ASSESSMENT OF THE STATE OF PRECIPITATION IN ALUMINUM CASTING A356.2 ALLOY USING NONDESTRUCTIVE MICROSTRUCTURE ELECTRONIC PROPERTY MEASUREMENTS

P. Kiattisaksri1, P.J. Gibbs1, K. Koenig1, E.A. Pfeif1, A.N. Lasseigne2, P.F. Mendez3, B. Mishra1 and D.L. Olson1

1Colorado School of Mines, Golden, Colorado2Generation 2 Materials Technology LLC, Firestone, Colorado3University of Alberta, Edmonton, Alberta, Canada

ABSTRACT. Application of nondestructive electronic property measurements to assess the strengthening of Mg2Si precipitates in aluminum A356.2 casting has been demonstrated. The results indicated the evolution of precipitation phase through over aging with increased cooling time or thickness of the cast part. The classical optimum precipitation behavior has been characterized and verified with thermal analysis, thermoelectric power, low frequency impedance, and micro hardness measurements. A proposed practice to use nondestructive electronic property measurement tools to assess aluminum casting for acceptable mechanical properties after mold shake out is presented.

Keywords: Nondestructive evaluation, Aluminum alloys, Precipitation hardening, TEP, Low frequency eddy current PACS: (81.70.-q, 81.30.-t, 72.15.Jf)

INTRODUCTION

Bulk electronic, magnetic and elastic properties have been used to evaluate the microstructure and phase stability of alloys [1]. Magnetic susceptibility was able to follow the GP zone formation and precipitation in Al-Cu alloys [2]. These magnetic measurements require destructive extraction of alloy specimens. Nondestructive thermoelectric power coefficient, TEP, measurements have been used to analyze microstructural evolution events [3]. These techniques offer industry an easy and rapid method to inspect manufactured materials to guarantee that they have met their specified criteria. Here correlations of nondestructive TEP and impedance measurements to the nucleation and growth of precipitates in a Al-Si-Mg alloy is presented.

Experimental Materials and Experimental Procedures

The alloy selected for this assessment is aluminum alloy A356.2 offering distinct microstructural features that are influenced by the casting thermal cycle. Table 1 gives a typical composition for this alloy. Alloy A356.2 is a hypoeutectic alloy and exhibits a mixture of pro-eutectic and eutectic constituents. This alloy has a modified eutectic solidification experience.

TABLE 1. Chemical compositions of aluminum cast alloy A356.2 in wt. pct.

Al Si Fe Cu Mn Mg Zn Ti V Na Ga Sr92.27 7.1 0.1 0 0 0.4 0 0.13 0.02 7E-04 0.01 0.001

Aluminum alloy A356.2 has the α-aluminum phase which is hardened by Mg2Si

precipitates. The addition of a small magnesium component activates age-hardening through precipitation of Mg2Si in the aluminum matrix. Alloy A356.2 is modified with additions of sodium and strontium resulting in a change in the eutectic structure from lamellar eutectic structure to fibrous eutectic structure [4]. Precipitation hardening of this alloy required specific thermal processing to precipitate finely dispersed precipitates throughout the material, which are on the order of 10 nm in size. The precipitation reactions usually take place by a series of transformations. For the A356.2 alloy the first phase is a coherent β″ monoclinic needle phase, then semicoherent hexagonal Mg2Si β′ rods, and finally β face-centered cubic Mg2Si platelet precipitates. These first two phases are Guinier-Preston (GP) zones of aluminum-magnesium-silicon alloys [5-7].

This aluminum alloy is produced by sand and permanent mold castings. The role of the casting process on precipitation in a casting on cooling is dependent on the shape and size of the casting because these features affect the rate and time for heat extraction from the mold cavity and thus the freezing times [8].

The experimental procedures are described for the making of the cast specimens. The first step is to develop a practice to sand cast aluminum alloy A356.2 in a manner to achieve a known thermal experience to be correlated to microstructure and electronic property measurements. This alloy is melted using a gas furnace at 300°C above its melting temperature. The crucible is removed from the furnace and the liquid metal is sparged at a temperature around 750°C with pure argon gas to reduce the hydrogen content in the melt and therefore reduce porosity [9]. The temperature during melting and crucible refining is continuously measured and recorded. When the crucible temperature is 100°C above the melting temperature the liquid alloy is poured into the mold.

The mold consists of a pouring basin, a down sprue and a right triangle-shaped pouring cavity. This shape will offer an increase in cooling time from the tip of the right triangle to thick end of the cavity. Eight thermocouples were placed in four sections of the mold as shown in Fig. 1. After cooling the casting was then sectioned into five 5 cm wide wedges each with distinct thermal experiences. The thermal experience of the specific located thermocouples was attained with a high speed digital data acquisition system (DAQ 5300) which simultaneously collected data with a maximum eight inputs. There are thermocouples placed in the center of the aluminum casting as well as near the edge. For characterization of the aluminum alloy A356.2 casting, specimens were extracted from the triangular sections and the thermal history was recorded.

Thermoelectric Power Coefficient Measurements

Thermoelectric power coefficient (TEP) measurements are performed by placing two copper probes held at a constant temperature difference( ΔT ) on the surface of the specimen. The constant temperature difference is achieved and maintained through heating cartridges in each side of the copper probes and with use of temperature controllers maintaining the constant temperature difference. The potential difference( ΔV )between the probes is measured with a high impedance nanovoltmeter. The thermoelectric power coefficient(S ) is calculated by

measuring the difference in potential between the copper probes at a constant temperature difference (dV/dT). During TEP measurements, the specimen material beneath the hot probe is being evaluated. TEP coefficient measurements require precise temperature controllers and accurate nanovoltmeters.

For thermoelectricity to occur, it is necessary to have a circuit composed of two different materials. The net difference between their thermoelectric properties can be measured. By comparing the relative Seebeck coefficient to standard electrode probe materials, the absolute Seebeck coefficient (ASC) of the alloy material can be written as:

Sa=ΔVΔT

−Scopper (1)

where Scopper is the reference Seebeck coefficient for copper. Thermoelectric power coefficient, Sa , with units represented as energy per temperature, carries information about the entropy of conductive electrons which suggests that it contains information of the materials state.

Low Frequency Impedance Measurements

An alternating magnetic field in a conducting material creates a circulating current or flow of electrons within the conducting material. These circulating currents, known as eddy currents, create a secondary magnetic a field in opposition to the initial alternating magnetic field. The response of the conductor to the initial alternating magnetic field is dependent on the frequency of the initial alternating magnetic field and the electrical conductivity (σ) and the magnetic permeability (μ) of the conducting material. The induced current density will be a maximum strength at the surface of metals and exponentially decrease with depth. The "standard depth of penetration" where the eddy current is 1/e (37 percent) of its surface value is inversely proportional to the square root of the electrical conductivity, the magnetic permeability and the frequency. The microstructure analysis of aluminum alloy specimens can be assessed by low frequency impedance measurements as a function of depth.

The low frequency impedance measurement is made with an Agilent E4980A Multi-Frequency LCR Meter. Low frequency impedance measurements are an induced current technique requiring an electronic oscillator that generates an AC waveform. The applications of this measurement depend on using various frequencies resulting in the variation of depth measurements. Voltage measurements consist of amplitude and phase difference measurements from the exciter coil current. The coil sensors must be configured for the particular application. These measurements require specific locations to obtain an accurate data to correlate with other experimental results such as microstructure and thermoelectric power results. The range of impedance frequencies were selected based on depth of penetration calculations.

RESULTS AND DISCUSSION An aluminum alloy A356.2 was cast in a dimensionally accurate, molded right triangle

sand cavity shape with thermocouples set up to measure specific locations. The similarity of cooling curves is the result of the high thermal conductivity of the aluminum casting even with the large variation in section size for the casting. Cooling curves relates the removal of heat from the part into the sand cast mold. The cooling rates of the eight cooling curves were calculated for temperatures between 600 and 400°C and are given in Table 2.

TABLE 2. Cooling rates (ΔT/Δt) of different positions in sand cast aluminum part.

Sample No.1 No.2 No.3 No.4 No.5 No.6 No.7 No.8Cooling rate (ΔT600-400) (°C/sec) 0.1433 0.0709 0.0434 0.0322 0.1572 0.0856 0.0587 0.0388

The cast product experiences the solidification time and then the time to cool to room temperature. The important considerations for the experimental design are that the cooling rates were slow enough to nucleate second particle (Mg2Si) in precipitate temperature. The experiment had a sufficiently large test matrix to achieve the variety of different cooling rates and a sufficient solute content to achieve measurable amount of Mg2Si precipitates.

The hardness test results for along center line and edge on the triangle casting which are the locations that thermoelectric power and low frequency impedance measurements were taken are seen in Fig. 2. This figure suggests that in cooling alloy A356.2 that precipitation of Mg2Si stable phase from supersaturated solid solution happens by a nucleation and growth process with significant optimum event and followed the classical overaging curves.

FIGURE 2. Typical hardness curves for aluminum alloy A356.2. for both across the center and along the side of the casting.

Electronic Property Measurements

Thermoelectric power coefficient and low frequency induced impedance measurements were made at the positions (Fig. 1 and Table 1) with known cooling rates and microstructure characterization. These two measurements are a function the same independent variables of electron concentration, electron scattering factors, and the effective electron mass, but have different mathematical functional forms. The thermoelectric power coefficients being measured with a nanovoltmeter has high internal impedance, resulting in the TEP measurements having effectively no current thus less sensitivity to scattering. The low frequency impedance is a current induced measurement and will offer the additional information on electron scattering.

Before the thermoelectric power measurements were performed the optimum temperature difference between the copper probes was determined to be 35°C. The thermoelectric power coefficient results as functions of positions across the casting are showed in Fig. 3. The relationship between the results for the first (No.1) and last (No.4) positions represent the smallest and largest cooling times respectively. There is a difference between thermoelectric power coefficients measured along center-line compared to the edge position. The results along the edge indicate that the TEP results correlate with hardness data which is consistent to precipitation strengthening and an overaging phenomenon in aluminum alloys.

No.7

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FIGURE 1. Schematic of the right triangle shaped pouring cavity indicating specific thermocouple positions

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FIGURE 3. Comparison of TEP coefficients at a 35°C temperature difference and impedance values to the Vickers hardness at center and edge measurements on aluminum alloy A356.2 casting.

Both hardness curves show the maximum values while TEP result shows the minimum value at center measurement. The impedance measurement also applied at positions along the edge of each sample. It is important to sense at the edge of the casting because it allows for non-contact non-destructive low frequency impedance technique on industrial castings. The surface measurement means that it is not necessary to cut out specimen and damage the industrial products. The frequency that best correlated to variations in a cast-microstructure was 80Hz.

However, if the impedance by the center measurements are compared with the impedance edge measurements, there is a different impedance result on the same position as shown in Fig. 3. This difference suggests that with two electronic property measurements that low frequency impedance pick up a potentially additional microstructural variation other than the just the precipitation of Mg2Si. The microstructure also contains the aluminum-silicon eutectic lamellar structure which may be experiencing partial modification of the silicon phase. The impedance measurements along the casting edge gave the similar information of the precipitation strengthening and overaging as did the TEP and hardness measurements (Fig. 3).

It is valuable to compare thermoelectric power coefficient measurements to the low frequency impedance measurements, as seen in Fig 4. Based on the free electron model the two primary electronic measurements, the mathematical expressions for TEP coefficient,(S ) , and electrical resistivity, , are different but related with same independent variables: The relationship of S/ρ, approximate by S/Z, is plotted to emphasize the scattering factor [10] as seen in Fig 5. The center readings are from positions in the casting that experiences the full spectrum of precipitation strengthening and overaging phenomenon. The readings from the edge of the casting, where the heat is extracted into the sand mold, have results representing one full aging cycle and the start of another cycle. The Nordheim-Gorter plot shows the expected precipitation and aging behavior for the edge probing. The S/Z values would be expected to decrease with the production of more precipitates and then increase with their reduction in number of precipitates with Ostwald ripening (overaging).

Note that the center probing has an opposite effect with a maximum rather than a minimum in the Nordheim-Gorter plot. The thermoelectric power coefficient and low frequency impedance data from center measurements of the thick-end sliced specimen of the casting are 180° out of phase with the low frequency impedance data and thermoelectric power coefficient data for the edge measurement.

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FIGURE 4. (left) Multiple results of thermoelectric power coefficient and low frequency Impedance measurements on aluminum alloy casting.

E volution of Precipitates

FIGURE 5. (right) the ratio of thermoelectric power coefficients and impedance measurements (S/Z) for aluminum alloy A356 casting as a function of a specific position.

The major difference in these two curves is that the edge probing experienced fairly similar time for the initiation of solidification at the mold wall, where the center line probe has solidification initiating along the line from triangle acute angle into the center of mold cavity at different times with each point has its separate precipitation event and time-reference. There are a number of microstructural evolution events that can cause the second aging peak behavior. One of the most obvious is the sequence of aging steps with specific phases that occurs in the early precipitation process, such as β″, β′, and β phase as seen in Table 3. This measured second overaging event in the thicker part of the casting could be the transition between β″ to β′ or β′ to β phase.

Another suggested explanation would be two different Mg2Si precipitation events. This second explanation suggests the thick-end sliced specimen is already overaged for Mg2Si precipitation in one of the two alpha constituents, proeutectic alpha and inter-dendritic alpha phases which have different silicon contents, thus making difference precipitate time dependency, One precipitation is from the (high silicon content) alpha phase in the eutectic constituents while the other precipitation is on the (lower silicon content) proeutectic alpha phase. At higher silicon content in alpha phase of the eutectic constituent will precipitate Mg2Si first and overaging behavior is followed by the (lower silicon content) proeutectic alpha precipitation of Mg2Si as seen in Fig. 6(A) and 6(B). This proposed second indication found for the center of thickest sliced specimen of cast aluminum. This second microstructural evolutionary indication should be verified.

TABLE 3. Selected precipitation methods in Al-Mg-Si alloys of commercial interested [11].

Al-Mg-Si Clusters of Mg and Si atoms. GP zones

GP zones solvus occurs at temperature that are normally higher than the ageing temperatures

β΄΄ monoclinica = 1.534 nm.b = 0.405 nm.c = 0.683 nm.

β = 106˚

Coherent needles, lie along <100>α

(010)β΄΄// (001)α ; [001] β΄΄// [310] α

β΄ hexagonal Mg2Si a = 0.705 nm.c = 0.405 nm.

Semi-coherent rods, lie along <100>α

(001)β΄ // (100)α ; [100] β΄ // [011] α

β face-centered cubic Mg2Si a = 0.639 nm. Platelets on {100}α . May transform directly from β

(001)β // (100)α ; [110] β // [100] α

Pro-eutectic alpha-constituents

Eutectic constituents

FIGURE 6(A). Aluminum-silicon phase diagram [www.globalsino.com]. FIGURE 6(B). Microstructures of A356.2 alloy

solidified at low cooling rate by Keller’s reagent [ASM Handbook Vol.9 2004].

The most meaningful data to be used for assessing the casting microstructural evolution is from the edge measurements with all the solidification event starts at once at the edge and the time to completing cool down controlled by the amount of hot metal adjacent to the edge probing positions. This observation described the situation that is very favorable for nondestructive low frequency impedance measurements to be a tool to assess the integrity of the aluminum castings.

The use of the low frequency impedance technique to assess the final microstructure to solidify in a casting, at a position of the center of the thickest section of the casting needs to be developed. A correlation between this internal microstructure and the edge probed low frequency impedance measurement needs to be determined to achieve an electronic property criterion for aluminum casting integrity.

RECOMMENDATIONS

1. Electronic property assessment of an aluminum casting using nondestructive thermoelectric power coefficients and low frequency impedance measurements can potentially be used for quality assurance for casted aluminum products.

2. These nondestructive electronic property measurements techniques achieve similar results as the classical hardness technique to assess the state of precipitation.

3. A microstructural correlation relative to the impedance signals on the edge of the casting to characterized microstructure at the center of the casting at specific distances from the perpendicular edge of the casting. Each type of aluminum alloy casting will likely require a specific calibration data set.

4. The two proposed models for two microstructural evolutionary events, the precipitation of Mg2Si in the alpha phase through its various precipitation stages or two Mg2Si precipitation events occurs at different cooling times in the two different alpha phases: (high silicon content) of alpha phase in the eutectic constituent and followed Mg2Si precipitation in proeutectic alpha phase (low silicon content) were proposed.

(A) (B)

ACKNOWLEDGEMENT

The authors acknowledge and appreciate the support of the American Bureau of Shipping.

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