved by continuous processes,which offer improved control and integrated automation of inspection, sawing and packaging (2).While the cost of the process is significant,quantification of the benefits is somewhat lacking.
The importance of billet ptrheating has also been recognized,with particular practices being recommended by various authors(3-6).Potential benefits from homogenization can be megated or improved by diffrtent preheating practices.
Both homogenization and billet preheating are believed to imfluence die pick-up and tearing, which can limit the extrusion speeds achievable.The exact mechanism by which this occurs is not clear.The types of intermetallic phases present,and the amount of elements in solution have all been linked to extrudability and mechanical properties(5,6).
Use of unhomogenized billet is quite common in Southeast Asian countries. Unhomogenized billet is available frim remelt casting favilities, and there may be interest in purchasing as-cast primary billet at a lower price for less demanding applications.Long preheat times (2-6) hours)are used in an attempt to compensate for unhoomognized billet,but fhe conditions used are not ideal for full homogenization.
In the current work, the idea of using billet preheating in place of homogenization has been investigated experimentally,and interpreted using electron microscopy techniques and phase diagram calculation methods based on Thermocalc software(7).The main purpose of this study was to determine the rffect of extended preheating on the performance of unhomogenized billet, however, the work also provides an improved understanding to the homogenization process and its benefits.
INVESTOGAION
The 6063 alloy used in the inestigation was cast at Comalco Research ＆ Technical Support using wagstaff single strand dual-jet mold technology (φ127mm billet).Use of grain erfiner and control of metal temperature ensured a uniform structure throughort the billet. The alloy composition was determined to be, in wt percent, 0.41Si, 0.19Fe,0.013Cu, 0.49Mg 0.04Mn, 0.003Cr,0.016Ti,0.002Zn,(others<0.03 each,<0.15 total)
Billet Preheating Trials
Billet slices were sectioned into segments for pweheating trials.Samples were heates in an air-circulation oven at a rate of 300-400℃/hour, then held at a temperature of 450,480, 510,540, 570 or 600℃ for hold times of 0,0.5 or 2hours. The samples were water-quenched and the hardness was measured after 24 hours natural aging using the Rockwell H scale(φ2mm/60kg indentation, average of 5 readings).This range of treatments encompasses two extremes:
Zero hold times at the lower temperatures(e.g. 480℃),equivalent to heating up unhomogenized billet and extruding immediately .
Two hours hold time at a high temperature (e,g, 570℃)/similar to a normal homogenization cycle(apart fron the cooldown rwte).
The microstructural changes expected were determined from equilibrium phase diagrams calculated using Thermocalc skftware. Excluding Fecontaining intermetallics, a phase diagram diagrsm in its simplest form for 6000 series alloys is shown in Figure 1, and indicates the:
Liquidus-the temperature at which the alloy is fully molten,
Solidus-temperature at which melting begins(incipient melting),and
Solvus- temperature at which the Mg and Si are in solution in the matrix.
Heat treating a material at temperature above the solvus (but below thesolidus),followed by eapid cooling or quenching, ensures that the maximum amount of mg and Si are retained in solution, for subsequent age-hatdening. The presence of Fe introduces a number of phases that dwpend on alloy conposit on and which were calculated for thd alloy studied.

Flgure 1. Al-Si-Mg-Fe phase diagram section for 0.5wt percent Mg,0.1wt percent Fe.Homogenization cooldown or quenching form solution treament is shownfor 6063.
The miceostuctural changes that occurred during the preheating cycles were exanined on selected samples using EDS(kevex)analysis on a JEOL Jxa-840 scanning microanalyser(accelerating voltage 15keV,current 4.5×10-9 A,nagnification 30-40K). method was previously developed whereby semi-quantitative EDS data could be used to identify α-Al7Fe3Si, α-Al12Fe3Si, α-Al15Fe3Si2, β-Al5FeSi, π-Al8FeSi6Mg3，Mg2Si and Si particles in 6000-series alloys. Approximately 20 particles were analvzed on each sample. A plot was made of Fe versus Si content for the particles, The expected composltion of AlFeSi phases was also plotted , then a line was drawn from this point to a point corresponding to matrix composition. This line represents the full range of possible compositions that could be measured for the phase , depending on its size relative to the microanalysis volume
The levels and distribution of Mg, Si, Mn and Fe in the samples were determined using X-ray wavelength dispersion, speotroscopy with a JEOL 8800Lautomated electron microprobe operating at 15keV.A profile was produced by moving the probe in 2μm steps across a brain. Two profiles were obtained for each sample.Matrix levels, as well as the composition of sooome of the larger phases present, were determined.
Extrusion and Mechanical Propertise
Following these trials, five preheating treatments were selected for extrusion and mechanical property testing:2 hours/570℃, 2hours/540℃,2hours/510℃, hours/480 and 0 hours/480℃.The billets were preheated in a purpose-designed furnace used for homogenization simulation work. The heat-up rate was 300℃/h, After the hold time , billets were cooled at 400℃/hour to reach 480℃,a typical extrusion temperature. For convenience, the billets were then aircooled (1800℃/hour),billet slices were taken as a control check, and the remainder of the billet was retained for later extrusion.
The extrusion was a 6×40mm solid section , with an extrusion ratio of 56∶1, A 880t Cheng Hua direct extrusion press was used . The extrudate speed was 10m/min , die temperature 425℃ and container temperature 445℃ . The billet was induction heated to 480℃and the extrudate fan cooled at ～200℃minute from an exit temperature of 500-510℃. Breakout pressures were measured and extrusion surfaces were visually inspected.
Heat treatment conditions evaluated included:
n T1-press-quenched, stretched 0.5 percent and naturally aged (24hours for hardness measurements).
n T5-as for T1,with 24 hours pre-aging at room temperature then ageing for 10 hours/185℃.
n T4-as for T1,then solution treated 1 hourd/520℃,cold water quenched and naturally aged (24 hours for hardness measurements).
n T6-as for T4 with 24h pre-aging at room temperature, then aging as for T5.
Mechanical testing of the extrusions included:
n Rockwell H hardness, for all heat treatments.
n Tensile testing according to AS1391-1991 in a NATA accredited laboratory, for the T5 heat treatments, and for the most extreme preheating with T6 hest treatment .
n Charpy(simple-beam) impact testing on a Mohr ＆ Federhauff X1745 machine, according to ASTM E23 for 5mm subsize(Type A), unnotched and v-notched samples, for the T5 heat treatment.
RESULTS AND DISCUSSION
Hardness of Preheated Billet
Results from the billet preheating trials are plotted in Figure 2 . The typical standard deviation of results was δ=0.7. The hardness response to the preheating depends on time and temperature. A maximum hardness of about 77 was reached , As the hold tine was reduced, the temperature required to reach the maximum hardness increased.
The maximum in hardness could be interpreted as indicating that all the available Mg and Si is in solution(that is, the treatment temperature was above the alloy solvus), thereby maximizing the water-quenched and naturally aged hardness. The time and temperature dependence indicate diffusion-controlled processes taking place.

Figure2. Hardness of billets, following simulated billet preheating with various hold temperatures and times (samples water quenched and naturally aged for 24 hours prior to testing).
Microstructure of As-Cast Billet
For the sa -cast material, the EDS analysis(Figure 3a)shows there are a number of phases present. A number of Mg-containing paeticles were found (open symbols). For Mg containing particles with no Fe, the Mg content suggested the coincidence of Mg2Si and Si phases.
The solubility of Mg in α-AlFeSi and β- AlFeSi should be very low. Particles containing Fe and Mg were therefore estimated to be one form of α-phase orβ-Al5FeSi coinciding with π-Al8FeSi6Mg3 or Mg2Si. The Mg2Si can form at the interface regions of these Fe-containing. intermetallics. Particles containing only Fe and Si are αAl15Fe3Si2 and β-Al5FeSi. All of the Fe-containing particles had an elevated level of Mn.
The microprobe profile(figure 3b) shows the variation of Mg. Si, Mn and Fe resulting from solidification of the as-cast microstructure. Both Mg and Si are strongly segregated, while Mn and Fe are relatively constant. The two peaks observed are consistent with the EDS analysis, the first being a Mg-containing particle and the second an α-Al15Fe3Si2 particle with some Mn.
Microstructure of Preheated Billet
Significant changes have already occurred in the microstructure following billet preheating for 2 hours/480℃. The predominant phases identified by EDS(Figure 3c), are α-Al8Fe2Si and β- Al5FeSi. Compared to the as-cast material, the form of the α-AlFeSi phase has shifted to a higher Fe/Si ratio. The few Mg-containing particles are an indeterminate mixture of Si, Mg2Si and AlFeSi (Mg) phases .The microprobe results(Figure 3d) show that segregation of Mg and Si has been removed.
Using EDS, bothα-Al8Fe2Si andβ- Al5FeSi phases were found for the 2 hours/510℃ and 2 hours/540℃ conditions, but on Mg-containing phases were recorded.
Following the Billet preheating treatment at 2 hours/570℃,the EDS results(Figure 3e)indicate only one phase is present, α-Al8Fe2Si. Note that the particles all contain some Mn substituted for Fe(average Fe/Mn ratio by weight of 20).
The matrix levels of the elements from all the microprobe profiles (Figures 3b, 3d ＆ 3f) are summarized in Table 1. After 2 hours/480℃, the solution levels of Mg and Si are quite high, but further solutionizing of elements occurred for 2 hores/570℃ with all the Mg-containing phases and Si particles dissolved. Also, all the AlFeSi particles transformed to the equilibrium form in this alloy (α-Al8Fe2Si),thereby releasing the excess Si as the Si:Fe ratio decreases The solution levels of Mg and Si fit well with the hardness results (Figure 2), the hardness after 2 hours/480℃ being Slughtly below the maximum..
In all conditions, most of the Mn is in the matrix, whereas most of the Fe is tied up in the AlFeSi phases. For the case of 2 hours/570℃, with (α-Al8Fe2Si) present, the amount of Si tied up in this phase is 0.04wt present , which accounts for the difference between the measured Si level and that in the alloy.
The results obtained by equilibrium calculations(using Thermocalc)of the phases in alloy 6063, as a function of temperature, were as follows.
The predicted solidus temperature is 621℃,well above the range of experimental conditions used(up to 600℃). The predicted solvus temperature is 511℃, with matrix levels of 0.49wt percent Mg, 0.37wt percent Si, 0.04wt percent Mn, 0.01wt percent Fe, which is in good agreement with the experimentally determined levels(Table 1).
The dominant equilibrium Fe-containing phase predicted is one form of AlFeSi(calculated to be Al7.1Fe1.9Si1.0).This agrees with the experimental results for the 2 hours/570℃ condition(Figure 3e). However, for 2 hours/540℃,2hours/510℃ and 2 hours/480℃ conditions both α-Al8Fe2Si, and β-Al5FeSi were found experimentally (also possibly π-Al8FeSi6Mg3 at hours/480℃).Either the predictions are incorrect ,or there has been insufficient time at the lower temperatures to reach equilibrium, given the relatively low diffusion coefficient for Fe in aluminum.
The experimental results can be used to estimate the alloy solvus for comparison with the predicted value.
For the 2 hours/480℃ condition, segregation of Mg and Si was eliminated according to the microprobe results, but according to EDS some Mg-containing particles were found for the 2 hours/510℃ condition. The solvus is therefore between 480℃ and 510℃.The measured matrix Mg level for 2 hours/480℃(Table 1),is quite close to the value for 2 hours /570℃. This suggests the solvus is slightly above 480℃.
The hardmess for 2 hours/480℃ is alose to the aximum (Figure 2). Extrapolation of the hardness results, at 450℃ and 480℃ suggests that the equilibrium solvus is -490℃, and this consistent with the microprobe and EDS results.
Optical micrographs taken for the various conditions indicate that the intermetallics are not fully spheroidized (even for 2 hours.570℃). it appears spheroidization in itself is not a useful indicator of the phases present in the alloy.
Summarizing the results for the microstructure and hardness or 6063, it appears that segregation can be eliminated, and high solution levels of Mg and Si achieved, with a range of temperatures and hold times, such as 2 hours/480℃,0.5 hours/510℃,or 0 hours/540. However, to achieve the maximum solugilith of Si,and to attain complete transformation of intermetallics to the equilibrium α-Al8Fe2Si phase, higher temperatures and /or longer times are required (2hours/570℃ certainly being adequate).

(a)EDS microanalysis of as-cast billet (b)microprobe analysis of as -vasr billet

(c) EDS for billet preheated 2 hrs/480℃ (d) microprobe for billet preheated 2 hrs.480℃

(e)EDS for billet preheated 2 hrs./570℃ (f)Microprobe for billet preheated 2 hrs./570℃
Figure 3.(a,c,e)EDS microanalyses of intermetallic particles Mg-containing particles shown as open symbols), and(b,d,f)microprobe step composition profiles across a grain.
Table 1.Average matrix solute levels, form the mocroprobe profiles(two per sample) compared to the nominal alloy content.

Condition Wt%Si Wt%Mg Wt%Mn Wt%Fe
As-cast 0.06*,0.07* 0.18,0.34 0.04,0.04 0.02,0.04
2h/480℃ 0.32,0.34 0.45,0.46 0.03,0.03 0.03,0.03
2h/570℃ 0.39,0.37 0.47,0.46 0.03,0.03 0.04,0.03
Nominal Alloy content 0.41 0.49 0.04 0.19
*minimum rather than average recorded, due to segregation
Extrusion Measurements
There was no trend observed for the breakout pressure as a function of billet preheating. The breakout pressure is known to be quite sensitive to billet temprtatures, and these temperatures were controlled quite alosely.
Previous work(1) has shown a significantly higher breakout pressure for as-cast material compared to fully homogenized material. One main difference between preheating unhomogenized billet fully homogenized billet is the absence of a cooling stage, which has a significant effect on breskout pressure and extrusion(4). Note that in the current study, the normal cooling stage of homogenization was effectively omitted, by using rapid quenching and induction heating.
There were no visible differences in extruded surface finish on the extrusions for the various billet preheating cycles.
Tensile Properties
The tensile properties of the T5 heat treated extrusions are shown in Figure 4. There is a small effect of billet preheating condition. The strengths, and to a lesser extent, the elongations, decrease as the preheat temperature and/or tine decrease.
The difference between the two extremes of preheating conditions (2 hours/570℃ and 0 hours/480℃ ),is 13 percent for tensile strength and 19 percent for yield strength and 8 percent for elongation. However, the T5 properties for all the preheat conditions meet the minimum standards for 6063-T5(YS:110Mpa,UTS:150Mpa,EL:8 percent, for <12mm thichness(8). Only the 0 hours/480℃preheat fails to reach the 6063-T6 minimum standards(YS:170Mpa,UTS:205Mpa,EL:8 percent, for<25mm thickness(8)).

Figure 4. Room temperature tensile properties for T5 heat treated extrusions.
Charpy Impact Testing
Results of the impact testing of T5 heat treated extruaions are shown in Figure 5.
Unnotched impact testingis in some respects similar to measuring the area under a stress-strain curve(work done). The higher the flow stress and/or elongation, the greater the energy to fracture the sample. The results reflect this, with a 12 precent difference in average unnotched inpact energy vetween the two extremes of preheating conditions.
The average notched impact energy increases by 21 percent, with lower temperature and/or time of preheating. The reason for this is not apparent.

Figure 5.Charpy impact test results for all extrusions following T5 heat treatment.
Comparison of Heat Treatments
For the lower temperature billet preheating conditions, the material reaches maximum temperature during extrusion for only very short time. This appears to be adequate to achieve minimum tensile properties Figure 4).
A comparison of the properties of press-quenched extrusions(T1 and T5 heat treatments)with the properties for subsequently solution treated samples (T4 and T6) shows whether solution treatment can compensate for any loss in properties from using interesting for applications, such as forgings, that are often fully solution-treated and aged. Hardness results for all heat treatment conditions are shown in Figure 6.
The T1 and T4 hardness is similar for all conditions. The T6 hardness is largely independent of billet preheating condition, while the T5 hardness drops slightly below the T6 hardness for lower temperature billet preheating. Thus the properties for all billet preheating conditions are almost fully compensated for by solution treating.
According to the microstructural work, the lower billet preheating temperatures are insufficient to fully solutionize Mg and Si. The exit temperatures during extrusion(500-510℃) were close to the solvus temperature for this alloy, but the tine was apparently insufficient to completely solutionize Mg and Si for the lowest preheat conditions. A solution the alloy for all billet preheating conditions. Therefore, it is likely that, with a higher extrusion temperature, the properties for T1 hest treatments could be improved with the lower billet treatments could be improved with the lower billet preheating temperatures.
Note that the difference in quenching rate between T6 (water-quenching)and T5 (fan cooling) is not critical in this alloy. Also note that a fine recrystallized grain structure was observed in the extrusions for all billet preheating conditions and heat treatments .

Figure 6. Comparison of press-quenched T1 and T5 hardness with fully solution treated T4 and T6 hardness for various billet preheating conditions,
The T6 tensile properties for the two extremes of preheating condition are condition are conpared in Figure 7 with the T5tensile properties for these conditions (form Figure 4).The trends in the T5 and T6hardness(Figure 6)are reflected in the tensile results of Figure 7. That is, the T5 tensile and yield strength decrease significantly. But the T6 tensile and yield strength are only slightly affected (4 percent decrease)by billet preheating conditions.

Figure 7. Comparison of average T5 and T6 tensile properties for the two extremes of billet preheating conditions.
Suitability of Unhomogenized billet
The performance of unhomogenized billet is quite promising and perhaps better than expected. High solution levels of Mg and Si, that are important for alloy properties, can be achieved with moderate billet preheating temperatures. The longer the billet preheating hold time, the lower the temperature that is required. The T1and T5 hardmess and tensile properties are close to the maxmum possible(i.e.compared to T4and T6 conditions).
For unhomogenized billet heated at low temperature and extruded without hold time(0 hours/480℃ preheat condition), there is a penalty in T5 properties. However, the properties still meet minimum standards. The unnotched impact properties are also lows, but the motched properties are(surprisingly) higher than those with the maximum preheating conditions.
If required, hardness and tensile properties can be increased to match those for the maximum preheating conditions using a post extrusion, standard solution treatment and aging. It nay also be possible to increase the T1 and T5 properties for unhomogenized billet by using higher extrusion temperatures, however the higher exit temperatures would then limit the maximum extrusion speed possible.
It appears that conventional gas preheaters, with reasonably long hot zones, could be adapted to achieve the types of bollet preheating suitable for alloy 6063, If higher preheating temperatures are desired to maximize the performance, there is billet heating equipment in the market(9) that quenches the billet prior to extrusion, to achieve the desired extrusion temperature and impart a taper(temperature gradient).This approach could be adapted for processing unhomogenized billet.
The use of lower billet preheating temperatures or times would mean that the Fe-containing intermetallic phases in the material have not fully transformed to the equilibrium α-Al8fe2Si phase. While the effects of this on strength and impact resistance have been studied, there may be an effect on extrudability For example, the size, shape, and type of intermetallic phase may influence die wear and die build-up(5).If the Mg and Si are not completely solutionized, the remaining Mg2Si/Si phases could have a negative effect on the maximum extrusion speed possible due to eutectic melting and pick-up(5). No significant effect of billet preheat conditions on breakout pressure or surface finish was found in the current work..
Another factor to consider is alloy conpositionVariants of 6063 alloy without Mn might have a reduced rate of transformation to the equilibriumα-Al8fe2Si phase, so that higher billet preheating temperatures or longer times would be erquird changes to the Mg and Si content may have some effect on, properties directly, but also indirectly by changing the solvus temperature. In addition different Fe contents will tie up different amounts of Si, which may impact final properties.
CONCLUSIONS
The alloy solvus temperature is a critical parameter for homogenization. Billet preheating,extrusion(relative to the exit temperature). And solution treatment. A number of techniques have been used to evaluate billet preheating:nardness, EDS microanalysis, microprobe profiles, and Thermocalc. The experimental results indicate the solvus is about 490℃ for alloy 6063, approximately 20℃ below the calculated value.
During billet preheating, the formation of Fe-containing intermetallics is strongly temperature dependent. The calculated equilibrium α- Al8fe2Si phasw was the only phase present in 6063 after 2 hours/570℃,but more than one intermetallic phase type was found at 540℃ and lower temperatures.
The use of unhomogenized biller leads to a decrease in T5 hardmess, tensile properties, and unnotched impact energy, although International minimum mechanical properties can still be met. As the billet preheating conditions approach those of conventional homogenization, the properties are close to maximum. The T5 notched impact energy and the T6 temsile properties are not adversely affected by using unhomogenized billet. The effect on extrudability still needs to be quantified.
ACKNOWLEDGEMENTS
The following assistance us gratefully acknowledged: Owen Parker for casting and extrusion of materials and Shane Shane Charles for heat treatments and testing (Comalco Research ＆ Technical Support) and John Heathcock for technical advice(Comalco Smelting).From the University of Queensland, Ron Rasch(Centre for Microscopy ＆ Microanalysis) for performing the microprobe analysis, Daniel Graham for sample preparation and analysis, Graham Ruhle for tensile testing, and Marc de Glas for Charpy testing.
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