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Observation of exothermic reaction during laser-assisted iron oxide coating on aluminum alloy
Materials Science and Engineering A 390 (2005) 404–413
Observation of exothermic reaction during laser-assisted iron oxide coating on aluminum alloy S. Nayaka , Hsin Wangb,d , Edward A. Kenikd , Ian M. Andersond , Narendra B. Dahotrea,c,d,∗ a
Department of Materials Science and Engineering, The University of Tennessee, Knoxville, TN 37932, USA b High Temperature Materials Laboratory, Oak Ridge National Laboratory, Oak Ridge, TN, USA c Materials Processing Group, Oak Ridge National Laboratory, Oak Ridge, TN, USA d Metals and Ceramics Division, Oak Ridge National Laboratory, Oak Ridge, TN, USA Received 1 July 2004; received in revised form 9 August 2004
Abstract Aluminum and Fe3 O4 reacts readily in what is known as thermite reaction to produce large amount of heat. Attempts were made to coat Feoxide on A319 cast aluminum alloy employing a high power laser to exploit this reaction. High-speed, high-resolution infrared thermography was employed to study the thermal conditions during the laser treatment. Parallel experiment using a less exothermic oxide (FeO) and the same substrate further emphasized development of higher temperature during highly exothermic reaction. The cooling rate calculated via both steady state and non-steady state relations were one order of magnitude different, which was supported by microstructural observations. Transmission electron microscopy revealed formation of aluminides as a result of reaction between iron oxide and aluminum alloys. © 2004 Elsevier B.V. All rights reserved. Keywords: Infrared thermography; Laser coating; A319; Thermite reaction; Iron oxides
1. Introduction Most of the widely used cast aluminum alloys belong to a class of Al–Si based composition. These alloys owe their excellent castability to silicon, which lowers melting point and improves fluidity. These alloys are particularly suitable for intricate, pressure-tight casting. Although these alloys are now proven substitute for cast iron for their lightweight, corrosion resistance and crack-resistance, they still suffer from unsatisfactory wear characteristics. Wear resistance is basically complex combination of mechanical properties. It is generally accepted that an Al–matrix composite with fine reinforcements would provide better wear resistance [1–3] than cast aluminum alloys. A composite is often advantageous over its constituents, as it can possess superior set of properties; even a combina∗
Corresponding author. Tel.: +1 865 974 3609; fax: +1 865 974 4115. E-mail address: [email protected] (N.B. Dahotre).
0921-5093/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2004.08.008
tion of seemingly conflicting properties. A particle reinforced metal matrix composite is suitable for many engineering applications, especially when strength and wear resistance are of prime concern. Easy melting due to eutectic reaction in Al–Si based alloys can be exploited for the purpose of surface modification via selective melting and mixing of ceramic powder in the melted surface region for improved properties However, one of the primary limiting factors in developing such a material is the relative poor wettability of most ceramics with aluminum and therefore, a weak bonding between matrix and the reinforcement. Since the load transfer from matrix to reinforcement is abrupt, the interface is a potential site for crack initiation and propagation. Another problem in synthesis of such composite materials is that the reinforcing particles tend to segregate owing to hydrophobic nature, lack of wettability, higher surface tension, different state of matter (liquid metal and solid ceramic) and (often) wide density difference. A non-uniform microstructure with segregated particles manifests in adverse mechanical properties. It is,
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Nomenclature cp G Gm I k q t T Tm Tmg dT/dt DT/Dt
∂T/∂t
ux X Y
specific heat temperature gradient maximum temperature gradient infrared intensity of individual pixel thermal conductivity extra heat flow term time temperature maximum temperature in an IR thermograph frame temperature corresponding to maximum temperature gradient (Gm ) ordinary temperature derivative with respect to time substantial derivative of temperature, this is a summation of partial derivative with respect to time and a quantity (∂T/∂x × ∂x/∂t) originated from the position dependence of temperature variation partial derivative of temperature with respect to time only (where the T is a function of position as well as time) laser traverse speed laser traverse direction direction perpendicular to laser traverse
Greek letters α heat diffusivity ε emissivity ρ density
therefore, desired to minimize the segregation by mechanical means such as stirring or other rapid material transport in bulk composite materials. Several efforts are being made to enhance wettability and bonding between the ceramic reinforcement and metal matrix. One such method is to select a material system in which the ceramic and the metallic constituents have potential for chemical reaction [4]. The chemical reaction serves three purposes: (1) often reaction between metal and ceramic can alter the wettability characteristics because of the resulting reaction product layers in interfaces. The free energy associated with such a reaction, provides energy required for work of liquid spreading on particle, (2) the phases formed in situ in metal-ceramic reaction provides further reinforcement. These particles are usually uniformly distributed, fine and well-bonded and therefore, effective reinforcements, (3) the extra heat generated as a result of reaction improve energy efficiency of the process. Therefore, in this work, a novel process is proposed to form a composite layer selectively on surfaces exploiting
405
the reaction induced interaction. The top layer, thus formed, shall provide protection against surface initiated deterioration. Thermodynamics provides basic guideline to select a material system for the composite layer. Thermodynamic data depicted in an Ellingham diagram indicates that the aluminum is capable of reducing a large number of oxides [5]. Oxides of iron are proven candidate in self-sustaining aluminothermic reaction as in thermite welding [6]. Fe3 O4 is considered suitable for its lower cost and high free energy of reaction. Such a reaction, besides improving wettability between oxide and aluminum matrix, would also provide extra energy for the process [7]. The combustion front can propagate deep into the substrate providing a much thicker coating if desired. Iron oxide can be reduced by aluminum via following reaction: 3Fe3 O4 + 8Al → 4Al2 O3 + 9Fe (G = −3095 kJ/mol of Al at 1173 K)
(1)
Resulting Fe can mix with molten Al and driven by the negative free energy values of several possible stoichiometric reactions between Fe and Al, Fe-aluminides can form. The other reaction products Al2 O3 formed in situ, Fe-aluminides and unreacted but well-bonded Fe3 O4 can together produce excellent reinforcement. A high-density laser beam when scanned over a Fe3 O4 precursor deposit layer on top of A319 alloy, it can melt only a confined volume of laser–material interaction. At the same time, the rest of the component remains at lower temperature. The high temperature gradient and very good thermal conductivity of A319 aluminum (k = 243 W m/K) induce rapid solidification. The surface layer thus melted and rapidly solidified would consist of a refined microstructure with the above-mentioned reinforcements. The explosive reaction kinetics and strong convective flow intrinsic to laser coating would ensure at least some degree of reaction and uniform distribution of reinforcements. The exothermic reactions that are occurring will reduce the total laser energy required for processing. This holds promise for improved efficiency of laser surface engineering. Therefore, laser-assisted exothermic reactions, like the ones considered here, are a step in the direction of synthesis of novel, far-from-equilibrium composite coating. It is, therefore, important to understand the thermal conditions during laser processing and its influence in consequent microstructure. The rapid speed of laser treatment and lack of any reliable direct high-speed response techniques have prevented this step. In light of this, in the present study, a novel infrared (IR) thermography technique is explored to determine thermal conditions particularly cooling rate and its correlation with microstructural evolution. Thus the emphasis of the present paper is confined to a description on high-speed, highresolution infrared thermography to study the development of thermal conditions during laser treatment and evolution of the corresponding microstructure. The additional details
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about the various aspects of the oxide coating with laser induced exothermic reaction are discussed and explained by the authors in several other publications in the open literature [8–15].
2. Experimental procedure 2.1. Materials Coupons of size 60 mm × 60 mm × 4 mm were machined out from A319 cast billet (Si 5.5–6.5, Cu 3.0–4.0, Mg <0.1, Mn <0.5, Fe <1.0, Zn <1.0, Ti <0.25, Ni <0.35, others <0.5, all in wt.%, balance Al). The coupons were ground, sand blasted and ultrasonically cleaned. Commercially available Fe3 O4 powders (99.5% purity, Reade Advanced Materials, Riverside, RI, average powder size <15 m) were used in the present study. The powder precursor made of 95 wt.% Fe3 O4 + 5 wt.% Si suspended in a 10 wt.% water based organic binder (proprietary formulation made from commercially available resins used in the paint industry) was spray deposited on A319 samples (60 mm × 60 mm × 4 mm). Presence of Si in the as-received material (∼6.5 wt.%) and addition of 5 wt.% Si to iron oxide powder was intended to prevent over heating of liquid metal pools as experienced in thermite reactions. (In thermite welding, ferro-alloys are added to cool the otherwise violent exothermic reaction [6].) The average precursor deposit thickness was 150 ± 15 m. Two sets of parallel experiments were also conducted to study the thermal conditions during surface coating, under similar laser processing parameter as those used during Fe3 O4 coating. In the first set of experiments, the coupons were deposited with FeO. FeO has similar thermal properties and reactivity as Fe3 O4 , however a modest free energy of reaction. Al reduces FeO as per following equation: 3FeO + 2Al → Al2 O3 + 3Fe (G = −775.7 kJ/mol of Al at 1173 K)
(2)
In the second set of parallel experiments, only laser surface melting without any precursor powder was conducted. These samples are referred to as ‘remelted’ samples. 2.2. Laser processing A 2 kW Rofin Sinar continuous wave Nd:YAG laser equipped with fiber optic beam delivery system was employed for laser treatment of the deposited samples. The lenses within the output-coupling module of the fiber optic delivery system were configured to produce a circular laser spot of 600 m diameter. The laser beam power was maintained at 2.0 kW. Two laser traverse speeds were used: 325 and 375 cm/min. The laser beam was traced in straight, overlapping stripes. The direction of laser traverse is taken as + X-axis and perpendicular to this is Y-axis (Fig. 1). Similarly, stripes can be traced for entire or se-
Fig. 1. Schematic showing the infrared thermography during laser processing.
lective coverage of the surface of the components as necessary. 2.3. Infrared thermography In the present study, a state-of-the-art, high-speed, and high sensitivity Raytheon Radiance HS IR imaging system was used to record the temperature during laser treatment. The IR camera had a 256 × 256 pixels focal-plane-array InSb detector, which was sensitive to thermal radiation in wavelength range of 3–5 m. The camera could be operated in a snapshot mode, and could be externally triggered. The temperature resolution of the camera was 0.015 ◦ C. The infrared camera system is capable of capturing thermographic frames at 1–144 Hz. In the present work, it is set up to capture infrared image at 30 Hz based on initial trials for optimum incoming IR signal. As seen in Fig. 1, the camera was focused on a location covering few tracks. The camera remained stationary while the laser beam traversed on the sample surface within the field of view of the camera. In addition, the IR imaging system can be coupled with the laser treatment system through a synchronizer to trigger the IR camera to record data at a certain time delay such that any phase lag of data recording was avoided. The IR thermography camera recorded the intensity of infrared light in digital form (Fig. 2). The pixel size was 20 m × 20 m. The infrared thermograph is an array of intensity values of pixels covering entire surface as a function of time. The recording (of three laser tracks) was done at 30 frames per second for both laser traverse speeds of 325 and 375 cm/min. Energy is emitted by a hot body, such as the laserprocessed samples, in form of electromagnetic radiation. A significant portion of the total energy emitted in the temperature range of the laser processing of usual metals would normally be in infrared range. The intensity of any particular wavelength is a function of temperature and emissivity (ε) only. In the current study, attempts were made to measure temperature from intensity value at a particular wavelength. Hence, experimental calibration was necessary. For calibrating the infrared intensity (I) thus measured for actual temperature (T), actual samples were heated from room
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Table 1 Maximum temperature, Tm (◦ C), observed during laser treatment Location
Maximum temperature, Tm (◦ C) (standard deviation %) Fe3 O4 coating
Laser track 1 Laser track 2 Laser track 3
FeO coating
Remelted sample
325
375
325
375
325
375
638 (3.0) 633 (2.3) 643 (2.4)
650 (1.8) 626 (2.7) 636 (1.8)
597 (0.7) 603 (2.5) 615 (1.8)
614 (0.6) 605 (2.1) 612 (1.7)
569 (1.3) 578 (1.9) 589 (2.5)
569 (2.0) 569 (2.1) 578 (3.2)
325 and 375 represents the speed values in cm/min.
temperature to 600 ◦ C in a closely controlled blackbody furnace. The sample was kept in the center of the furnace opening resembling a blackbody. As the temperature of the furnace was raised, the IR intensity was recorded corresponding to the furnace temperature. The samples were heated at the highest possible heating rate so that off-equilibrium condition is maintained. The temperature of the sample was trailing behind furnace temperature during heating. Hence, the intensity was recorded with IR camera along with temperature with a rapid response thermocouple (coupled with the control thermocouple to reduce error) attached to the sample. The IR intensities were recorded in the range 100–600 ◦ C for the furnace (maximum setting temperature of the furnace) and 100–480 ◦ C for the sample, as it was difficult to maintain a good thermocouple–sample contact beyond 480 ◦ C as the sample started softening (melting point is 485 ◦ C). Care was taken to keep camera setting (filters, lenses, distance between camera and sample, etc.) identical to those used for recording during laser treatments. The raw data were used to determine temperature from the IR intensity value. More details of these particular efforts are available in other reference [8]. The general characterization of Fe3 O4 coated samples included cross-sectional examination of the coating and interface using optical BX60M Olympus microscope and Philip (FEI) XL30 FEG scanning electron microscope. Further, transmission electron microscopy was carried out using Tecnai-30 TEM to confirm any reaction that might have taken place in case of Fe3 O4 coating.
The intensity, I, versus temperature, T, determined in furnace heating experiments, was analyzed. It was established that the emissivity (ε) of all three samples was very close. Experimentally, it was not possible to go beyond the explored temperature range (100–600 ◦ C for the furnace and 100–485 ◦ C for the samples). It was found that the relation between I versus T was a fourth order polynomial. To find out T of the sample for a corresponding I, the value of I was substituted in fourth order polynomial relations. High correlation factor of the empirical polynomials (and both for blackbody and samples) indicate that the sample was very close to blackbody as far as emission of infrared radiation is concerned. Considering above factors and the ease and simplicity of use, it was considered appropriate to extrapolate the I–T relation for the sample beyond the experimental range to determine temperature from the intensity. As depicted in Fig. 2, the intensity of each pixel is recorded every 1/30 s. By using the empirical calibration relation developed in above-mentioned efforts [8], temperature can be calculated of the entire pixel array or of any particular set of points. To determine the maximum temperature (Tm ) during laser treatment several (10) such readings were taken in equal intervals and the average of values (T calculated from
Fig. 2. Schematic showing the pixels of a infrared thermograph during laser processing.
Fig. 3. Temperature profile across the laser interaction region in the direction of laser traverse (laser speed 325 cm/min).
3. Results and discussion 3.1. Temperature from infrared intensity
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Fig. 4. Temperature profile across the laser interaction region in the direction of laser traverse (laser speed 325 cm/min).
I) are presented in Table 1. The standard deviation associated with all the measurements presented was below 3.5% despite of rapid events intrinsic to laser processing and IR thermography (e.g., plume and vapor obstruction of field of view) suggesting that the experiments were well controlled and reproducible. The temperature of laser remelted sample and iron oxide on A319 can provide information if there is additional heat due to exothermic reactions (Eqs. (1) and (2)). Fig. 3 shows a typical temperature profile across the laser interaction region in the direction of laser traverse, i.e., X-axis as illustrated in Fig. 2. The temperature profile of FeO coating was found to be lower than Fe3 O4 coating. The precursor thickness, laser absorption, laser beam size and thermal characteristics of both oxides are comparable, but the amount of exotherm associated with the heat of reduction reaction in case of Fe3 O4 is much higher than that of
FeO. Hence, it can be concluded that the higher temperature and bigger area of interaction in case of Fe3 O4 coating than FeO and remelted samples (Fig. 3) were due to this strong exotherm of the thermite reaction. The temperature indicates a peak ahead of laser path. This can be explained as follows. As the laser moves in X-direction, rapid heating of the substrate takes place. Due to rapid speed of the process, higher superheating is required before melting can occur. Once it starts melting, the temperature drops a little due to absorption of latent heat of fusion and/or convective heat transfer within the melt-pool. Similarly, Fig. 4 shows a typical temperature profile across the laser interaction region in the direction perpendicular to laser traverse, i.e., Y-axis as illustrated in Fig. 2. The temperature is lower in the middle region than the periphery of laser interaction volume. This is because of the temperature due to superheating. Again, the temperature corresponding to Fe3 O4 coating was higher than FeO coating, which in turn was higher than remelted sample. Also, as can be seen from the width of melt region (in Figs. 3 and 4 and confirmed with melt track measurements), the volume of melt region under direct laser–material interaction is larger for Fe3 O4 coating than FeO coating, which in turn was higher than remelted sample. It can be seen from Table 1 that the maximum temperature recorded in several (10) frames (of the same coating and speed), Tm , is highest in case of Fe3 O4 coating followed by FeO coating and remelted sample for both the laser speeds. Even though both oxides undergo reduction reaction, Fe3 O4 coating exhibit higher maximum temperature, Tm due to extra heat supplied by the strong thermite reaction (Eq. 1) and greater readiness of Fe3 O4 to react with Al. The remelted sample exhibited lowest maximum temperature, Tm . For the given coating (Fe3 O4 or FeO) and substrate combination, the maximum temperature Tm did not show any distinctly
Fig. 5. Infrared thermograph of the (a) remelted sample and (b) Fe3 O4 coating.
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Table 2 Maximum temperature gradient during cooling, Gm (◦ C/m) Axis
Location
Maximum temperature gradient during cooling, Gm (◦ C/m) Fe3 O4 coating 325
X
Y
Laser track 1 Laser track 2 Laser track 3 Laser track 1 Laser track 2 Laser track 3
FeO coating 375
3.4 × 3.0 × 106 2.9 × 106 2.7 × 106 2.2 × 106 2.1 × 106 106
Remelted sample
325
3.5 × 3.1 × 106 3.1 × 106 1.9 × 106 1.9 × 106 1.9 × 106 106
375
3.1 × 2.9 × 106 2.8 × 106 2.2 × 106 2.1 × 106 2.1 × 106
325
3.1 × 3.0 × 106 3.1 × 106 2.3 × 106 2.1 × 106 2.1 × 106
106
375
3.0 × 2.6 × 106 2.3 × 106 1.9 × 106 1.9 × 106 1.9 × 106
106
3.1 × 106 3.0 × 106 2.8 × 106 2.0 × 106 1.9 × 106 2.0 × 106
106
325 and 375 represents the speed values in cm/min.
noticeable trend for a given traverse speed within a range employed in the present study. Fig. 5 represents instantaneous infrared thermograph for (a) remelted sample and (b) Fe3 O4 coating. The iron oxide coating indicated a vapor tail following the laser spot. The iron oxide reacts violently with aluminum around the laser spot and as a result produces the comet-like vapor tail that follows the laser spot. As hypothesized earlier, the thermite reaction is triggered by laser providing higher temperature (Table 1 and Figs. 3 and 4), larger melt region (Figs. 3 and 4) and direct reaction related activities as evident from Fig. 5. Such a phenomenon is also expected to affect thermal dynamics of laser treatment process. 3.2. Thermal dynamics: gradient and cooling rates Typical sets of instantaneous temperature distribution for different samples in X- and Y-directions (determined from the array of intensity values of the pixels in a typical thermograph) are presented in Figs. 3 and 4. From these figures, it is clear that the temperature gradient around the periphery of instantaneous laser spot is very high (difference of >250 ◦ C in just 100 m distance). Additionally laser travels at relatively rapid speeds (325 and 375 cm/min). The high thermal gradient and rapid traverse speeds translate to high cooling rates and severe thermal conditions. The solidification microstructure and hence the properties are strongly influenced by the cooling rate, which in turn, is primarily dictated by the temperature gradient (G), dT/dx or dT/dy. Hence, it was essential to estimate the temperature gradient, especially during cooling. From the temperature versus position data for a known laser traverse speed, it is possible to determine the temperature gradient and subsequently the cooling rate. The temperature profiles in X- and Y-directions were different (Figs. 3 and 4) as laser beam traversed linearly only in X-direction. Therefore, the cooling rates were calculated for both X- and Y-directions. Firstly, the temperature-position data is incremental (pixel by pixel) and therefore, discrete temperature gradients, G were calculated. The temperature gradient during cooling thus calculated can vary strongly as a function of position due to rapid speed of laser treatment. (For example, laser travels >91 pixels in the interval of consecutive two IR thermography frames.) As the solidification
front propagates opposite to heat extraction direction and is always associated with latent heat of fusion, the temperature gradient is highest close to the solidification front, which is hereafter called as maximum temperature gradient, Gm . The value of Gm was determined by selecting a highest value among all discrete G values in an IR thermograph frame and presented in Table 2. It can be seen that the maximum temperature gradients, Gm were highest for Fe3 O4 coating (Table 2). However, the gradient in Y-axis is though significantly different from sample to sample, does not follow any obvious trend. Several such frames were analyzed and found to possess minimal variations (<5%). The increase in laser traverse speed induces an increase in temperature gradient in X-direction as the moving heat source superimposes additional thermal gradient over that caused by a static heat source. As explained above, the maximum temperature gradient is of interest and hence the temperature corresponding to maximum temperature gradient, Tmg in an IR frame was calculated. Tmg was determining by finding temperature corresponding to position of Gm . The average values are presented in Table 3. It can be noted that the temperature corresponding to maximum gradient, Tmg is different (significantly lower) than the maximum temperature, Tm , recorded (Table 1). This is obvious as the highest temperature, Tm , corresponds to over-heated liquid within the melt-pool and the maximum temperature gradient occurs on the periphery of the meltpool where the solidification front is and hence corresponding temperature, Tmg . Higher temperature gradient, G is expected to provide higher cooling rate for different samples. The limited temporal and spatial resolution of infrared thermography combined with rapid speed of laser treatment pose challenge to Table 3 Temperature corresponding to maximum gradient, Tmg (◦ C) Location
Laser track 1 Laser track 2 Laser track 3
Temperature of maximum gradient, Tmg (◦ C) Fe3 O4 coating
FeO coating
Remelted sample
325
375
325
375
325
375
469 528 512
482 492 488
461 479 467
469 453 470
445 457 448
439 466 448
325 and 375 represents the speed values in cm/min.
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Table 4 Cooling rate calculated assuming non-steady state heat transfer, DT/Dt (◦ C/s) Axis
Non-steady state cooling rate, DT/Dt (◦ C/s)
Location
Fe3 O4 coating
FeO coating
325 X
Y
375
4.3 × 4.4 × 106 4.5 × 106 2.5 × 106 2.7 × 106 2.2 × 106
Laser track 1 Laser track 2 Laser track 3 Laser track 1 Laser track 2 Laser track 3
325
5.2 × 4.2 × 106 3.9 × 106 1.1 × 106 1.3 × 106 1.9 × 106
106
Remelted sample 375
4.5 × 2.7 × 106 2.0 × 106 1.8 × 106 1.3 × 106 1.2 × 106
106
325
3.4 × 2.0 × 106 4.4 × 106 2.6 × 106 1.7 × 106 1.8 × 106
106
106
375
6.8 × 5.9 × 106 2.8 × 106 2.0 × 106 1.8 × 106 2.3 × 106 106
5.4 × 106 4.0 × 106 5.0 × 106 1.9 × 106 2.2 × 106 1.8 × 106
325 and 375 represents the speed values in cm/min.
track 1 to track 3. Also, the corresponding standard deviations for a particular track were within 5%. This is an indication of close to steady state. It is often possible for such moving origin (heat source) problem to attain steady state (time independent), meaning the temperature profile around the laser–matter interaction remains unchanged. Therefore, attempts were made to calculate the cooling rates assuming a steady state heat transfer for a moving heat source (origin), which is, by its very nature, a problem easy to solve. The surface cooling rate DT/Dt in the direction opposite to laser traverse under steady state (∂T/∂t = 0) can be estimated as per steady state relation for the discrete data, in this case given in Eq. (4)
determine the cooling rate from IR intensity data. However, mathematical models are available to different degree of complexities to estimate cooling rates [8,16–19]. If the sample being laser treated is large enough compared to instantaneous laser interaction volume, and/or the sample is adequately cooled, the condition can be described by heat transfer equations for moving heat source (origin). If a body is moving relative to a frame of reference at speed ux and conducting heat primarily in the direction of motion (X-direction), then the equation in that reference frame (for constant properties) is given by the ‘substantial derivative’ instead of the partial derivative [8,16,17] that accounts for both spatial as well as temporal variations. When the temporal partial derivative is non-zero, then the problem is more generic in nature and is known as non-steady state heat transfer, i.e., the temperature profile around the origin (laser spot) varies with time. In the finite difference terms, the non-steady state cooling rate can be estimated from the pixel array double derivative (with respect to position) of temperature in X- or Y-directions and can be expressed as follows [8,20]: DT G q (T/x) q =α + =α + Dt x cp ρ x cp ρ
(3a)
DT G q (T/y) q =α + =α + Dt y cp ρ y cp ρ
(3b)
T T x T (4) = = ux t x t x where ux , is the laser traverse velocity. The above all temperature derivatives (DT/Dt, dT/dt, and ∂T/∂t) are mathematical (theoretical) description of infinitesimal variations. However, as mentioned earlier the IR thermography data is discrete in nature. For this distinction, T/t has been used. The thermography technique used here had a finite temporal resolution of t = 1/30 s and spatial resolution (pixel) of x = 20 m. The steady state cooling rates corresponding to maximum temperature gradient, Gm (from Table 2) and laser traverse velocity ux = 325 or 375 cm/min were calculated using Eq. (4). As seen in this equation, the cooling rate calculation depends upon rectilinear velocity (ux ) of the origin (heat source). No such term can be accounted for in Y-direction. Therefore, the cooling rates for each track in X-direction were only calculated and are presented in Table 5. They are about one order of magnitude lower than the corresponding cooling rates for non-steady state (Table 4). In non-steady state,
The cooling rates thus estimated in both X- and Y-directions are presented in Table 4. As expected the cooling rate in Xdirection is higher than that in Y-direction due to the effect of laser traverse. From Tables 1–4, it can be seen that the maximum temperature Tm , maximum temperature gradient Gm , temperature of maximum gradient Tmg , and non-steady state cooling rate DT/Dt change only very gradually from Table 5 Cooling rate calculated assuming steady state heat transfer, dT/dt (◦ C/s) Location
Steady state cooling rate, dT/dt (◦ C/s) Fe3 O4 coating 325
Laser track 1 Laser track 2 Laser track 3
FeO coating 375
1.9 × 1.7 × 105 1.8 × 105 105
325
2.2 × 2.0 × 105 1.9 × 105
325 and 375 represents the speed values in cm/min.
105
Remelted sample 375
1.7 × 1.6 × 105 1.5 × 105 105
325
2.0 × 1.9 × 105 1.6 × 105 105
375
1.6 × 1.3 × 105 1.1 × 105 105
1.9 × 105 1.8 × 105 1.8 × 105
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Fig. 6. Optical micrographs: microstructure of (a) as-received A319, (b) the remelted A319, (c) FeO coating and (d) Fe3 O4 coating at same magnification.
generally the instantaneous temperature gradient is higher compared to steady state and correspondingly it is expected to indicate higher cooling rate. It is difficult to accurately determine whether the thermal dynamics is closer to steady state or non-steady state. How-
ever, both predicted cooling rates are high (>105 ◦ C/s) for all the samples and processing conditions considered in this work. As postulated previously, such high cooling rate and off-equilibrium conditions in addition to violent thermite reaction expected to result in generating a microstructure with
Fig. 7. Transmission electron micrograph of laser induced iron oxide coating showing Fe3 O4 particle within cell.
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refined matrix grains reinforced with well-bonded iron oxide and/or the reaction products. 3.3. Microstructure evolution The cast alloys are most sensitive to the solidification rate. The high cooling rate associated results in highly refined microstructures in all laser treated samples. Fig. 6 presents the optical micrograph of (a) as-received A319 and (b) remelted sample. The laser-processed samples can be seen to possess very similar appearance. The oxide particles are not apparent in Fig. 6(c) and (d) perhaps due to their readiness to react with aluminum leaving behind little remnant. Eq. (3) (nonsteady state) is based on a steeper temperature gradient than that corresponding to Eq. (4) (steady state). Hence, these two relations provide the natural limits (upper and lower, respectively) of the surface cooling rate. Both values of the upper and lower limits being very high (>105 C/s), it is expected that the actual cooling rate responsible for microstructural refinement is some intermediate value, and therefore, the solidification rate in laser treatment is also expected to refine microstructure to great extent.
Fig. 7 illustrates a particle of Fe3 O4 incorporated in the aluminum cell. Energy dispersive spectroscopy (EDS) and diffraction pattern confirms that the particle is iron oxide. The matrix seen here is essentially cellular dendrite structure with the Si and CuAl2 phases occupying the intercellular region. However, there were much fewer oxide particles seen than expected. Even though the average particle size of the oxide used in precursor was about 15 m, the particles found under TEM rarely exceeded 1 m. This can be attributed to physical and chemical disintegration of oxide particles during laser processing. Under transmission electron microscope, such oxides were found to have reacted with aluminum. Fig. 8 shows a typical reaction zone. The oxide and aluminum has reacted along the interface. The interfacial long precipitate was analyzed using EDS and diffraction pattern and found to be Fe-aluminide (FeAl). Formation of such a reaction product in the interface between iron oxide and aluminum matrix ensures good bonding (reaction induced bonding) between the reinforcing ceramic and the matrix. In case of complete reaction, the FeAl is also well-bonded with the matrix and provide effective reinforcing [21]. The Fe3 O4 phase in Fig. 8 shows almost a ring-like diffraction pattern
Fig. 8. Transmission electron micrograph of reaction between a Fe3 O4 particle and Al matrix.
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indicating the precipitate is consisted of ultrafine grains. Iron oxides are known to be extremely tolerant to defects owing to multi-valance of Fe. The severe thermal conditions in laser processing and thermite reaction can produce ultrafine grains by physical fragmentation and/or introduction of high defect density. If the iron oxide particles eventually fragment and separate and/or react, the resulting ultrafine particles can provide very good reinforcement of the matrix.
4. Conclusion The iron oxides (Fe3 O4 and FeO) precursor reacted with A319 substrate during laser treatment. The temperature estimated using highly resolved (spatial) IR thermography technique revealed that the thermite reactions between iron oxide and aluminum produced higher heat of reaction compared to the remelted sample. The extent of heat of reaction between aluminum and iron oxide (higher for Fe3 O4 compared to FeO) is in line with observed temperature. The transmission electron microscopy indicated reactions between Al and Fe-oxide particles and physical disintegration of oxide particles. The reaction products present as precipitate on the interface was found to be FeAl, which might have formed as a result of subsequent reaction between Al and Fe formed in thermite reaction. In principle it is possible to utilize the heat of exothermic reaction for efficient surface engineering of aluminum alloys. Also, such a reaction induced wetting between the matrix and reinforcing ceramics shall result in an improved composite material.
Acknowledgements The authors acknowledge the infrared thermography work sponsored by the Assistant Secretary of Energy Efficiency and Renewable Energy, Office of Transportation Technologies, as part of the High Temperature Materials Laboratory User Program, and transmission electron microscopy facil-
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ity at the Oak Ridge National Laboratory through the Shared Research Equipment (SHaRE) User Center at the Oak Ridge National Laboratory, managed by UT-Battelle, LLC, for the U.S. Department of Energy under Contract No. DE-AC0500OR22725.
References [1] F.S. Pettit, Surface Engineering of Non-ferrous Metals, 10th ed., ASM International, 1996. [2] J.H. Tylczak, A. Oregon, Friction, Lubrication and Wear Technology, 9th ed., ASM International, Materials Park, OH, 1992, pp. 184– 189. [3] A.K. Prasada Rao, Karabi Das, B.S. Murty, M. Chakraborty, Wear 257 (1–2) (2004) 148–153. [4] M.G. Nicholas, N. Eustathopoulos, B. Drevet, Wettability at High Temperature, Pergamon Press, New York, 1999, pp. 93–97. [5] H.J.T. Ellingham, J. Soc. Chem. Ind. 63 (1944). [6] B. Schwartz, Welding, Brazing, and Soldering, vol. 6, 10th ed., ASM International, Materials Park, OH, 2000, 291–293. [7] J.J. Moore, H.J. Feng, Prog. Mater. Sci. (1995) 39–243. [8] S. Nayak, Laser induced surface modification of aluminum alloys, Ph.D. Dissertation, The University of Tennessee, 2004. [9] S. Nayak, N.B. Dahotre, J. Miner. Met. Mater. Soc. (JOM) 56 (1) (2004) 46–48. [10] S. Nayak, N.B. Dahotre, Surf. Coat. Technol., in press. [11] S. Nayak, L. Reister, H.M. Meyer III, N.B. Dahotre, J. Mater. Res. 18 (4) (2003) 833–839. [12] S. Nayak, N.B. Dahotre, Adv. Eng. Mater. 5 (4) (2003). [13] S. Nayak, N.B. Dahotre, J. Miner. Met. Mater. Soc. (JOM) 54 (9) (2002) 39–41. [14] A. Dasari, S. Nayak, R.D.K. Misra, O.O. Popoola, N.B. Dahotre, Mater. Sci. Technol. 18 (2002) 11–18. [15] N.B. Dahotre, S. Nayak, O.O. Popoola, J. Miner. Met. Mater. Soc. (JOM) 53 (2001) 44–46. [16] D. Rosenthal, Weld. J. (Suppl. 20) (1941) 220–234. [17] D. Rosenthal, Trans. Am. Soc. Mech. Eng. 68 (1946) 849–858. [18] H.S. Carslaw, J.C. Jaeger, Conduction of Heat in Solid, Oxford Clarendon Press, Oxford, 1962. [19] Modeling of Casting and Welding Processes III, TMS, Warrendale, PA, 1986. [20] S. Nayak, H. Wang, N.B. Dahotre, Mater. Sci. Technol., in press. [21] S. Nayak, H.M. Meyer III, N.B. Dahotre, Surf. Eng. 20 (1) (2004) 48–52.
Observation of exothermic reaction during laser-assisted iron oxide coating on aluminum alloy S. Nayaka , Hsin Wangb,d , Edward A. Kenikd , Ian M. Andersond , Narendra B. Dahotrea,c,d,∗ a
Department of Materials Science and Engineering, The University of Tennessee, Knoxville, TN 37932, USA b High Temperature Materials Laboratory, Oak Ridge National Laboratory, Oak Ridge, TN, USA c Materials Processing Group, Oak Ridge National Laboratory, Oak Ridge, TN, USA d Metals and Ceramics Division, Oak Ridge National Laboratory, Oak Ridge, TN, USA Received 1 July 2004; received in revised form 9 August 2004
Abstract Aluminum and Fe3 O4 reacts readily in what is known as thermite reaction to produce large amount of heat. Attempts were made to coat Feoxide on A319 cast aluminum alloy employing a high power laser to exploit this reaction. High-speed, high-resolution infrared thermography was employed to study the thermal conditions during the laser treatment. Parallel experiment using a less exothermic oxide (FeO) and the same substrate further emphasized development of higher temperature during highly exothermic reaction. The cooling rate calculated via both steady state and non-steady state relations were one order of magnitude different, which was supported by microstructural observations. Transmission electron microscopy revealed formation of aluminides as a result of reaction between iron oxide and aluminum alloys. © 2004 Elsevier B.V. All rights reserved. Keywords: Infrared thermography; Laser coating; A319; Thermite reaction; Iron oxides
1. Introduction Most of the widely used cast aluminum alloys belong to a class of Al–Si based composition. These alloys owe their excellent castability to silicon, which lowers melting point and improves fluidity. These alloys are particularly suitable for intricate, pressure-tight casting. Although these alloys are now proven substitute for cast iron for their lightweight, corrosion resistance and crack-resistance, they still suffer from unsatisfactory wear characteristics. Wear resistance is basically complex combination of mechanical properties. It is generally accepted that an Al–matrix composite with fine reinforcements would provide better wear resistance [1–3] than cast aluminum alloys. A composite is often advantageous over its constituents, as it can possess superior set of properties; even a combina∗
Corresponding author. Tel.: +1 865 974 3609; fax: +1 865 974 4115. E-mail address: [email protected] (N.B. Dahotre).
0921-5093/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2004.08.008
tion of seemingly conflicting properties. A particle reinforced metal matrix composite is suitable for many engineering applications, especially when strength and wear resistance are of prime concern. Easy melting due to eutectic reaction in Al–Si based alloys can be exploited for the purpose of surface modification via selective melting and mixing of ceramic powder in the melted surface region for improved properties However, one of the primary limiting factors in developing such a material is the relative poor wettability of most ceramics with aluminum and therefore, a weak bonding between matrix and the reinforcement. Since the load transfer from matrix to reinforcement is abrupt, the interface is a potential site for crack initiation and propagation. Another problem in synthesis of such composite materials is that the reinforcing particles tend to segregate owing to hydrophobic nature, lack of wettability, higher surface tension, different state of matter (liquid metal and solid ceramic) and (often) wide density difference. A non-uniform microstructure with segregated particles manifests in adverse mechanical properties. It is,
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Nomenclature cp G Gm I k q t T Tm Tmg dT/dt DT/Dt
∂T/∂t
ux X Y
specific heat temperature gradient maximum temperature gradient infrared intensity of individual pixel thermal conductivity extra heat flow term time temperature maximum temperature in an IR thermograph frame temperature corresponding to maximum temperature gradient (Gm ) ordinary temperature derivative with respect to time substantial derivative of temperature, this is a summation of partial derivative with respect to time and a quantity (∂T/∂x × ∂x/∂t) originated from the position dependence of temperature variation partial derivative of temperature with respect to time only (where the T is a function of position as well as time) laser traverse speed laser traverse direction direction perpendicular to laser traverse
Greek letters α heat diffusivity ε emissivity ρ density
therefore, desired to minimize the segregation by mechanical means such as stirring or other rapid material transport in bulk composite materials. Several efforts are being made to enhance wettability and bonding between the ceramic reinforcement and metal matrix. One such method is to select a material system in which the ceramic and the metallic constituents have potential for chemical reaction [4]. The chemical reaction serves three purposes: (1) often reaction between metal and ceramic can alter the wettability characteristics because of the resulting reaction product layers in interfaces. The free energy associated with such a reaction, provides energy required for work of liquid spreading on particle, (2) the phases formed in situ in metal-ceramic reaction provides further reinforcement. These particles are usually uniformly distributed, fine and well-bonded and therefore, effective reinforcements, (3) the extra heat generated as a result of reaction improve energy efficiency of the process. Therefore, in this work, a novel process is proposed to form a composite layer selectively on surfaces exploiting
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the reaction induced interaction. The top layer, thus formed, shall provide protection against surface initiated deterioration. Thermodynamics provides basic guideline to select a material system for the composite layer. Thermodynamic data depicted in an Ellingham diagram indicates that the aluminum is capable of reducing a large number of oxides [5]. Oxides of iron are proven candidate in self-sustaining aluminothermic reaction as in thermite welding [6]. Fe3 O4 is considered suitable for its lower cost and high free energy of reaction. Such a reaction, besides improving wettability between oxide and aluminum matrix, would also provide extra energy for the process [7]. The combustion front can propagate deep into the substrate providing a much thicker coating if desired. Iron oxide can be reduced by aluminum via following reaction: 3Fe3 O4 + 8Al → 4Al2 O3 + 9Fe (G = −3095 kJ/mol of Al at 1173 K)
(1)
Resulting Fe can mix with molten Al and driven by the negative free energy values of several possible stoichiometric reactions between Fe and Al, Fe-aluminides can form. The other reaction products Al2 O3 formed in situ, Fe-aluminides and unreacted but well-bonded Fe3 O4 can together produce excellent reinforcement. A high-density laser beam when scanned over a Fe3 O4 precursor deposit layer on top of A319 alloy, it can melt only a confined volume of laser–material interaction. At the same time, the rest of the component remains at lower temperature. The high temperature gradient and very good thermal conductivity of A319 aluminum (k = 243 W m/K) induce rapid solidification. The surface layer thus melted and rapidly solidified would consist of a refined microstructure with the above-mentioned reinforcements. The explosive reaction kinetics and strong convective flow intrinsic to laser coating would ensure at least some degree of reaction and uniform distribution of reinforcements. The exothermic reactions that are occurring will reduce the total laser energy required for processing. This holds promise for improved efficiency of laser surface engineering. Therefore, laser-assisted exothermic reactions, like the ones considered here, are a step in the direction of synthesis of novel, far-from-equilibrium composite coating. It is, therefore, important to understand the thermal conditions during laser processing and its influence in consequent microstructure. The rapid speed of laser treatment and lack of any reliable direct high-speed response techniques have prevented this step. In light of this, in the present study, a novel infrared (IR) thermography technique is explored to determine thermal conditions particularly cooling rate and its correlation with microstructural evolution. Thus the emphasis of the present paper is confined to a description on high-speed, highresolution infrared thermography to study the development of thermal conditions during laser treatment and evolution of the corresponding microstructure. The additional details
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about the various aspects of the oxide coating with laser induced exothermic reaction are discussed and explained by the authors in several other publications in the open literature [8–15].
2. Experimental procedure 2.1. Materials Coupons of size 60 mm × 60 mm × 4 mm were machined out from A319 cast billet (Si 5.5–6.5, Cu 3.0–4.0, Mg <0.1, Mn <0.5, Fe <1.0, Zn <1.0, Ti <0.25, Ni <0.35, others <0.5, all in wt.%, balance Al). The coupons were ground, sand blasted and ultrasonically cleaned. Commercially available Fe3 O4 powders (99.5% purity, Reade Advanced Materials, Riverside, RI, average powder size <15 m) were used in the present study. The powder precursor made of 95 wt.% Fe3 O4 + 5 wt.% Si suspended in a 10 wt.% water based organic binder (proprietary formulation made from commercially available resins used in the paint industry) was spray deposited on A319 samples (60 mm × 60 mm × 4 mm). Presence of Si in the as-received material (∼6.5 wt.%) and addition of 5 wt.% Si to iron oxide powder was intended to prevent over heating of liquid metal pools as experienced in thermite reactions. (In thermite welding, ferro-alloys are added to cool the otherwise violent exothermic reaction [6].) The average precursor deposit thickness was 150 ± 15 m. Two sets of parallel experiments were also conducted to study the thermal conditions during surface coating, under similar laser processing parameter as those used during Fe3 O4 coating. In the first set of experiments, the coupons were deposited with FeO. FeO has similar thermal properties and reactivity as Fe3 O4 , however a modest free energy of reaction. Al reduces FeO as per following equation: 3FeO + 2Al → Al2 O3 + 3Fe (G = −775.7 kJ/mol of Al at 1173 K)
(2)
In the second set of parallel experiments, only laser surface melting without any precursor powder was conducted. These samples are referred to as ‘remelted’ samples. 2.2. Laser processing A 2 kW Rofin Sinar continuous wave Nd:YAG laser equipped with fiber optic beam delivery system was employed for laser treatment of the deposited samples. The lenses within the output-coupling module of the fiber optic delivery system were configured to produce a circular laser spot of 600 m diameter. The laser beam power was maintained at 2.0 kW. Two laser traverse speeds were used: 325 and 375 cm/min. The laser beam was traced in straight, overlapping stripes. The direction of laser traverse is taken as + X-axis and perpendicular to this is Y-axis (Fig. 1). Similarly, stripes can be traced for entire or se-
Fig. 1. Schematic showing the infrared thermography during laser processing.
lective coverage of the surface of the components as necessary. 2.3. Infrared thermography In the present study, a state-of-the-art, high-speed, and high sensitivity Raytheon Radiance HS IR imaging system was used to record the temperature during laser treatment. The IR camera had a 256 × 256 pixels focal-plane-array InSb detector, which was sensitive to thermal radiation in wavelength range of 3–5 m. The camera could be operated in a snapshot mode, and could be externally triggered. The temperature resolution of the camera was 0.015 ◦ C. The infrared camera system is capable of capturing thermographic frames at 1–144 Hz. In the present work, it is set up to capture infrared image at 30 Hz based on initial trials for optimum incoming IR signal. As seen in Fig. 1, the camera was focused on a location covering few tracks. The camera remained stationary while the laser beam traversed on the sample surface within the field of view of the camera. In addition, the IR imaging system can be coupled with the laser treatment system through a synchronizer to trigger the IR camera to record data at a certain time delay such that any phase lag of data recording was avoided. The IR thermography camera recorded the intensity of infrared light in digital form (Fig. 2). The pixel size was 20 m × 20 m. The infrared thermograph is an array of intensity values of pixels covering entire surface as a function of time. The recording (of three laser tracks) was done at 30 frames per second for both laser traverse speeds of 325 and 375 cm/min. Energy is emitted by a hot body, such as the laserprocessed samples, in form of electromagnetic radiation. A significant portion of the total energy emitted in the temperature range of the laser processing of usual metals would normally be in infrared range. The intensity of any particular wavelength is a function of temperature and emissivity (ε) only. In the current study, attempts were made to measure temperature from intensity value at a particular wavelength. Hence, experimental calibration was necessary. For calibrating the infrared intensity (I) thus measured for actual temperature (T), actual samples were heated from room
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Table 1 Maximum temperature, Tm (◦ C), observed during laser treatment Location
Maximum temperature, Tm (◦ C) (standard deviation %) Fe3 O4 coating
Laser track 1 Laser track 2 Laser track 3
FeO coating
Remelted sample
325
375
325
375
325
375
638 (3.0) 633 (2.3) 643 (2.4)
650 (1.8) 626 (2.7) 636 (1.8)
597 (0.7) 603 (2.5) 615 (1.8)
614 (0.6) 605 (2.1) 612 (1.7)
569 (1.3) 578 (1.9) 589 (2.5)
569 (2.0) 569 (2.1) 578 (3.2)
325 and 375 represents the speed values in cm/min.
temperature to 600 ◦ C in a closely controlled blackbody furnace. The sample was kept in the center of the furnace opening resembling a blackbody. As the temperature of the furnace was raised, the IR intensity was recorded corresponding to the furnace temperature. The samples were heated at the highest possible heating rate so that off-equilibrium condition is maintained. The temperature of the sample was trailing behind furnace temperature during heating. Hence, the intensity was recorded with IR camera along with temperature with a rapid response thermocouple (coupled with the control thermocouple to reduce error) attached to the sample. The IR intensities were recorded in the range 100–600 ◦ C for the furnace (maximum setting temperature of the furnace) and 100–480 ◦ C for the sample, as it was difficult to maintain a good thermocouple–sample contact beyond 480 ◦ C as the sample started softening (melting point is 485 ◦ C). Care was taken to keep camera setting (filters, lenses, distance between camera and sample, etc.) identical to those used for recording during laser treatments. The raw data were used to determine temperature from the IR intensity value. More details of these particular efforts are available in other reference [8]. The general characterization of Fe3 O4 coated samples included cross-sectional examination of the coating and interface using optical BX60M Olympus microscope and Philip (FEI) XL30 FEG scanning electron microscope. Further, transmission electron microscopy was carried out using Tecnai-30 TEM to confirm any reaction that might have taken place in case of Fe3 O4 coating.
The intensity, I, versus temperature, T, determined in furnace heating experiments, was analyzed. It was established that the emissivity (ε) of all three samples was very close. Experimentally, it was not possible to go beyond the explored temperature range (100–600 ◦ C for the furnace and 100–485 ◦ C for the samples). It was found that the relation between I versus T was a fourth order polynomial. To find out T of the sample for a corresponding I, the value of I was substituted in fourth order polynomial relations. High correlation factor of the empirical polynomials (and both for blackbody and samples) indicate that the sample was very close to blackbody as far as emission of infrared radiation is concerned. Considering above factors and the ease and simplicity of use, it was considered appropriate to extrapolate the I–T relation for the sample beyond the experimental range to determine temperature from the intensity. As depicted in Fig. 2, the intensity of each pixel is recorded every 1/30 s. By using the empirical calibration relation developed in above-mentioned efforts [8], temperature can be calculated of the entire pixel array or of any particular set of points. To determine the maximum temperature (Tm ) during laser treatment several (10) such readings were taken in equal intervals and the average of values (T calculated from
Fig. 2. Schematic showing the pixels of a infrared thermograph during laser processing.
Fig. 3. Temperature profile across the laser interaction region in the direction of laser traverse (laser speed 325 cm/min).
3. Results and discussion 3.1. Temperature from infrared intensity
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Fig. 4. Temperature profile across the laser interaction region in the direction of laser traverse (laser speed 325 cm/min).
I) are presented in Table 1. The standard deviation associated with all the measurements presented was below 3.5% despite of rapid events intrinsic to laser processing and IR thermography (e.g., plume and vapor obstruction of field of view) suggesting that the experiments were well controlled and reproducible. The temperature of laser remelted sample and iron oxide on A319 can provide information if there is additional heat due to exothermic reactions (Eqs. (1) and (2)). Fig. 3 shows a typical temperature profile across the laser interaction region in the direction of laser traverse, i.e., X-axis as illustrated in Fig. 2. The temperature profile of FeO coating was found to be lower than Fe3 O4 coating. The precursor thickness, laser absorption, laser beam size and thermal characteristics of both oxides are comparable, but the amount of exotherm associated with the heat of reduction reaction in case of Fe3 O4 is much higher than that of
FeO. Hence, it can be concluded that the higher temperature and bigger area of interaction in case of Fe3 O4 coating than FeO and remelted samples (Fig. 3) were due to this strong exotherm of the thermite reaction. The temperature indicates a peak ahead of laser path. This can be explained as follows. As the laser moves in X-direction, rapid heating of the substrate takes place. Due to rapid speed of the process, higher superheating is required before melting can occur. Once it starts melting, the temperature drops a little due to absorption of latent heat of fusion and/or convective heat transfer within the melt-pool. Similarly, Fig. 4 shows a typical temperature profile across the laser interaction region in the direction perpendicular to laser traverse, i.e., Y-axis as illustrated in Fig. 2. The temperature is lower in the middle region than the periphery of laser interaction volume. This is because of the temperature due to superheating. Again, the temperature corresponding to Fe3 O4 coating was higher than FeO coating, which in turn was higher than remelted sample. Also, as can be seen from the width of melt region (in Figs. 3 and 4 and confirmed with melt track measurements), the volume of melt region under direct laser–material interaction is larger for Fe3 O4 coating than FeO coating, which in turn was higher than remelted sample. It can be seen from Table 1 that the maximum temperature recorded in several (10) frames (of the same coating and speed), Tm , is highest in case of Fe3 O4 coating followed by FeO coating and remelted sample for both the laser speeds. Even though both oxides undergo reduction reaction, Fe3 O4 coating exhibit higher maximum temperature, Tm due to extra heat supplied by the strong thermite reaction (Eq. 1) and greater readiness of Fe3 O4 to react with Al. The remelted sample exhibited lowest maximum temperature, Tm . For the given coating (Fe3 O4 or FeO) and substrate combination, the maximum temperature Tm did not show any distinctly
Fig. 5. Infrared thermograph of the (a) remelted sample and (b) Fe3 O4 coating.
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Table 2 Maximum temperature gradient during cooling, Gm (◦ C/m) Axis
Location
Maximum temperature gradient during cooling, Gm (◦ C/m) Fe3 O4 coating 325
X
Y
Laser track 1 Laser track 2 Laser track 3 Laser track 1 Laser track 2 Laser track 3
FeO coating 375
3.4 × 3.0 × 106 2.9 × 106 2.7 × 106 2.2 × 106 2.1 × 106 106
Remelted sample
325
3.5 × 3.1 × 106 3.1 × 106 1.9 × 106 1.9 × 106 1.9 × 106 106
375
3.1 × 2.9 × 106 2.8 × 106 2.2 × 106 2.1 × 106 2.1 × 106
325
3.1 × 3.0 × 106 3.1 × 106 2.3 × 106 2.1 × 106 2.1 × 106
106
375
3.0 × 2.6 × 106 2.3 × 106 1.9 × 106 1.9 × 106 1.9 × 106
106
3.1 × 106 3.0 × 106 2.8 × 106 2.0 × 106 1.9 × 106 2.0 × 106
106
325 and 375 represents the speed values in cm/min.
noticeable trend for a given traverse speed within a range employed in the present study. Fig. 5 represents instantaneous infrared thermograph for (a) remelted sample and (b) Fe3 O4 coating. The iron oxide coating indicated a vapor tail following the laser spot. The iron oxide reacts violently with aluminum around the laser spot and as a result produces the comet-like vapor tail that follows the laser spot. As hypothesized earlier, the thermite reaction is triggered by laser providing higher temperature (Table 1 and Figs. 3 and 4), larger melt region (Figs. 3 and 4) and direct reaction related activities as evident from Fig. 5. Such a phenomenon is also expected to affect thermal dynamics of laser treatment process. 3.2. Thermal dynamics: gradient and cooling rates Typical sets of instantaneous temperature distribution for different samples in X- and Y-directions (determined from the array of intensity values of the pixels in a typical thermograph) are presented in Figs. 3 and 4. From these figures, it is clear that the temperature gradient around the periphery of instantaneous laser spot is very high (difference of >250 ◦ C in just 100 m distance). Additionally laser travels at relatively rapid speeds (325 and 375 cm/min). The high thermal gradient and rapid traverse speeds translate to high cooling rates and severe thermal conditions. The solidification microstructure and hence the properties are strongly influenced by the cooling rate, which in turn, is primarily dictated by the temperature gradient (G), dT/dx or dT/dy. Hence, it was essential to estimate the temperature gradient, especially during cooling. From the temperature versus position data for a known laser traverse speed, it is possible to determine the temperature gradient and subsequently the cooling rate. The temperature profiles in X- and Y-directions were different (Figs. 3 and 4) as laser beam traversed linearly only in X-direction. Therefore, the cooling rates were calculated for both X- and Y-directions. Firstly, the temperature-position data is incremental (pixel by pixel) and therefore, discrete temperature gradients, G were calculated. The temperature gradient during cooling thus calculated can vary strongly as a function of position due to rapid speed of laser treatment. (For example, laser travels >91 pixels in the interval of consecutive two IR thermography frames.) As the solidification
front propagates opposite to heat extraction direction and is always associated with latent heat of fusion, the temperature gradient is highest close to the solidification front, which is hereafter called as maximum temperature gradient, Gm . The value of Gm was determined by selecting a highest value among all discrete G values in an IR thermograph frame and presented in Table 2. It can be seen that the maximum temperature gradients, Gm were highest for Fe3 O4 coating (Table 2). However, the gradient in Y-axis is though significantly different from sample to sample, does not follow any obvious trend. Several such frames were analyzed and found to possess minimal variations (<5%). The increase in laser traverse speed induces an increase in temperature gradient in X-direction as the moving heat source superimposes additional thermal gradient over that caused by a static heat source. As explained above, the maximum temperature gradient is of interest and hence the temperature corresponding to maximum temperature gradient, Tmg in an IR frame was calculated. Tmg was determining by finding temperature corresponding to position of Gm . The average values are presented in Table 3. It can be noted that the temperature corresponding to maximum gradient, Tmg is different (significantly lower) than the maximum temperature, Tm , recorded (Table 1). This is obvious as the highest temperature, Tm , corresponds to over-heated liquid within the melt-pool and the maximum temperature gradient occurs on the periphery of the meltpool where the solidification front is and hence corresponding temperature, Tmg . Higher temperature gradient, G is expected to provide higher cooling rate for different samples. The limited temporal and spatial resolution of infrared thermography combined with rapid speed of laser treatment pose challenge to Table 3 Temperature corresponding to maximum gradient, Tmg (◦ C) Location
Laser track 1 Laser track 2 Laser track 3
Temperature of maximum gradient, Tmg (◦ C) Fe3 O4 coating
FeO coating
Remelted sample
325
375
325
375
325
375
469 528 512
482 492 488
461 479 467
469 453 470
445 457 448
439 466 448
325 and 375 represents the speed values in cm/min.
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Table 4 Cooling rate calculated assuming non-steady state heat transfer, DT/Dt (◦ C/s) Axis
Non-steady state cooling rate, DT/Dt (◦ C/s)
Location
Fe3 O4 coating
FeO coating
325 X
Y
375
4.3 × 4.4 × 106 4.5 × 106 2.5 × 106 2.7 × 106 2.2 × 106
Laser track 1 Laser track 2 Laser track 3 Laser track 1 Laser track 2 Laser track 3
325
5.2 × 4.2 × 106 3.9 × 106 1.1 × 106 1.3 × 106 1.9 × 106
106
Remelted sample 375
4.5 × 2.7 × 106 2.0 × 106 1.8 × 106 1.3 × 106 1.2 × 106
106
325
3.4 × 2.0 × 106 4.4 × 106 2.6 × 106 1.7 × 106 1.8 × 106
106
106
375
6.8 × 5.9 × 106 2.8 × 106 2.0 × 106 1.8 × 106 2.3 × 106 106
5.4 × 106 4.0 × 106 5.0 × 106 1.9 × 106 2.2 × 106 1.8 × 106
325 and 375 represents the speed values in cm/min.
track 1 to track 3. Also, the corresponding standard deviations for a particular track were within 5%. This is an indication of close to steady state. It is often possible for such moving origin (heat source) problem to attain steady state (time independent), meaning the temperature profile around the laser–matter interaction remains unchanged. Therefore, attempts were made to calculate the cooling rates assuming a steady state heat transfer for a moving heat source (origin), which is, by its very nature, a problem easy to solve. The surface cooling rate DT/Dt in the direction opposite to laser traverse under steady state (∂T/∂t = 0) can be estimated as per steady state relation for the discrete data, in this case given in Eq. (4)
determine the cooling rate from IR intensity data. However, mathematical models are available to different degree of complexities to estimate cooling rates [8,16–19]. If the sample being laser treated is large enough compared to instantaneous laser interaction volume, and/or the sample is adequately cooled, the condition can be described by heat transfer equations for moving heat source (origin). If a body is moving relative to a frame of reference at speed ux and conducting heat primarily in the direction of motion (X-direction), then the equation in that reference frame (for constant properties) is given by the ‘substantial derivative’ instead of the partial derivative [8,16,17] that accounts for both spatial as well as temporal variations. When the temporal partial derivative is non-zero, then the problem is more generic in nature and is known as non-steady state heat transfer, i.e., the temperature profile around the origin (laser spot) varies with time. In the finite difference terms, the non-steady state cooling rate can be estimated from the pixel array double derivative (with respect to position) of temperature in X- or Y-directions and can be expressed as follows [8,20]: DT G q (T/x) q =α + =α + Dt x cp ρ x cp ρ
(3a)
DT G q (T/y) q =α + =α + Dt y cp ρ y cp ρ
(3b)
T T x T (4) = = ux t x t x where ux , is the laser traverse velocity. The above all temperature derivatives (DT/Dt, dT/dt, and ∂T/∂t) are mathematical (theoretical) description of infinitesimal variations. However, as mentioned earlier the IR thermography data is discrete in nature. For this distinction, T/t has been used. The thermography technique used here had a finite temporal resolution of t = 1/30 s and spatial resolution (pixel) of x = 20 m. The steady state cooling rates corresponding to maximum temperature gradient, Gm (from Table 2) and laser traverse velocity ux = 325 or 375 cm/min were calculated using Eq. (4). As seen in this equation, the cooling rate calculation depends upon rectilinear velocity (ux ) of the origin (heat source). No such term can be accounted for in Y-direction. Therefore, the cooling rates for each track in X-direction were only calculated and are presented in Table 5. They are about one order of magnitude lower than the corresponding cooling rates for non-steady state (Table 4). In non-steady state,
The cooling rates thus estimated in both X- and Y-directions are presented in Table 4. As expected the cooling rate in Xdirection is higher than that in Y-direction due to the effect of laser traverse. From Tables 1–4, it can be seen that the maximum temperature Tm , maximum temperature gradient Gm , temperature of maximum gradient Tmg , and non-steady state cooling rate DT/Dt change only very gradually from Table 5 Cooling rate calculated assuming steady state heat transfer, dT/dt (◦ C/s) Location
Steady state cooling rate, dT/dt (◦ C/s) Fe3 O4 coating 325
Laser track 1 Laser track 2 Laser track 3
FeO coating 375
1.9 × 1.7 × 105 1.8 × 105 105
325
2.2 × 2.0 × 105 1.9 × 105
325 and 375 represents the speed values in cm/min.
105
Remelted sample 375
1.7 × 1.6 × 105 1.5 × 105 105
325
2.0 × 1.9 × 105 1.6 × 105 105
375
1.6 × 1.3 × 105 1.1 × 105 105
1.9 × 105 1.8 × 105 1.8 × 105
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Fig. 6. Optical micrographs: microstructure of (a) as-received A319, (b) the remelted A319, (c) FeO coating and (d) Fe3 O4 coating at same magnification.
generally the instantaneous temperature gradient is higher compared to steady state and correspondingly it is expected to indicate higher cooling rate. It is difficult to accurately determine whether the thermal dynamics is closer to steady state or non-steady state. How-
ever, both predicted cooling rates are high (>105 ◦ C/s) for all the samples and processing conditions considered in this work. As postulated previously, such high cooling rate and off-equilibrium conditions in addition to violent thermite reaction expected to result in generating a microstructure with
Fig. 7. Transmission electron micrograph of laser induced iron oxide coating showing Fe3 O4 particle within cell.
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refined matrix grains reinforced with well-bonded iron oxide and/or the reaction products. 3.3. Microstructure evolution The cast alloys are most sensitive to the solidification rate. The high cooling rate associated results in highly refined microstructures in all laser treated samples. Fig. 6 presents the optical micrograph of (a) as-received A319 and (b) remelted sample. The laser-processed samples can be seen to possess very similar appearance. The oxide particles are not apparent in Fig. 6(c) and (d) perhaps due to their readiness to react with aluminum leaving behind little remnant. Eq. (3) (nonsteady state) is based on a steeper temperature gradient than that corresponding to Eq. (4) (steady state). Hence, these two relations provide the natural limits (upper and lower, respectively) of the surface cooling rate. Both values of the upper and lower limits being very high (>105 C/s), it is expected that the actual cooling rate responsible for microstructural refinement is some intermediate value, and therefore, the solidification rate in laser treatment is also expected to refine microstructure to great extent.
Fig. 7 illustrates a particle of Fe3 O4 incorporated in the aluminum cell. Energy dispersive spectroscopy (EDS) and diffraction pattern confirms that the particle is iron oxide. The matrix seen here is essentially cellular dendrite structure with the Si and CuAl2 phases occupying the intercellular region. However, there were much fewer oxide particles seen than expected. Even though the average particle size of the oxide used in precursor was about 15 m, the particles found under TEM rarely exceeded 1 m. This can be attributed to physical and chemical disintegration of oxide particles during laser processing. Under transmission electron microscope, such oxides were found to have reacted with aluminum. Fig. 8 shows a typical reaction zone. The oxide and aluminum has reacted along the interface. The interfacial long precipitate was analyzed using EDS and diffraction pattern and found to be Fe-aluminide (FeAl). Formation of such a reaction product in the interface between iron oxide and aluminum matrix ensures good bonding (reaction induced bonding) between the reinforcing ceramic and the matrix. In case of complete reaction, the FeAl is also well-bonded with the matrix and provide effective reinforcing [21]. The Fe3 O4 phase in Fig. 8 shows almost a ring-like diffraction pattern
Fig. 8. Transmission electron micrograph of reaction between a Fe3 O4 particle and Al matrix.
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indicating the precipitate is consisted of ultrafine grains. Iron oxides are known to be extremely tolerant to defects owing to multi-valance of Fe. The severe thermal conditions in laser processing and thermite reaction can produce ultrafine grains by physical fragmentation and/or introduction of high defect density. If the iron oxide particles eventually fragment and separate and/or react, the resulting ultrafine particles can provide very good reinforcement of the matrix.
4. Conclusion The iron oxides (Fe3 O4 and FeO) precursor reacted with A319 substrate during laser treatment. The temperature estimated using highly resolved (spatial) IR thermography technique revealed that the thermite reactions between iron oxide and aluminum produced higher heat of reaction compared to the remelted sample. The extent of heat of reaction between aluminum and iron oxide (higher for Fe3 O4 compared to FeO) is in line with observed temperature. The transmission electron microscopy indicated reactions between Al and Fe-oxide particles and physical disintegration of oxide particles. The reaction products present as precipitate on the interface was found to be FeAl, which might have formed as a result of subsequent reaction between Al and Fe formed in thermite reaction. In principle it is possible to utilize the heat of exothermic reaction for efficient surface engineering of aluminum alloys. Also, such a reaction induced wetting between the matrix and reinforcing ceramics shall result in an improved composite material.
Acknowledgements The authors acknowledge the infrared thermography work sponsored by the Assistant Secretary of Energy Efficiency and Renewable Energy, Office of Transportation Technologies, as part of the High Temperature Materials Laboratory User Program, and transmission electron microscopy facil-
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ity at the Oak Ridge National Laboratory through the Shared Research Equipment (SHaRE) User Center at the Oak Ridge National Laboratory, managed by UT-Battelle, LLC, for the U.S. Department of Energy under Contract No. DE-AC0500OR22725.
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