-液相烧结(课堂PPT)

液相烧结的早期观点：液相的作用主要是一种物理过程，液相出现时由于毛细管压液相的作用主要是种物理过程液相出现时由于毛细管压力的作用，将引起坯体中粉粒或尖凸部分在液相中溶解，通过扩散而在另一部分的淀析；由于润湿作用而使固-液界面能下降。

Ref: 网络课件液相烧结的早期观点：液相的作用主要是一种物理过程，液相出现时由于毛细管压液相的作用主要是种物理过程液相出现时由于毛细管压力的作用，将引起坯体中粉粒或尖凸部分在液相中溶解，通过扩散而在另一部分的淀析；由于润湿作用而使固-液界面能下降。

¾近期的研究发现：近期的研究发现在绝大多数情况下，液相烧结都伴随着不同形式的化学反应。

这种化学反应的作用力远比毛细管压力大，其所引起的体系自由能的下降，也比表面（包括界面）自由能的下降要大几个数量级，的降也表括自的降大个数故有人将这类有反应作用的烧结称之为活化烧结（Activated sintering）。

§3-6 液相与活化烧结一、液相烧结中的物理作用过程前提条件：¾体系中必须具有一定的液相含量；体系中必须具有一定的液相含量¾固相物质在液相中必须具有明显的溶解度；¾液相必须能较好地润湿固相物质。

产生的物理效应：1．润滑效应——液相对固粒的润滑作用，使粉粒之间的摩擦减小，便于粉粒作相对运动，可使成型时留下的内应力下降，粉粒堆集度也有所改善。

2．毛细管压力与粉粒的初次重排当液相能很好润湿固相时，粉粒间的大多数空隙都将被液相填充，形成毛细管状液膜。

直径为0.1-1微米的毛细管在可在般陶瓷粉粒间产生110Mpa左右的压强，如此大的压可在一般陶瓷粉粒间产生1-10Mpa强加上液相的润滑作用，使胚体成型后粉粒重新排布，达到更紧密的空间堆积。

这被称为烧结过程中的初次重排。

5．熟化与外形适应（Ripening with shape accommodation）——奥氏熟化（Ostwald Ripening）d ti）奥氏熟化（O t ld Ri i）在可传质媒介中的颗粒大现象在可传质媒介中的颗粒长大现象。

液相烧结的早期观点：液相的作用主要是一种物理过程，液相出现时由于毛细管压液相的作用主要是种物理过程液相出现时由于毛细管压力的作用，将引起坯体中粉粒或尖凸部分在液相中溶解，通过扩散而在另一部分的淀析；由于润湿作用而使固-液界面能下降。

Ref: 网络课件液相烧结的早期观点：液相的作用主要是一种物理过程，液相出现时由于毛细管压液相的作用主要是种物理过程液相出现时由于毛细管压力的作用，将引起坯体中粉粒或尖凸部分在液相中溶解，通过扩散而在另一部分的淀析；由于润湿作用而使固-液界面能下降。

¾近期的研究发现：近期的研究发现在绝大多数情况下，液相烧结都伴随着不同形式的化学反应。

这种化学反应的作用力远比毛细管压力大，其所引起的体系自由能的下降，也比表面（包括界面）自由能的下降要大几个数量级，的降也表括自的降大个数故有人将这类有反应作用的烧结称之为活化烧结（Activated sintering）。

§3-6 液相与活化烧结一、液相烧结中的物理作用过程前提条件：¾体系中必须具有一定的液相含量；体系中必须具有一定的液相含量¾固相物质在液相中必须具有明显的溶解度；¾液相必须能较好地润湿固相物质。

产生的物理效应：1．润滑效应——液相对固粒的润滑作用，使粉粒之间的摩擦减小，便于粉粒作相对运动，可使成型时留下的内应力下降，粉粒堆集度也有所改善。

2．毛细管压力与粉粒的初次重排当液相能很好润湿固相时，粉粒间的大多数空隙都将被液相填充，形成毛细管状液膜。

直径为0.1-1微米的毛细管在可在般陶瓷粉粒间产生110Mpa左右的压强，如此大的压可在一般陶瓷粉粒间产生1-10Mpa强加上液相的润滑作用，使胚体成型后粉粒重新排布，达到更紧密的空间堆积。

这被称为烧结过程中的初次重排。

5．熟化与外形适应（Ripening with shape accommodation）——奥氏熟化（Ostwald Ripening）d ti）奥氏熟化（O t ld Ri i）在可传质媒介中的颗粒大现象在可传质媒介中的颗粒长大现象。

1第四章烧结4.1 4.1 概述概述烧结是粉末冶金生产过程中最基本的工序之一烧结是粉末冶金生产过程中最基本的工序之一。

烧结是粉末和粉末压坯烧结是粉末和粉末压坯，，在适当温度和气氛下加热所发生的现象或过程所发生的现象或过程。

2按烧结过程有无明显的液相出现和烧结系统的组成分为和烧结系统的组成分为：：1）单元系烧结2）多元系固相烧结3) 3) 多元系液相烧结多元系液相烧结3粘结阶段颗粒的原始接触点或面转变成晶体结合颗粒的原始接触点或面转变成晶体结合，，即通过成核即通过成核、、结晶长大等原子过程形成烧结颈等原子过程形成烧结颈。

烧结体密度烧结体密度、、烧结体强度烧结体强度、、导电性等的变化烧结颈长大阶段原子向颗粒结合面迁移原子向颗粒结合面迁移，，烧结颈扩大烧结颈扩大，，颗粒间距缩小颗粒间距缩小，，晶粒长大，晶界越过孔隙移动晶界越过孔隙移动。

烧结体密度烧结体密度、、烧结体强度等的变化闭孔隙球化和缩小阶段烧结体致密度达到烧结体致密度达到90%90%90%以上以上以上，，孔隙闭合后孔隙闭合后，，孔隙形状趋于球形并缩小缩小。

4.2 4.2 烧结的基本过程烧结的基本过程41）烧结为什么会发生烧结为什么会发生？？2）烧结是怎样进行的烧结是怎样进行的？？4.34.3 烧结理论的两个最基本的问题51）烧结为什么会发生烧结为什么会发生？？烧结是系统自由能减低的过程。

•由于颗粒结合面的增大和颗粒表面的平直化，粉末体的总表面积和总表面自由能减小•粉末体内孔隙的总体积和总表面积减小•粉末内晶格畸变的消除62）烧结是怎样进行的烧结是怎样进行的？？烧结的机构和动力学问题，研究烧结过程中各种物质迁移方式以及速率。

7单元系烧结是指:纯金属或有固定成分的化合物的粉末在固态下的烧结，不会出现新组成物或者新相，也不会出现凝聚状态的改变。

4.4 4.4 单元系烧结单元系烧结8一、烧结温度和时间•单元系的烧结主要机构是扩散和流动构是扩散和流动。

Materials Chemistry and Physics67(2001)85–91Liquid phase sintering of aluminium alloysG.B.Schaffer∗,T.B.Sercombe1,R.N.Lumley2Department of Mining,Minerals and Materials Engineering,The University of Queensland,Brisbane,Qld4072,AustraliaAbstractThe principle that alloys are designed to accommodate the manufacture of goods made from them as much as the properties required of them in service has not been widely applied to pressed and sintered P/M aluminium alloys.Most commercial alloys made from mixed elemental blends are identical to standard wrought alloys.Alternatively,alloys can be designed systematically using the phase diagram characteristics of ideal liquid phase sintering systems.This requires consideration of the solubilities of the alloying elements in aluminium, the melting points of the elements,the eutectics they form with aluminium and the nature of the liquid phase.The relative diffusivities are also important.Here we show that Al–Sn,which closely follows these ideal characteristics,has a much stronger sintering response than either Al–Cu or Al–Zn,both of which have at least one non-ideal characteristic.©2001Elsevier Science B.V.All rights reserved. Keywords:A-metals;B-sintering1.IntroductionPowder metallurgy(P/M)can be used to make high strength and high stiffness aluminium alloys[1–10].Indeed, room temperature tensile strengths in excess of800MPa have been reported[11],which is approaching the theoreti-cal limit for aluminium[12].However,these alloys are not produced to near net shape and are therefore expensive to fabricate,which limits their use to niche applications in the aerospace industry.Conventional press-and-sinter P/M is an exemplary net shape process and therefore offers inexpen-sive manufacturing.The commercially available alloys are based on research that was done in the late1960s to early 1970s[13–16]and there has only been sporadic activity in thefield since then[17–23].This early work concentrated on wrought alloy compositions and the alloys were not designed to be sintered.Because sintering is the step in the P/M process that is most responsible for the development of strength and other properties,it is not surprising that current commercial alloys do not meet the requirements of many load bearing applications for which they may otherwise be suitable.Pressed-and-sintered alloys therefore require sub-stantial improvement before widespread use is likely.This ∗Corresponding author.Tel.:+61-7-3365-4500;fax:+61-7-3365-3888. E-mail address:g.schaffer@.au(G.B.Schaffer).1Present address:IRC in Materials,The University of Birmingham, Edgbaston B152TT,UK.2Present address:CSIRO Manufacturing Science and Technology, Private Bag33,Clayton,South MDC,Vic.3169,Australia.paper reviews recent work at The University of Queens-land in which the traditional compositional restraints have been relaxed.We begin,however,with a discussion of the ubiquitous oxidefilm.2.The surface oxideAluminium is always covered by an oxide.The thickness of the oxide is dependant on the temperature at which it formed and the atmosphere in which it is stored,particularly the humidity.Fresh oxide on bulk aluminium at room tem-perature is widely reported as being10–20Åthick[24–27]. The thickness on atomised powder can vary from50to150Å[28–31].The oxide on aluminium is usually amorphous [28,31,32]and hydrated[27,31,33,34]with an adsorbed wa-ter layer[35,36].The oxide crystallises to␥-Al2O3on pro-longed annealing at temperatures above350◦C[32,37,38]. Similar transformations occur in bulk alumina[39].The oxide prevents solid state sintering in low melting point metals[40],including aluminium[41],but not in all metals[42–44].This has been explained in terms of the rel-ative diffusion rates through the oxide and the metal,for metals with stable oxides[45–47].The use of liquid phases is an alternative to solid state sintering.An essential require-ment for effective liquid phase sintering is a wetting liquid [48].The wettability of a solid by a liquid is determined by the work of adhesion,W a,[49,50]:W a=γlv(1+cosθ)=γsv+γlv−γsl(1)0254-0584/01/$–see front matter©2001Elsevier Science B.V.All rights reserved. PII:S0254-0584(00)00424-786G.B.Schaffer et al./Materials Chemistry and Physics67(2001)85–91 whereγlv is the surface tension of the liquid–vapour inter-face,γsv the surface tension of the solid–vapour interface,γsl the solid–liquid interfacial tension andθthe contactangle.A liquid is said to wet a solid when cosθ>0.Highmelting point metal oxides are generally poorly wetted byliquid metals,except above the wetting threshold,a tem-perature beyond which W a increases sharply[50].Liquidaluminium is not therefore expected to wet alumina nearthe melting point of the metal.Indeed,the contact angle isvariously given as∼103◦at900◦C[51],∼160◦at800◦C[52]or∼162◦at950◦C[53],although this is dependant onthe partial pressure of oxygen and the presence of an oxidefilm on the molten metal[54].It has been suggested that theAl–CuAl2eutectic can wet Al2O3at600◦C[19].However,neither Mg,Ce nor Ca additions to molten Al reduce thecontact angle sufficiently to produce wetting[51,52].Sincethe work of adhesion of liquid metals on oxide surfacesincreases with the free energy of formation of the metaloxide,it is unlikely that Cu will be efficacious.It is there-fore apparent that the oxide on aluminium is a barrier tosintering and needs to be disrupted or otherwise removed.The oxidation of a metal,M,may be represented asM+O2↔MO2(2)The free energy of formation, G,of the oxide is given byG=−R T ln K1(3)where R is the gas constant,T the temperature in kelvin andK1the equilibrium constant given byK1=(P O2)−1(4)where P O2is the partial pressure of oxygen when reaction(1)is at equilibrium.For aluminium at600◦C,a P O2<10−50atm is required to reduce the oxide[55].Atmospheres containing hydrogen are often used in powder metallurgy. Hydrogen can reduce a metal oxide by the reaction:MO+H2↔M+H2O(5) The equilibrium constant for this reaction,K5,is given byK5=P H2OP H2(6)where P H2and P H2Oare the partial pressure of hydrogen andwater vapour,respectively.The ratio of partial pressures can be converted to the dew point,effectively the water vapour content.A dew point of≤−140◦C at600◦C is required to reduce Al2O3[56].Neither a dew point of−140◦C nora P O2of10−50atm is physically attainable and thereforealuminium cannot be sintered in conventional atmospheres. Magnesium is highly reactive and the free energy of for-mation of its oxide is more negative than that of the oxides of aluminium.Magnesium therefore has the potential to act as a solid reducing agent in this system.A possible reaction is3Mg+4Al2O3↔3MgAl2O4+2Al(7)which is a partial reduction reaction.This reaction is ob-served in studies of the oxidation behaviour of Al–Mg alloys[57,58]and at bonding interfaces in metal matrix composites[59–63].Detailed analytical transmission elec-tron microscopy(Fig.1)indicates that spinel crystallites are indeed present in a sintered Al–Mg alloy.The reac-tion may be facilitated during sintering by diffusion of the magnesium through the aluminium matrix and will be ac-companied by a change in volume,creating shear stresses in thefilm,ultimately leading to its break up.This will propitiate diffusion,wetting and therefore sintering.It has been shown that the sintering of aluminium is enhanced in the presence of magnesium[23,64,65].More recently,X-ray photoelectron spectroscopy indicated that the surface oxide can be reduced in the presence of magne-sium,which exposes fresh metal and facilitates the subse-quent formation of AlN on exposed surfaces in a nitrogen atmosphere[66].The effect that magnesium has on sinter-ing can be shown by dilatometry(Fig.2).An addition of >0.15%Mg causes shrinkage.The microstructures of the Al–Sn system show that liquid tin only wets aluminium in the presence of magnesium,when the dihedral angle is very sharp.Without magnesium,the dihedral angle is obtuse and the liquid is exuded during sintering(Fig.3).By promoting sintering,magnesium also affects the mechanical proper-ties(Fig.4).The large increase in strength and ductility at0.15%Mg is a direct consequence of improved inter-particle bonding and densification following oxide rupture. The excess magnesium at concentrations>0.15%remains in solution in the aluminium,causing expansion by the Kirkendall effect and solid solution hardening.It is apparent that the oxide is not the barrier to the sin-tering of aluminium that it is traditionally considered to be. Other factors must therefore be the cause of the poor sinter-ing response.3.Alloy designAlloys are generally designed to accommodate the manu-facture of goods made from them as much as the properties required of them in service.It is for this reason that sintered steels,for example,often contain copper or phosphorous in addition to carbon and nickel.Similarly,cast aluminium al-loys are different to forging alloys which are different again to extrusion alloys.However,with one exception[67],this principle does not appear to have been applied to pressed and sintered P/M aluminium,although Savitskii only exami-ned binary alloys,the oxide phase was not reduced and no allowance was made in the thermal cycle for the transient nature of the sintering liquids.The compositions of the cur-rent commercial alloys are compared to standard wrought material in Table1.It is noteworthy that the compositions of the P/M alloys are essentially identical to those of the wrought material.It is therefore not surprising that their sin-tering response is poor.G.B.Schaffer et al./Materials Chemistry and Physics 67(2001)85–9187Fig.1.(a)Transmission electron micrograph of a sintered Al–2.5%Mg alloy,showing a multitude of spinel crystallites.The inset shows the selected area diffraction pattern from this region;it can be indexed to spinel.(b)EDS spectra from (a)showing that the fine crystallites contain significantly more magnesium and oxygen than does the aluminium matrix (c)[80].Fig.2.Dilatometry curves for Al–x Mg alloys,where x is 0,0.15and 1.5wt.%Mg showing the effect of trace additions of magnesium to aluminium cause shrinkage during sintering [80].Fig.3.Exuded liquid on the surface of an Al–8Sn alloy after sintering at 620◦C.88G.B.Schaffer et al./Materials Chemistry and Physics 67(2001)85–91Fig.4.As sintered tensile properties for Al–Mg alloys sintered 30min at 620◦C [80].Instead of producing sintered alloys which simply mimic existing wrought alloys,it is preferable to develop alloys that are specifically designed to be sintered.Based on an under-standing of fundamental liquid phase sintering phenomena,German and co-workers [68,69]recognised that it is possi-ble to define certain ideal phase diagram characteristics.The key features of an ideal liquid phase sintering system are as follows:•The additive should have a lower melting point than the base.The alternative is a low melting point eutectic which is less advantageous because liquid formation does not occur spontaneously on heating.•The solubility of the additive in the base should be low because this ensures that the additive remains segregated to particle boundaries and maximises the liquid volume.•While the base should be soluble in the liquid,it is not necessary for the base to be soluble in the solid pletely miscible liquids ensures that mass transport is not constrained.In addition,the base should also have a high diffusivity in the liquid.This ensures high rates of mass transport and therefore rapid sintering.3.1.The Al–Cu systemCopper is one of the primary alloying elements for alu-minium,based largely on the substantial age hardening re-sponse of Al–Cu alloys.They are arguably the most widely studied P/M alloys [15,19–22,70,71];they are certainly the most widely used.The binary phase diagram is shown inTable 1The composition and properties of sintered aluminium alloys and the equivalent wrought alloys [79]Alloy TypeComposition Density (%)T6propertiesCuSi Mg UTS (MPa)εf (%)6061Wrought 0.30.6110031012601P/M 0.250.619423222014Wrought 4.40.80.510048313201P/M4.40.80.5933230.5Fig.5.It has two of the ideal features:there is a single liquid phase in which aluminium is continuously soluble and the maximum solid solubility of copper in aluminium is 5.65%at 548.2◦C.However,the melting point of copper is almost double that of aluminium.The liquid phase forms as a eutec-tic between (Al)and Al 2Cu,this is shown in Fig.6.Because there is some solid solubility of copper in aluminium,the liquid is partially transient.The sequence of events during sintering of Al–Cu is:•interdiffusion takes place on heating from room tempe-rature and a series of Al–Cu intermetallics form;•the first liquid forms at 548◦C on Al–Al 2Cu (θ)bound-aries;•Cu is drawn from the liquid into solution in the aluminium and is replaced by dissolution of the intermetallics,which are replenished in turn by solid state diffusion from adjacent Cu particles;•the intermetallics disappear when all the Cu is completely dissolved;•all the liquid is absorbed into the Al particles if the Cu content is low,although most alloys retain some liquid throughoutsintering.Fig.5.The Al–Cu phase diagram [81].G.B.Schaffer et al./Materials Chemistry and Physics67(2001)85–9189Fig.6.Optical micrograph of an Al–5.5Cu alloy quenched from575◦C showing the eutectic liquid forming between the Al matrix and the Al2Cu phase.The major problem in this system is that the diffusivity of copper in aluminium is almost5000times faster than that of aluminium in copper.The diffusivity,D,of Cu in Al at600◦C is5.01×10−9cm2s−1whereas the diffusivity of Al in Cu is1.14×10−12cm2s−1[72].While the faster diffusivity of copper in aluminium enhances the rate of homogenisation, it causes expansion via the Kirkendall effect.Sintering of the Al–Cu system is therefore dependent on the process variables,particularly the copper particle size and the heating rate[73,74].This is non-ideal.3.2.The Al–Zn systemThe Al–Zn system(Fig.7)shows some of the characte-ristics of an ideal liquid phase sintering system in that zinc has a lower melting point than aluminium,no intermediate phases form and there is complete miscibility in the liquid. However,the solid solubility ratio is non-ideal.The maxi-mum solid solubility of zinc in aluminium is83.1%,while the maximum solid solubility of aluminium in zinc is1.2%. The liquid phase during sintering of Al with Zn is therefore highly transient and Al–Zn alloys are extremely process sensitive.Fast heating rates and coarse zinc particle sizes enhance sintering[73].Wherefine zinc particles are used, the zinc dissolves in the aluminium before substantial quan-tities of liquid phase can form.Where coarse zinc particles are used,the aluminium becomes locally saturatedbeforeFig.7.The Al–Zn phase diagram[81]. homogenisation is achieved.Hence the additive forms a liquid which aids sintering.High heating rates also favour liquid formation because the opportunity for diffusion to oc-cur before melting is minimised and the reaction is delayed to higher temperatures where the equilibrium solubility is smaller and therefore local saturation can occur more easily. The7000alloys have the greatest response to age hard-ening of the conventional aluminium alloys and are there-fore used as high strength forgings in the aerospace industry. Because zinc is a poor sintering aid,however,these alloys, which contain3–8%Zn,do not have a good sintering re-sponse either.The high vapour pressure of zinc also gives rise to additional porosity in these alloys,particularly when elemental powders are used[75].It is therefore necessary to use master alloy powders[76]or microalloying additions in order to achieve acceptable sintered properties[77].3.3.The Al–Sn systemAn examination of the binary aluminium phase diagrams indicates that Al–Sn is perhaps the only one which exhibits almost all of the features of an ideal system(Fig.8).The melting point of tin(232◦C)is considerably lower than that of aluminium(660◦C)and there are no intermetallic phases. Tin is sparingly soluble in solid aluminium:the maximum solid solubility is<0.15%.Aluminium is completely soluble in liquid tin and no immiscible liquids form.In addition,the diffusivity of Al in liquid Sn is faster than the diffusivity of either Cu or Zn in liquid Sn and aboutfive times greater than the self diffusivity of liquid Sn[78].In the presence of magnesium,tin is indeed a very ef-fective sintering aid.This is illustrated in Fig.9,which is a densification contour map for the Al–Sn–Mg system.The densification is a function of the green density,sintered den-sity and theoretical density and is a measure of the sintering response:positive values indicate shrinkage,negative values indicate expansion;full density is achieved at a value of1.90G.B.Schaffer et al./Materials Chemistry and Physics 67(2001)85–91Fig.8.The Al–Sn phase diagram [81].The closely spaced,parallel contour lines at low magnesium concentrations indicate that small quantities of magnesium are required to activate the system.The widely spaced,gen-tly sloping contour lines at higher magnesium concentra-tions indicate that the system is relatively insensitive to Sn concentration and Mg levels greater than the critical con-centration.At a tin concentration of 8%,the sintered density approaches 99%of theoretical.The Al–Sn–Mg system,having been designed to be sintered,is also effective for uncompacted powder,i.e.alu-minium can be gravity sintered to near full density.This facilitates free form fabrication and rapid prototyping.By combining the flexibility of free forming and the easy sin-tering of the Al–Sn–Mg system,functionally graded metal matrix composites can also be manufactured (Fig.10).The mechanical properties of the Al–Sn system,however,are low because tin does not provide muchstrengthening.Fig.9.Map showing densification of sintered aluminium as a function of magnesium and tin concentration.Each contour represents a densification of0.2.Fig.10.The macrostructure of a section of one tooth of a functionally graded,freeform fabricated gear.The centre is the Al–8Sn–4Mg alloy;the surface contains a 10wt.%loading of alumina [82].Copper could be incorporated,but sintering of the quater-nary system becomes complicated,partly because of the formation of immiscible liquid phases.4.ConclusionsSintering of aluminium has always been considered to be problematical because of the oxide film present on the sur-face of the powder particles.However,trace additions of magnesium react with the oxide to form spinel.This breaks up the oxide,which facilitates sintering.It is therefore ap-parent that in contradiction to the standard paradigm,the properties of pressed-and-sintered aluminium alloys are not limited by the “oxide problem”.Aluminium P/M alloys can be improved without recourse to hot working or master al-loy powders if their design is based on an understanding of the underlying sintering processes and the characteris-tics of an ideal liquid phase sintering system.The Al–Sn system is 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