an analysis of intermetallic bonding between a ring carrier and an aluminum piston alloy

An Analysis of Intermetallic Bonding between a Ring Carrierand an Aluminum Piston Alloy

SRECKO MANASIJEVIC, NATALIJA DOLIC, ZDENKA ZOVKO BRODARAC,MILE DJURDJEVIC, and RADOMIR RADISA

This paper presents the results of an analysis of the formation of an intermetallic bond betweena ring carrier and an aluminum piston alloy. The ring carrier is made of austenitic cast iron (Ni-Resist) to increase the wear resistance of the first ring groove and is applied in highly loadeddiesel engines. The most important thing is that the Ni-Resist (ferrous) must be bonded with anon-ferrous piston material during the casting of the piston. A metallographic investigationusing an optical microscope in combination with the SEM/EDS analysis of the quality of theintermetallic bonding layer was done. The test results show that if the proper conditions are met,then the preparation of the ring carrier can be made successfully, as can the formation of themetal connection between the two materials of different qualities.

DOI: 10.1007/s11661-014-2415-x� The Minerals, Metals & Materials Society and ASM International 2014

I. INTRODUCTION

PISTONS are made mostly of aluminum alloys(Al-Si-Cu-Ni-Mg), which represent a special group ofindustrial alloys that are used in the automotive industrydue to a combination of good casting and mechanicalproperties,[1–5] high strength at elevated temperatures[e.g., up to 623 K (350 �C)],[1,2] and also resistance tosudden temperature changes.[1,3–5] Different types ofpiston alloys contain various amounts of major andminor alloying elements. The usual content of alloyingelements is 11 to 23 wt pct Si, 0.5 to 3 wt pct Ni, 0.5 to5.5 wt pct Cu, 0.6 to 1.3 wt pct Mg, up to 1.0 wt pct Fe,and up to 1 wt pct Mn.[3–5] The mechanical properties ofaluminum piston alloys are defined by their micro- andmacrostructures.

Depending upon the engine type and operating condi-tions, there are different design solutions for pistons,[5]

namely mono-metallic aluminum pistons, pistons with aring carrier, pistons with a cooling channel, pistons with acooling channel and a ring carrier, pistonswith controlledexpansion, pistons with controlled spreading and a ringcarrier, pistons with an iron coating, forged pistons, splitpistons, pistons with the first groove reinforced andpistons with an undulating mantle, etc.

This paper discusses pistons for highly loaded dieselengines with a ring carrier. In order to meet therequirements for lower fuel consumption, the meaneffective pressure of diesel engines for commercialvehicles is growing. These are the engines that must be

designed to withstand high loads for long periods ofwork, especially in the critical areas of pistons that arethermally highly loaded. Today’s modern aluminumpistons with a ring carrier can meet these requirements.The ring carrier is specially designed for the formationof the first piston ring groove in highly loaded dieselengines. The ring carriers of standard features are madeof austenitic cast iron (Ni-Resist) in order to increase thewear-resistance of the first ring groove, especially inengines with high loads.[5–10] Austenitic gray ironcastings are used primarily for their resistance to heat,corrosion, and wear, as well as controlled expansion,temperature stability, castability, and machinability.The Ni-Resist ring groove inserts are manufactured intopistons to increase engine and piston life. They can alsoimprove the air tightness of the piston in the cylinder,increase the efficiency of combustion, reduce emissionsand air pollution, and contribute to the environment.These inserts are not presented to strengthen the

piston. In reality, as they represent a discontinuity in thestructure of the piston, they are a potential weakness.The ring carrier must have a very similar coefficient ofthermal expansion with the aluminum piston alloy. Ifthis were not the case, then rapid and massive temper-ature fluctuations experienced in the piston crownwould result in the piston alloy and the alfin insertexpanding and contracting at very different rates andvalues. This would cause a failure of the bond betweenthe two materials. A ring carrier with high nickelcontent can be cast in the piston using metallurgicalconnections and can reduce the temperature at thepiston head and increase the wear resistance of thepiston and, therefore, prolong its lifetime. In themeantime, it can improve the air tightness of the pistonin the cylinder and make fuel burn more complete inorder to protect the environment from exhaust emis-sions. The ring carrier is particularly resistant to weardue to friction. This becomes an advantage at highoperating temperatures and pressures.

SRECKO MANASIJEVIC, Research Associate, and RADOMIRRADISA, Program Manager, are with the LOLA Institute, Belgrade,Serbia. Contact e-mail: [email protected] NATALIJA DOLIC,Assistant Professor, and ZDENKA ZOVKO BRODARAC, AssistantProfessor, are with the Faculty of Metallurgy, University of Zagreb,Sisak, Croatia. MILE DJURDJEVIC, Program Manager-R&D, iswith the Nemak Europe, Linz, Austria.

Manuscript submitted November 21, 2013.

METALLURGICAL AND MATERIALS TRANSACTIONS A

The ring carrier is produced by centrifugal casting.Manufacturers use very different shapes for the ringcarrier to help lock it into the casting. The ring carrier isring-shaped with different cross-sections. Most often, itsoutside diameter is 70 to 200 mm, and its inner diameteris 50 to 180 mm.[5] The real strength of the insert in thepiston is the result of a molecular or atomic bondbetween the insert and the piston materials. Producersalso often use other improvements, such as a cooled ringcarrier, the insertion of pin bore bushings, etc.

When a conventional die casting procedure is used topour the liquid Al-Si alloy into the mold over the insert,the latter is simply embedded in the light alloy aftersolidification. In effect, the iron and aluminum oxidelayers prevent the establishment of a direct contactbetween the two metals and no real interfacial bondexists. Using a special casting procedure derived fromthe so-called ‘‘Al-Fin process’’ (alfin), it has beenpossible to form an intermetallic bond at the insert/alloy interface.[5–11]. This has resulted in a lightercomponent with improved functionality.

The alfin process is a method for preparing a ferroussurface for intermetallic bonding.[5–10] The alfin bondingprocess is commonly used to bond a non-ferrous Alalloy and a ferrous alloy—such as aluminum dieselengine pistons and Ni-Resist ring carriers—directlytogether. During the piston casting, the ring carrier issoaked by the alfin bond process, which results in astrong connection with the piston material. During thisprocess, an intermetallic layer composed of FexAly isformed on the border between the two different mate-rials by the diffusion of atoms.[5–10]

The previous papers focused on analyzing the growthof an intermetallic compound layer between solid ironand molten aluminum,[11] the reaction between solid Aland solid Fe as well as the influence of the Al-Feintermetallic compound layers on interfacial bonding,[12]

phase equilibria, and structural investigations in the Al-Fe-Si system,[13] and the effects of variations in the alloycontent and the machining parameters on the overallstrength.[14]

The aim of this paper was to conduct a detailedanalysis of the intermetallic bonding layer that is formedbetween the ring carrier and the piston.

II. MATERIALS AND METHODS

The tests were performed on an Ø89 mm piston for adiesel engine, type OM604, with a turbocharger. The

mass of the piston casting with a pouring system andfeeder was 1275 g, and the mass of the piston castingwas 868 g. A cross-section of the investigated pistoncasting with its macrostructure is shown in Figure 1.The piston includes a cooling channel. A salt (NaCl)core was used to form the cooling channel in the pistoncasting.Piston alloys (Table I) of an approximately eutectic

composition were used to analyze the formation of theintermetallic bond between the ring carrier and thepiston alloy. The chemical compositions of the ringcarriers used in this case, according to the well-knownmanufacturers Mahle, KS, Federal Mogul, and a localpiston manufacturer Serbian Concern Petar Drapsin-Mladenovac (PDM), Serbia,[5] are also given in Table I.The hardness of the ring carrier in this case is 140 to 150HBS (the standard is 120 to 160 HBS).[5]

The ring carrier materials were melted in inductionfurnaces at 1723 K (1450 �C). Ring carriers were thenproduced by a centrifugal casting machine. The ringcarrier castings were machined into their final shape.After their preparation (degreasing and alfining), these

Table I. Chemical Composition of the Experimental Alloy (Weight Percent)

AlloyElement/Chemical Composition

Si Cu Ni Mg Fe Mn Cr Ti Zr V Al

AlSi13Cu4Ni2Mg 13.05 3.80 2.01 0.90 0.52 0.19 0.09 0.07 �0.03 �0.01 residual

Ni Cu C Cr Si Mn Fe

Sample of the ring carrier 15.10 6.32 2.81 2.21 1.89 1.23 residual

Fig. 1—Cross-section of the investigated piston casting with its mac-rostructure and indicated sampling point.


ring carriers were placed in a piston mold, and themolten piston material was poured in (in this case, at1023 K ± 278 K (750 �C ± 5 �C)/6.50 and 13 minutes).The piston mold temperature was 518 K to 548 K(245 �C to 275 �C).

The melting of the alloy for piston casting wasperformed in a tub-like electro-resistant inclined fur-nace—type RIO 750—with a power of 80 kW and acapacity of 120 kg/h. The preparation of the Al alloy wasperformed in an electro-resistant muffler-like furnacewith a black lead muffler—type RIO 250—with a powerof 85 kW. For improving its mechanical properties,during preparation, the piston casting was exposed tomelt-treatment processes (refining, modification, anddegasification). All three operations were performed at

998 K ± 278 K (725 �C ± 5 �C). The temperature of themelt was measured using a Ni-Cr-Ni digital pyrometer.The casting of the investigated pistons was performed

on a semi-automatic machine subject to the manufactur-ing conditions in the PDM, Serbia. The ring carrier wasinserted manually. The machine has an automatic cycleoperation. Casting machines tilt by 20 to 0 deg duringcasting (in this case, they started tilting 7 seconds afterthe start of casting). After being removed from the tool,the piston castings were air hardened (the air pressure was4 bar), and the heat treatment was performed after theremoval of the pouring system and feeder and washingout of the salt cores (stabilization at 493 K (220 �C) for4 hours). A ‘‘CER Cacak EPC 200/300’’ furnace with acapacity of 3,000 kg/h and a maximum temperature of

Fig. 2—Microstructure of the ring carrier-alfin bond-piston.


623 K (350 �C) was used for the stabilization. Thetemperature in the furnace was maintained within theprescribed narrow limits ±278 K(±5 �C) with goodatmosphere control.

An optical microscope (Olympus GX51) with a mag-nification of up to 10009 was used for visualization andcollecting the data for determining the microstructure ofthe material. Samples for metallographic analysis wereacquired by an Olympus DP70 color digital camera(12.5 megapixel resolution). Metallographic samples werecut out from the piston, polished and electrolyticallyetched in a HF reagent for 15 seconds. The samples wereobserved under a scanning electron microscope (SEM)TESKAN VEGA TS5136LS equipped with energy dis-persive spectrometer (EDS, Bruker).

III. RESULTS AND DISCUSSION

A. Well-Formed Alfin Bond

After casting and machining, the ring carrier wasdegreased by ultrasound and held in the alloy cast at themelt temperature for a certain time (alfin process).Afterward, the ring carrier was inserted into the moldusing a specially designed gripper. The molten Al-Sialloy was poured into the mold over the ring carrier.This ring carrier was observed to embed with the Alalloy after solidification. Next, the piston was subjectedto machining to create the final shape. Figure 2 showsthe microstructure of the piston cross-section, where thesuccessful intermetallic bond between the ring carrierand the piston alloy is shown. Figure 2(a) shows a

Fig. 3—The intermetallic bonding layer: (a) SEM, 91000; (b) EDS mapping of all elements and EDS mapping; (c) Fe; (d) C; (e) Ni; (f) Cu; (g)Si; (h) Mg; and (i) Al.


Fig. 4—Analysis of the intermetallic bonding layer: (a) SEM; (b) EDS analysis changes in all elements; (c) EDS analysis, changes in Al and Fe;and (d) EDS identification of FeAl3 (point 2).


cross-section of the piston casting with its macrostruc-ture and the points where the samples were taken. Themicrostructure of the ring carrier is shown in thefollowing figures: Figure 2(b) with a magnification of2009; and Figure 2(e) shows a SEM analysis with amagnification of 20009. In this case, the microstructureof the ring carrier consists of lamellar graphite distribu-tion in austenite: ASTM type A or B, size 4 to 6.

The microstructure of the alfin bond is shown in thefollowing figures: Figure 2(c) with a magnification of2009, and Figure 2(f) shows a SEM analysis with amagnification of 10009. The alfin bonding processbegins with the growth of an Al alloy intermetallicsurface layer on the ring carrier by the immersion of thering carrier into a molten Al alloy bath.[6] The bathtemperature is 1033 K ± 278 K (760 �C ± 5 �C) in thiscase. The immersion time is determined according to therequired layer thickness and the manufacturer’s internalprocedure (15 minutes in this case). The thickness of theintermetallic layer increases with the alfin bath temper-ature and immersion time. Next, the ring carrier was

uniformly coated with a thin liquid piston alloy layer (70to 150 lm thick) and pulled out of the alloy bath; in asecond step, the hot aluminized ring carrier was imme-diately placed in a preheated metallic mold, and a freshliquid piston alloy was poured into that mold. To obtaina bond at the ring carrier/alloy interface, it was essentialat this stage to transfer the insert into the mold and pourthe fresh alloy over it at a sufficiently fast rate (up to10 seconds). This is the time during which the ringcarrier cools down to the temperature [973 K to 993 K(700 �C to 720 �C)] of liquid piston alloy. If the inserttransfer is carried out at a sufficiently fast rate, then thetransient interface phenomena taking place during thisshort period of time are reversible (except for the growthof a thin Al2O3 oxidation layer at the surface of the alloyfilm exposed to the air, which cannot be avoided). Whenthe fresh alloy is rapidly poured into the mold, thisAl2O3 oxide layer is, however, not strong enough towithstand the shear stresses generated by the liquidflowing onto it and eddying in the mold.[5]

The microstructure of the aluminum piston alloy isshown in Figure 2(d) with a magnification of 2009, andFigure 2(g) shows a SEM analysis with a magnificationof 30009. As in the analyses so far, it has been shownthat Al-Si piston alloys have different structures depend-ing on the content of silicon and other alloying elements.Taking into account that the Si crystals are a primaryphase in eutectic and hypereutectic piston alloys, thesequence of solidification is thus L fi (Si), L fi (Al)+(Si), L fi (Al)+ (Si)+Y, and L+Y fi (Al)+ (Si)+Y, where Y is (e-Al3Ni, h-Al2Cu, M-Mg2Si, d-Al3Cu-Ni(Al3Ni2), c-Al7Cu4Ni, T-Al9FeSi, b-Al5FeSi, p-Al8-FeMg3Si6, and Q-Al5Cu2Mg8Si6) properties, while theprimary aluminum phase also appears in the micro-structure in hypoeutectic alloys.[1–5,15–19]

The intermetallic bond between the ring carrier andpiston must be able to withstand prolonged exposure tothe high-pressure and high-temperature environment ofthe diesel engine. Weak cohesion may result in thedebonding of the ring carrier. This debonding causesdevastating damage to the piston while the engine is in

Table II. Concentration of the Elements in Measuring Point

PointElement-Series

Element(Wt Pct)

Atom(at. pct)

Errors(Pct)

1 C-K 1.89 8.15 0.5Fe-K 71.41 66.33 2.0Si-K 1.89 3.49 0.1Mg-K 0.36 0.76 0.1Ni-K 14.59 12.89 0.5Cu-K 8.12 6.64 0.3Cr-K 1.73 1.73 0.1

2 Al-K 59.61 72.58 2.6SI-K 6.10 7.14 0.3Fe-K 31.79 18.70 0.9

3 Al-K 88.38 92.97 4.1Si-K 1.62 1.64 0.1Mg-K 0.85 0.99 0.1Cu-K 3.99 1.78 0.2Fe-K 5.15 2.62 0.2

Fig. 5—Defects appearing in the ring carrier area: (a) piston; and (b) microstructure, 9100.


service. For the aforementioned reasons, these processesshould be controlled precisely. For this reason, inaddition to characterization by optical and SEM micro-scopy, a qualitative analysis of the phases in theintermetallic bonding layer was conducted by EDSscanning electron microscopy.

In the first step, an EDS mapping of the intermetallicbonding layer was made. In addition to the conventionalSEM image, the EDS mapping provides a meaningfulpicture of the element distribution of a surface. TheEDS mapping was done in order to better identify thephases. Additionally, the EDS mapping also providesuseful information to predict the possible phases wherethe key elements show higher contrast. Figures 3(a)shows a SEM analysis, while Figure 3(b) shows an EDSmapping of all the elements in the intermetallic bondinglayer. Figure 3(c) to (i) shows the EDS mapping of otherimportant elements.

In the second step, a line EDS analysis of elementdistribution in the intermetallic bonding layer was made.The results of this analysis are shown in Figure 4. Thepoint of line analysis is indicated in Figure 4(a), whilethe results of changes are shown in Figure 4(b) for allelements and in Figure 4(c) only for Al and Fe. Basedon the obtained results, it can be seen that the Feconcentration decreases and that the Al concentrationincreases uniformly along the analyzed ring carrier-alfinbond-piston line, depending on the presence of otheralloying elements (i.e., there is no discontinuity).Figure 4(d) shows the corresponding EDS spectra forthe phase identified in the intermetallic bonding layer.The EDS results were used to identify the stoichiometryfor the particular phases based on the data reported inthe literature. The alfin bond is a real bond, which has achemical composition close to FeAl3 and is formed bythe Fe and Al alloy (Figure 4(d)). The identified phasecoincides with the Al-rich phases of the Fe-Al binaryphase diagram.[20] Figure 4(c) shows that in the linearanalysis, there is a change in the concentration of Fe andAl. In accordance with the Fe-Al binary phase diagram,other phases can be expected in addition to the identifiedFeAl3 (Fe2Al5) phase. This will be part of furtherresearch by applying new techniques.

The EDS analysis of the intermetallic bonding layerwas conducted in 3 points as shown in Figure 4(b). Theresults of the EDS analysis are given in Table II.

B. Immersion Time�Intermetallic Layer Thickness

The thickness of the intermetallic bonding layer wasmeasured. The results show that the thickness of thediffusion layer formed in the piston alloy for theimmersion time of 6.5 minutes is within the range of14.60 to 45.17 lm (the average is 31.19 lm), while forthe immersion time of 13 minutes, it is within the rangeof 32.15 to 97.65 lm (the average is 76.01 lm).

C. Poorly Formed Alfin Bond

All the requirements defined in the instructions shouldbe accomplished or else different types of defects orerrors will appear. The serious problems that can be

Fig. 6—Analysis of the intermetallic bonding layer: (a) SEM; (b)EDS analysis, changes in all elements; (c) changes in Al and Fe; and(d) EDS identification of FeAl3.


caused by bond defects can be seen in the literature.[5]

Many potential problems in ring carriers occur laterduring mechanical processing (because the ring carrierand the piston casting are processed together). Thefollowing are the most common defects that occur: themovement of the ring outside the tolerance limits,porosity near the ring carrier, porosity in the ringcarrier, the absence of the alfin bond between the twomaterials of different qualities (piston alloy and Ni-Resist), and the formation of an oxide layer. Figure 5(a)shows the defects of a poorly formed alfin bond, easilyobserved after machining. Figure 5(b) shows the defectof a failure to form an alfin bond with a magnification of1009. This piston was immediately removed fromfurther stages of the manufacturing process, and thecause of the appearance of the defect was analyzed.

In addition to the visible flaws in the piston shown inFigure 5, other defects may also appear in the areaaround the ring carrier. An EDS line analysis was madeto identify such defects in this case. Using this analysis,the distribution of the elements in the alfin bond wasdetermined (Figure 6). The point of line analysis isindicated in Figure 6(a), while the results of changes areshown in Figure 6(b) for all elements, and in Figure 6(c)only for Al and Fe. Based on the obtained results, it canbe seen that the Fe and Al concentrations did notchange uniformly along the analyzed ring carrier-alfinbond-piston line, as in Figure 4, and that there is adiscontinuity. It can be seen in Figure 6(c) that theintermetallic bonding layer was destroyed in the indi-cated area, which is manifested by a decrease in theconcentrations of all the elements in Figure 6(b) as wellas in the singled-out elements, Fe and Al, in Figure 6(c).Figure 6(d) shows that the EDS analysis detected thepresence of an intermetallic bonding layer (the FeAl3intermetallic phase was formed), but that the bond nearthe ring carrier itself was destroyed.

IV. CONCLUSIONS

Based on the analysis of the experimental test resultspresented in this paper it can be concluded that

1. an intermetallic bond was formed between the ringcarrier and the aluminum piston alloy;

2. the thickness of the intermetallic layer increaseswith the immersion time for the investigated tem-perature of 1023 K (750 �C);

3. the EDS analysis detected that a FeAl3 intermetallicphase was formed. Moreover, the analysis showsthat some other FexAly phases (which will be the

subject of further research) can appear in the inter-metallic bonding layer, which corresponds to theequilibrium compositions dictated by the Fe-Al bin-ary phase diagram;

The results presented in this paper are only anintroduction to further ongoing research related to thephase composition of the intermetallic bonding layerstudied by the XRD analyses and the testing of themechanical strength of the bond.

ACKNOWLEDGMENTS

This work is financially supported by the Ministryof Science and Technical Development, Serbia.

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an analysis of intermetallic bonding between a ring carrier and an aluminum piston alloy

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