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QUENCHING FUNDAMENTALS QUENCHING OF ALUMINUM ALLOYS: COOLING RATE, STRENGTH, AND INTERGRANULAR CORROSION A Cooling time-temperature luminum is solution treated at temperatures generally in quench-hardenable wrought alloys
  HEAT TREATING PROGRESS ã NOVEMBER/DECEMBER 200925 Cooling time-temperature data are not routinely shown, and correlations of cooling rate data, strength,and intergranular corrosionwith either residual stress or distortion are rarely reported together. This article addresses this issue. Patricia Mariane Kavalcoand Lauralice C. F. Canale* University of São PauloSão Carlos, SP, Brazil George E. Totten, FASM** Portland State UniversityPortland, Oreg. *Member of ASM International**Member of ASM International and member, ASM Heat Treating Society luminum is solution treatedat temperatures generally inthe range of 400 to 540°C (750to 1000°F). During solutiontreatment, some alloying ele-ments are re-dissolved to produce asolute-rich solid solution. The objec-tive of this process is to maximize theconcentration of hardening elementsincluding copper, zinc, magnesium,and (or) silicon in the solid solution.The concentration and rate of dissolu-tion of these elements increases withtemperature. Therefore, solutionizingtemperatures are usually near the liq-uidus temperature of the alloy [1,2] .If an aluminum alloy is slowlycooled from an elevated temperature,alloying elements precipitate and dif-fuse from solid solution to concentrateat the grain boundaries, small voids,on undissolved particles, at disloca-tions, and other imperfections in thealuminum lattice as shown in Fig. 1 [2] .For optimal properties, it is desirableto retard this diffusion process andmaintain the alloying elements in solidsolution. This is done by quenchingfrom the solution temperature. Forquench-hardenable wrought alloys(2xxx, 6xxx, and 7xxx) and casting al-loys such as 356, this is accomplished by the quenching process. The objec-tive is to quench sufficiently fast toavoid undesirable concentration of thealloying elements in the defect andgrain boundary structure while at thesame time not quenching faster thannecessary to minimize residualstresses, which may lead to excessivedistortion or cracking. After quenching,aluminum alloys are aged, and duringthis process, a fine dispersion of ele-ments and compounds are precipi-tated that significantly increase mate-rial strength. The diffusion process andprecipitation kinetics vary with thealloy chemistry.The cooling process of age-harden-able aluminum alloys not only affectsproperties such as strength and duc-tility, but it also affects thermal stresses.Thermal stresses are typically mini-mized by reducing the cooling ratefrom the solutionizing temperature.However, if the cooling rate is too slow,undesirable grain boundary precipita-tion will result. If the cooling rate is too A QUENCHING FUNDAMENTALS QUENCHING OF ALUMINUM ALLOYS: COOLING RATE, STRENGTH, AND INTERGRANULAR CORROSION Fig. 1 — Schematic illustration of the solid diffusion processes that may occur during solution heat treatmentof aluminum. AgedAged  low   c ooled  uen hed At  s olution  h eat  t reating   temperatureAt  r oom  t emperatureAfter  a ging  fast, there is an increased propensityfor distortion. Therefore, one of the pri-mary challenges in quench-process de-sign is to select quenching conditionsthat optimize strength while mini-mizing distortion, and at the same timeensure that other undesirable proper-ties are not obtained, such as intergran-ular corrosion, which is also coolingrate dependent.Bates and Totten have addressed theselection of quenchants and quenchingconditions that will optimize materialstrength and minimize the potentialfor distortion [3] . However, although it iswell-known that properties such ascorrosion resistance are also coolingrate dependent, the problem of quenching system evaluation with re-spect to strength, distortion, and cor-rosion are rarely evaluated together.The objective of this article is to pro-vide an overview of intergranular cor-rosion (IGC) of aluminum alloys andto illustrate the effect of cooling rate on both strength and IGC. Pitting and Intergranular Corrosion Pitting is the most common corro-sion process encountered with alu-minum alloys, and is a major cause invariations in the grain structure be-tween adjacent areas on the metal sur-faces in contact with a corrosive envi-ronment. Pitting results in theformation of very small holes (pits)inthe surface, which are covered bywhiteor gray powder-like depositsappearing as blotches on the surface [4] .Intergranular (intercrystalline) cor-rosion occurs most commonly in thefollowing aluminum alloys: Al-Cu-Mg(2xxx); Al-Mg (5xxx), which is similarto the Al-Cu-Mg alloys; Al-Mg-Si(6xxx); and Al-Zn-Mg-Cu (7xxx) [5,6] .(The 2xxx, 6xxx, and 7xxx series areheat treatable). IGC refers to a selectivedissolution of the surface grain- boundary zone, and typically, thegrains below the surface zone are notattacked (Fig. 2). As discussed above,upon cooling from the solutionizingtemperature, alloying elements mayconcentrate at the grain boundaries toform intermetallic compounds thatdiffer electrochemically from the adja-cent matrix and the metal adjacenttothe grain boundaries [7,8] .The critical temperature range andtime (s) for the transition of pitting tointergranular corrosion is shown bythe so-called TTP(time-temperature-property) or C-curve illustrated in Fig.3 [9] .The C-curve for 2024-T4 shows thechange in corrosion behavior by cor-relating the critical temperature rangewhere precipitation was fastest. In-creased intergranular corrosion for2024-T4 will be favored if cooling ratesare excessively slow during quenching.Similar behavior can be shown forother aluminum alloys. Therefore, it isimportant that cooling rates duringquenching be sufficiently fast to avoidthis undesirable behavior.As the IGC process continues, exfo-liation will result, which refers to thelifting of thesurfacegrains caused byexpansion due to increasing volumeof the corrosion products accumulatinginthe subsurface grain boundaries [10] .It has been reported that exfoliation re-sults in severely reduced structuralstrength, plasticity, and fatigue. Exfo-liation in aircraft aluminum alloy struc-tural materials is most often observedwith extrusions, where the grain thick-nesses are often less than the rolledforms [4] . Electrochemical Behavior of IGC Processes IGC is caused by the formation of amicrogalvanic cell between the inter-metallic compounds formed in thegrain boundary during cooling andthe adjacent metal. These intermetalliccompounds may be either anodic orcathodic, with respect to the adjacentmetal, dependingontheir composi-tion. Figure 4 illustrates these two situ-ations [11] . In one case, noble (inactive)alloying elements may precipitate inthe grain boundaries leaving a de-pleted zone adjacent to the grain boundary, which is electrochemicallyactive (Fig. 4a). Conversely, electro-chemically active alloying elementsmay precipitate at the grain boundary,and then the metal adjacent to the 26 HEAT TREATING PROGRESS ã NOVEMBER/DECEMBER 2009 200   m200   m Fig. 2 — Sample cross section after corrosion testillustrating intermetallic deposition in the grainboundaries in the surface region (a); Microstructure of sample cross section (b). Corrosion testing according to BS ISO 11846:1995method B. Sample material is a model AlMgSi aluminium alloy (6000 series) extruded using aresearch extrusion press with a reduction ratio of34:1. Sample dimensions = 78 × 2.7 mm. Source:G. Svenningsen, Norwegian University of Scienceand Technology, Trondheim, Norway.  a b Fig. 3 — Curve indicating cooling rate dependent mechanism of corrosion attack on aluminum alloy 2024-T4sheet. K700   600   5000.1  1 1 0  1 00  1 000Critical  t ime,  s Predominantly  p ittingPredominantly  i ntergranular480   425   370   315   260   205   150 Tmaue    ° C  grain boundary will be noble (Fig. 4b).Note that corrosion behavior has also been shown to be due to microstruc-tural changes from the heat treatmentprocess, which will not be discussedhere. The reader is referred to Refer-ence 12 for more detailed discussion.To illustrate this process, considerIGC occurring in 2xxx or 7xxx alloysthat would be caused by the loss of copper or sufficient magnesium inareas near the grain boundaries tocreate an anodic electrochemical po-tential. The electrochemical potential(referred to as electromotive force, orEMF) for various aluminum alloysprovided in Table 1 shows that thepresence of copper in solid solutionwith aluminum makes it more ca-thodic [13] . The cathode is the positiveelectrode in an electrochemical circuit,and, therefore, it is the electrode thatgains electrons or electrons flow fromthe anode (which possesses the morenegative potential) to the cathode(more positive potential). An alu-minum alloy containing 4% copper insolid solution has an EMF of -0.69 V.However, copper concentrations in thegrain boundaries may reduce the EMFto -0.84 V, making it more anodic.Grain boundary corrosion may alsooccur when the grain boundary pre-cipitates are more anodic than the ad- jacent solid solution. For example, Mg 2 ,Al 3 , MgZn 2 and Al x -Zn x Mg are morecathodic than CuAl 2 and AlxCu x Mg [14] .When two dissimilar metal com-pounds with different electron affini-ties (EMF values) are connected, thereis a potential for electrons to pass fromthe material with the smaller affinityfor electrons (anode – the more nega-tive pole) to the material with thegreater affinity for electrons (cathode– the more positive pole). Apotentialdifference between the materials willincrease until equilibrium is achieved.This equilibrium potential is definedas the potential that balances the dif-ference between the propensity of thetwo metals to gain or lose electrons. IGC Control The degree of IGC may be con-trolled by the selection of the temperand maximizing the cooling rate thatwill provide minimum distortion. Forexample, the T4 and T6 temper condi-tions are typically selected when op-timum resistance to IGC is required [15] .Schuler reported that the criticalcooling rates (cooling rates between750 and 550°F) for the 7xxx series to be<400°F/s and 1000°F/s for the 2xxx se-ries to achieve optimal resistance toIGC. The data in Table 2 show the ef-fect of cooling rate on IGC for 50 mmAA7075 round bar [16] . The depth of at-tack was consistently greater towardthe center of the round bar as thecooling rate decreased.Other factors affecting IGC includetransfer rate from the furnace to thequench, air entrainment in the quen-chant, and the ratio of sectionmass/surface area. However, these fac-tors, in the final analysis, affect coolingrates, and, therefore, IGC.At this point, it is important to note HEAT TREATING PROGRESS ã NOVEMBER/DECEMBER 200927 Fig. 4 — Illustration of two potential IGC processes. Grain  A D epleted  z one  G rain  B Noble  p articles  A  ctive  p articles a D epleted  z one  d issolved  ( b  A  ctive  p articles  d issolved   preferentially  p referentially Table 1 —Electrode potentials of aluminum solid solutions and constituents Solid-solution composition Potential, V (0.1 N Calomel Scale) 1  (Ag-Mg) (Mg 5 Al 8 ) -1.24 Al + Zn+Mg (4% MgZn 2 Solid Solution) -1.07 Al+Zn (4% Zn Solid Solution)-1.05  (Zn-Mg)(MgZn 2 ) -1.05 Al+Zn (1% Zn Solid Solution) -0.96 Al+Mg (7% Mg Solid Solution) -0.89 Al+Mg (5% Mg Solid Solution)-0.88 Al+Mg (3% Mg Solid Solution) -0.87  Al-Mn (Mn-Si 6 ) -0.85 Aluminum (99.95%) -0.85 Al+Mg+Si (1% MgSi 2 ) -0.83 Al+Si (1% Si Solid Solution) -0.81 Al+Cu (2% Cu Solid Solution) -0.75 (Al+Cu) (CuA l2 )-0.73 Al+Cu (4% Cu Solid Solution) -0.69  (Al-Fe)(FeA l3 ) -0.56 NiA l3 -0.52 Silicon -0.26 1. Measured in an aqueous solution of 53 g NaCl + 3 g water per liter at 25°C. [The order of the EMF values in the table indi-cates the ability of a compound to reduce any compound metal below it. The values are reduction potentials. The  (Ag-Mg)(Mg 5  Al 8 ) compound at the top of the list has the most negative number, which indicates that it is the strongest reducing agentin the series shown. The strongest oxidizing agent is silicon with the least negative (most positive) EMF potential.] Source: Ref 1. Cao   o eems    i n   p ce  that often in the industry, cooling be-havior of various quench media is de-termined using an Inconel 600 probeaccording to ISO 9950 or ASTM D6200.However, the thermal conductivity of Inconel 600 is much less than that of aluminum as shown in Table 3 [17] .Clearly, the low thermal conductivityof Inconel 600 versus aluminum ren-ders this probe to relatively insensitiveto the cooling properties experienced by an aluminum alloy duringquenching.Silver probes are also used to eval-uate quench severity exhibited by dif-ferent quenchants. Because of the simi-larity of the thermal characteristics of silver and aluminum, and because of the significantly lower oxidation ten-dency for silver relative to aluminum,the cooling behavior of AlMgSiCu anda silver (99.5 %) probe was compared [17] .Thermal conductivity (   ) and spe-cific heat capacity of various materialsare provided in Table 3. Thermal con-ductivity is a measure of the rate of propagation of temperature change ina body and is related to the specificheat capacity by:  =   x C  p where  is thermal diffusivity, C p is spe-cific heat capacity,   is thermal conduc-tivity, and   is density of the material.Tensi, et. al., compared coolingcurves recorded during quenching of an aluminum (AlMgSiCu) and a silverspecimen (Ag 99.5) in a Type I water-soluble polymer quenchant solution.The Type I aqueous polymer quen-chant concentration was 10% byvolume and the bath temperature was25°C. The temperature of both probematerials when quenched was 520°C.Both probes were cleaned with 600 gritabrasive paper before each test. Thecooling curves obtained are shown inFig. 5. 28 HEAT TREATING PROGRESS ã NOVEMBER/DECEMBER 2009 Table 2 —Effect of cooling rate on maximum intergranular  penetration of 7075 as a function of cooling rate Cooling rate, °C/s Sample ID(50 mm diam bar)Location(a)Depth of attack, mm A53Surface0.46Center0.56B50Surface0.30Center0.86C30Surface0.46Center0.61D17Surface0.74Center1.09 (a) Surface: within 3.2 mm of cylinder surface. Center: within 3.2 mm of centerline of the cylinder. Source: Ref 16. Table 3 —Thermal conductivity and specific heat capacity  for different materials MaterialThermal conductivity, m 2 s-1Specific heat, kJ kg-1 K-1 Aluminum 99.595 × 10 -6 0.896Silver 99.5174 × 10 -6 0.235Nickel14 × 10 -6 0.448CrNi Steel(a)4 × 10 -6 0.477Inconel 600(b)4 × 10 -6 0.465( a) Austenitic stainless steel SAE 30304. (b) Nickel based alloy. Fig. 5 — Comparison of the cooling processes of a cylindrical AlMgSiCu probe (15 mm diam. × 45 mm) with those of a silver probe; cooled into a 10% solution of a water-soluble polymer at 25ºC (temperatures recorded at the geometric center of the probe); solution treating temperature is 520ºC for the AlMgSiCu probe; annealingtemperature is 800ºC for the silver probe; (a) changes in temperature and conductivity as a function of time; (b) cooling rate as a function of temperature.   0  1 0  2 0  3 0Immersion  t ime,  s 100   0 Ca    ( G    % 800   600   400   200   0 Tmaue    ( T    ° C  a T e Ag  9 9.5AlMgSiCuT e =  T  s Ag  9 9.5AlMgSiCuT s t s AlMgSiCu   t s Ag  9 9.50  2 00  4 00  6 00  8 00Temperature  ( TZ ,  ° C300   250   200   150   100   50   0 Cn   r ae    K s  b Ag  9 9.5    A  lMgSiCuA   Grain    d irectionA  D isc  s ectioned  a nd  p olished   through  d iameter  a t  p lane  “ A”  a fter  e xposure
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