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Enhanced reactivity of Ni-Al reactive material formed by cold spraying combined with cold-pack rolling
Accepted Manuscript Enhanced reactivity of Ni-Al reactive material formed by cold spraying combined with cold-pack rolling Huilin Zhao, Chengwen Tan, Xiaodong Yu, Xianjin Ning, Zhihua Nie, Hongnian Cai, Fuchi Wang, Yan Cui PII:
S0925-8388(18)30171-3
DOI:
10.1016/j.jallcom.2018.01.170
Reference:
JALCOM 44634
To appear in:
Journal of Alloys and Compounds
Received Date: 23 October 2017 Revised Date:
10 January 2018
Accepted Date: 12 January 2018
Please cite this article as: H. Zhao, C. Tan, X. Yu, X. Ning, Z. Nie, H. Cai, F. Wang, Y. Cui, Enhanced reactivity of Ni-Al reactive material formed by cold spraying combined with cold-pack rolling, Journal of Alloys and Compounds (2018), doi: 10.1016/j.jallcom.2018.01.170. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT Enhanced reactivity of Ni-Al reactive material formed by cold spraying combined with cold-pack rolling
Huilin Zhaoa,b, Chengwen Tana,b,c,*, Xiaodong Yua,b,c, Xianjin Ninga,b, Zhihua Niea,b,
a
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Hongnian Caia,b, Fuchi Wanga,b, Yan Cuid
School of Materials Science and Engineering, Beijing Institute of Technology,
b
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Beijing 100081, China
National Key Laboratory of Science and Technology on Materials under Shock and
c
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Impact, Beijing Institute of Technology, Beijing 100081, PR China
Laboratory of Advanced Materials Behavior Characteristics, Beijing Institute of
Technology and Institute of Space Medico-Engineering, Beijing 100081, PR China Central Iron& Steel Research Institute, Beijing 100081, PR China
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d
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*Corresponding author.
E-mail address: [email protected] (C. Tan).
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ACCEPTED MANUSCRIPT Abstract Near-fully-dense (porosity<1%) Ni-Al reactive bulk materials were obtained by cold spraying and large deformation without crack was successfully achieved by the subsequent gradual cold-pack rolling, and the largest total thickness reduction could
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reach 95%. The influences of cold-pack rolling on the reactivity of the cold-sprayed Ni-Al reactive materials were investigated. The material reactivity was addressed through the solid-state reaction threshold temperature in thermal explosion mode
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conducted by differential scanning calorimetry and combustion wave velocity in self-propagating mode measured by a high-speed camera. For the cold-sprayed
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materials with rolling treatment, the onset of solid diffusion reaction shifted to a lower temperature range by 30-40°C in the DSC experiment. An obvious increase in the combustion reaction reactivity of the cold-sprayed samples after treatment with cold-pack rolling was demonstrated. The combustion wave velocities for the materials
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with fifteen gradual rolling passes could reach 130-140 mm/s. The combustion reaction velocity showed a linear dependence on the statistic results of contact area per unit volume(Sv) between Ni and Al reactants and an expression was proposed,
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giving an activation energy of 129 kJ/mol.
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Keywords: Nickel aluminum; Cold spraying; Pack rolling; Reaction velocity; Microstructural evolution
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ACCEPTED MANUSCRIPT 1. Introduction An aluminum- (Al-) metal mixture is one class of reactive materials in which a combustion reaction can be ignited using different methods, such as shock loading [1,2] and local or global heating [3,4]. The highly exothermic nature of Al-metal
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mixtures, associated with combustion reactivity, is sufficient for military applications, such as reactive fragments [5], shaped-charge liners [6], or penetrating warheads [7], and supplies additional energy and enhances damage effects. Of all the Al-metal
with
the
following
characteristics:
a
high
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mixtures, nickel-aluminum (Ni-Al) is an optimal exothermic mixture combination heat
of
reaction
per
unit
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mass/volume(1381J/g), adiabatic reaction temperature (1910K), and low cost [8]. Ni-Al reactive materials can be fabricated through a top-down mechanical process or a bottom-up deposition method. The mechanical processes include cold-isostatic pressing [9.10],cold rolling[11-14], and swaging[15,16]. Mechanical
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processing usually has the advantages of low cost and simple processing. Meanwhile, some disadvantages which would limit the application in reactive fragment are inevitable. For the cold-isostatic pressing, the fraction of the theoretical maximum
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density (TMD) of samples produced in this way is usually low, e.g., 80% [9]. High
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porosity and will decrease the reactivity and reduce the energy density. In the cold rolling, the severe plastic deformation can bonds the alternating Al and Ni sheets together and reduce the average bilayer thickness in the laminate. However, the two materials will deform at the different rate due to their different hardness. Cracks can be generated and become more prevalent as the extent of rolling increases,leading to lower materials utilization compared to the cold pressing. Prof. Timothy Weihs and co-workers[16] explored the rotary swaging of elemental powders for fabricating reactive rods. This mechanical method works by rapidly reducing the diameter of the 3
ACCEPTED MANUSCRIPT tube and increases the total packing density of the composite with little fracture. Like other mechanical methods, the compacted samples obtained by swaging are also limited to relatively simple geometry. Physical vapor deposition (PVD), especially magnetron sputtering, is a most
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method for multilayer films with the average reactant spacing on a nanometer scale [17-27]. The Ni-Al nanofoils have been studied extensively by Rogachev[19-22] and Weihs’ group[23-27]. Vapor deposition can be used to form dense multilayer nanofoils
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with unique reactive characteristics (e.g., a combustion velocities in the range of 10-100 m s−1 ,according to the recent review by Rogachev [19]). However, the
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geometry of sputtered materials is limited to foils.The high capital costs and the lower throughputs limit the application of reactive fragment in which the efficiency of bulk production is required.
The cold-gas dynamic spray process, or simply cold spraying, is an alternative
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deposition technique for rapidly manufacturing large quantities of sensitive energetic materials with high reactivity and low porosity [28,29].When the powders impinge on the base material, bonding will take place entirely through the expensive localized
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deformation due to the dynamic impact, which is similar to the mechanical
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interlocking[30,31].Compared to the PVD or the mechanical methods, an attractive advantage is the possibility to obtain complex shapes[32]. Meanwhile, the use of an inert carrier gas also limits oxidation of deposited materials [33]. The potential makes it an ideal candidate for the bulk production of reactive materials used in the military. Normally, considering the application for the reactive fragments, reaction threshold temperature and self-propagating reaction speed under static ignition conditions are important measurements for the reactivity in thermal explosion mode and self-propagating reaction mode, respectively. The diffusion distance and 4
ACCEPTED MANUSCRIPT surface-to-volume ratio of the contact area between reactants play an important role in the reactivity. The powders used in the cold-spray process are usually on the micrometer scale. A micrometer-scale microstructure means a relatively lower contact area compared with the nanoscale. Therefore, the cold-sprayed blanks should be
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further refined through severe plastic deformation to increase the contact area between reactants. However, the mechanical combination of the powders makes the room-temperature ductility of Ni-Al mixtures poor [34], limiting their further
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deformation. Pack rolling, as the most efficient way to fabricate brittle sheets, is a highly efficient means of obtaining product [35]. Pack rolling, which involves the
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rolling of “difficult-to-form” materials covered on all sides by a material of similar strength, has been widely used in fabricating magnesium alloys [36] or TiAl-based alloys [37], which exhibit poor formability at room temperature. The cold-sprayed blank will be in a three-dimensional compressive stress state provided by the pack,
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which could minimize fracture within the sample and thus leads to higher materials utilization compared to cold rolling. Therefore, the methods using cold spray combined with pack rolling will have many advantages. One is through the high
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velocity impact of spray processing breaking surface oxides of Al powders and
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enhancing bonding between powders. The second is through the adding of the pack leading to large deformation without crack and control the degree of microstructural refinement. There can be no doubt that the technique of High energy Ball Milling could enhance the reactivity and ignition sensitivity, which have been shown by Mukasyan and co-workers[38, 39]. However, owing to the cold welding, the size of the particles will increase and the powders should be sieved to isolate the particles 5-45µm size range for the cold spraying, reducing the powder utilization ratio and productivity. On the other hand, the process of intensive interaction between the 5
ACCEPTED MANUSCRIPT particles and milling media will cause the powder to flatten, and the irregular flaky powders with low fluidity will reduce the spray efficiency. In the present work, the influences of cold-pack rolling on the reactivity of the cold-sprayed Ni-Al reactive materials were investigated to obtain basic data for the
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future application as reactive fragment. The Ni-Al composites were fabricated by cold spraying combined with cold-pack rolling. The reactivity of these materials is addressed through the two basic modes of a combustion reaction. In thermal
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explosion mode, the threshold of the reaction is measured by differential scanning calorimetry (DSC). In self-propagating reaction mode, combustion wave velocity was
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measured by a high-speed camera. A combustion front quenching method was used to arrest the self-propagating combustion reaction. The phase analysis was performed using X-ray diffraction (XRD), and the microstructural evolution in the specimen after the combustion front quenching was observed and analyzed using scanning
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electron microscopy (SEM) in backscattered electron mode (BSE) and by energy-dispersive X-ray (EDX) spectrometry.
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2. Experimental procedures
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2.1. Materials manufacturing
High-purity spherical powders of Al and Ni measuring 5−30 µm in diameter
were deposited by cold spraying (KM-CDS3.2). The operating parameters used are presented in Table 1. The cold-sprayed bulk material is shown in Fig. 1(a). A near fully dense structure, in which porosity is less than 1%, was obtained , with the density, using the Archimedes method, measuring 5.17 g/cm3; the stoichiometric atomic ratio of Ni-Al approached 1:1. Fig. 1(b) shows a SEM image in BSE mode of the composites prior to cold-pack rolling, with the Ni particles (bright areas) 6
ACCEPTED MANUSCRIPT surrounded in a matrix of deformed Al. It should be noted that some of the Ni powders have agglomerated in the Al matrix (indicated by arrows) during spraying, resulting in a non-uniform microstructure of the cold-sprayed material, as shown in
Table 1 Cold spray process operating parameters.
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regions A and B.
Working gas
Stagnation temperature T(℃)
Pressure(MPa)
Substrate
Standoff distance(mm)
Ni+Al
He
150
0.6
6061-Al
15
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Sample
Fig. 1. (a) Bulk Ni-Al reactive material produced by cold spray; (b) cross-section
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view of cold-sprayed sample.
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The cold-sprayed (CS) sample was removed from the substrate on which it was deposited and Ni-Al composite sheets measuring 40×30×3 mm3 were cut from it and encapsulated in a 6061-Al alloy can. The pack design is shown in Fig. 2. The CS plate with the pack was rolled on a mill with a roller dimension of Φ250×400 mm2 for 3, 6, 9, 12, and 15 gradual 20% thickness reductions, respectively. We refer to the cold-rolled sheets using the number of thickness reductions; that is, CR3, CR6, CR9, CR12, and CR15.
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Fig. 2. (a) Schematic and (b) photograph of pack design.
Under the precise control of rolling parameters, including speed, reduction for
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each pass, and so on, a crack-free Ni-Al composite sheet can be fabricated through different gradual cold-rolling passes. The cold-pack-rolled Ni-Al sheets are shown in
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Figs. 3(a)–3(d). Owing to the characteristics of pack rolling, the two ends of the cold-sprayed bulk were extended into the 6061Al-alloy packing materials, and a crack-free sheet with large deformation was achieved. The total thickness reduction were in the range of 50%-95%. During compression loading, the rigid Ni particles
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move in the Al matrix [Figs. 3(e)–3(h)]. Such gradual 20% rolling reductions appear to produce a more uniform microstructure, which agrees with the results reported
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previously by Stover et al. [12] in Ni/Al reactive foils.
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Fig. 3. (a)–(d) macro-photographs of CR3, CR6, CR9, and CR15, respectively; (e)
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and (f) corresponding microstructure of rolling cross-section.
Contact area per unit volume (Sv) between Ni particles and the Al matrix was
characterized on the scanning images, using Image-Pro plus 6.0 software (Media Cybernetics, Inc., Rockville, MD, USA), to measure the extent of contact between the two reactants; Sv is expressed in µm−1. The measurements were made on each of 20 different cross-section microstructure images (200×) obtained by uniform random 9
ACCEPTED MANUSCRIPT sampling. The measurements for all the sheets are shown in Fig. 4. The statistical results showed an increase in Sv between the Ni particles and the Al matrix with more rolling numbers. Given the micrometer-sized reactants studied here, and the relatively low remperature increase during cold rolling, the effect of pre-reaction should be
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negligible.
Fig. 4. Dependence of contact area per unit volume on the number of rolling passes.
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2.2. Ignition threshold temperature
The material reactivity in thermal explosion mode was verified through DSC experiments carried out for the CS, CR6, and CR15 materials in a temperature range
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of 25–1000°C at a heating rate of 10°C/min under an Ar flow of 20 mL/min with a
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10-mg sample filling an Al2O3 DSC pan. Further analyses in the phase evolution were studied by controlled annealing method under the same heating conditions as in the DSC process.
2.3. Combustion wave velocity In the case of self-propagating reaction mode, the material reactivity of CS, CR6, and CR15 was addressed through measurements of the combustion wave velocity. 10
ACCEPTED MANUSCRIPT Samples were cut into rectangular strips along the rolling direction with dimensions of 40mm×10mm×0.1mm, which were then placed in a free-hanging form. Combustion reactions were ignited with flame at one end of the strips in the air atmosphere. The reaction was observed using a Phantom v.125 high-speed camera (Vision Research,
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Inc., Wayne, NJ, USA) set at 3200 fps. The video was then analyzed to measure the combustion wave propagation velocity based on the speed at which luminous front
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moved through the sample.
2.4. Combustion front quenching test
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Another sheet was sandwiched between two steel blocks, which function as heat sinks, keeping it partially exposed, and the reaction was initiated at the bare end, as shown in Fig. 5(a). A sample with an extinguished reaction front was obtained [Fig. 5(b)]. Phase analysis was performed by XRD, the microstructures were observed by
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SEM (the microscope was equipped with a backscattered-electron detector) and
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chemical analysis was carried out using EDX spectrometry.
Fig. 5. (a) Schematic of the quenching experiment; (b) photograph of quenched sample.
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ACCEPTED MANUSCRIPT 3. Experimental results 3.1. Ignition threshold temperature Multiple DSC scans were run on each of materials with a heating rate of 10°C/min,
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as shown in Fig. 6. There are three separated peaks in all of the curves (marked by letters). All three materials show two broad exotherms between 400 and 600°C, and the double-bottomed exotherm in the CS materials gradually diminishes. In this
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temperature range, these reactions were controlled by solid-state diffusion, and with increasing rolling number, the peaks of the first exotherm shifted to a lower
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temperature range by 30-40°C. The sharp exotherm at 640°C corresponding to the solid-liquid reaction, namely the thermal explosion reaction, is associated with the melting reaction of Al. In the DSC experiments, the relative magnitudes of the
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solid-state and solid-liquid reaction exotherms will correspond to the extent of the reaction. Considering to this, it is evident that the CS materials exhibit mostly a liquid-state reaction, and a small amount of reaction in the solid state. While, CR15
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materials exhibit the significantly largest solid-state reaction and only limited
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liquid-state reaction. The results indicated that the solid-state diffusion reaction were more prominent in the materials with rolling treatment. To identify the reaction products associated with each exothermic peak, the three
samples were heated up to 450°C, 500°C, 550°C, and 580°C respectively, and then quenched, using the same heating rate as in the previous DSC runs. The XRD patterns and cross-section of these quenched materials are shown in Figure 7. The as-sprayed sample annealed to 450°C only contained Ni and Al (Fig. 7a). In contrast, in the CR6 and CR15 materials that was annealed to 450°C, Al3Ni layers can be observed in the 12
ACCEPTED MANUSCRIPT SEM images(EDS analysis suggests that the Ni/Al atomic ratio is approximately 1/3). All the materials quenched at 500°C (peak A) consist of three phases: Ni, Al and Al3Ni (Fig. 7b). This indicates that the first exothermic peak observed in DSC (peak A)
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corresponds to the formation of Al3Ni. It has been shown that materials annealed to just before and after peak B (570°C) consist of the phases Ni, Al3Ni2, Al3Ni and Al. Due to the small amount of Al in the CR6 and CR15, it could not be identified from
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the XRD scan. Therefore, DSC peak positions A and B in the curve of the samples are
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associated with the exothermic formation of Al3Ni and Al3Ni2, respectively. This is also agreement with the previous reports [12]. The intensities of the intermediate product Al3Ni (Fig. 7b) and Al3Ni2 (Fig. 7d) peaks in the XRD scans may correspond to the extent of solid-state diffusion reaction after heating up to the same temperature
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number.
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within same time frame, and the degree was higher in the materials with more rolling
Fig. 6. DSC curves of the three materials.
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Fig.7. XRD patterns and SEM images of the three materials subjected to DSC scans up to (a) 450°C, (b) 500°C, (c) 550 and (d) 580°C.
3.2. Wave rate of combustion reaction
Fig. 8 illustrates the combustion wave propagation as a function of time. In this study, the materials were ignited with flame, and a pre-combustion region will appear
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at the end of ignition. After the self-propagating reaction occurred, the combustion wave propagates forward and then the propagating velocity tends to a macroscopically steady-state value at the distance of about 15-20mm from the
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ignition end. The combustion velocity was measured using the combustion wave in
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the steady-state stage, regardless of the pre-combustion stage. We refer to the beginning time of the steady-state self-propagating reaction as the zero point. The samples were hung to reduce conductive heat losses from the sample to the sample holder by decreasing the contact area between the two. Obviously, for the CR15 material, the largest extent of changes in shape and the tense flame indicated the enhanced reactivity achieved through cold rolling. A linear fit was made to the combustion wave growth distance versus time plots. The region of ignition and changes in shape were not taken into account when calculating the velocity. 15
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Fig. 8. Image sequence of combustion wave propagation and wave growth distance
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versus time for (a) CS, (b) CR6, and (c) CR15.
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The measured wave velocities of the combustion reaction for the CR6 and CR15 materials range from 100 to 110 mm/s and from 130 to 140 mm/s, respectively. For the cold-sprayed sheets, the velocities vary between 60 and 100 mm/s. The experimental results for the cold-sprayed materials are more dispersed than those of the cold-rolled samples. The velocities of all samples in this study are higher than those of the cold-pressed samples reported by Munir and Anselmi-Tamburini [40] in which the velocities were usually lower than 50 mm/s.
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ACCEPTED MANUSCRIPT 4. Discussion
4.1. Enhanced reactivity in Solid-state reaction
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In the thermal explosion mode, the influences of cold rolling on the enhanced reactivity were illustrated by the decreased onset temperature of the solid-state diffusion and the more extent within the same time frames of heating in the DSC.
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These results would be attributed to the accumulation of the mechanical energy and increased contact surface produced by the rolling, respectively. In the process, rolling
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treatment would play a similar role as mechanical activation produced by ball milling[41] or other mechanical methods such as swaging[16]. When the samples were heated under the given heating rate, the reaction were
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controlled by the solid diffusion below the eutectic temperature(640°C), and Al3Ni will be the first intermediate product, forming a layer between the reactants(Fig.7b). In the process, the reserve of the mechanical energy after cold rolling will act as the
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driving force, and result in a lower onset temperature of the interaction between the
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initial reactants, showing the shift of the first peak in DSC curves (Fig.6). Then, the solid-state reaction continues as the Al diffuses through to the intermediate product interface, and the Al3Ni2 will also form in a layer-by-layer manner. However, the Al3Ni has one semi-coherent interface with Al and this interface has a relatively high energy [42]. The intermediate product layer may act as a barrier and the stored mechanical energy may have been realized in the formation of Al3Ni. Therefore, the shift of second peak cannot be distinguished.
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ACCEPTED MANUSCRIPT In addition, there can be approximate square relationships between the time required for solid-state diffusion reaction and the diameter of initial particles at the same temperature adopted by Thadhani[1].The particles size also corresponds the
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diffusion distance, which is inversely proportional to the contact surface at a certain volume. Severe plastic deformation of the cold-sprayed materials occurred during the rolling (Fig.3), and as a result, the contact surface increases, as shown by the statistic
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measurements (Fig.4). Therefore, after rolling treatment, more intimate mixing made
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the solid-state reaction favored in the DSC, showing the weakness of the subsequent liquid-state reaction at eutectic temperature. 4.2. Reaction mechanism
It has been shown that the use of cold spraying combined with a pack rolling
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process allows fabrication of bulk samples with nearly full density and is proven to be beneficial for enhancing the reactivity via decreasing the reaction threshold
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temperature or increasing the combustion reaction velocity, for example. To obtain complementary information about the reaction, SEM and XRD were
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performed on the samples after quenching. Microstructural evolution is illustrated in Fig. 9 by showing a CS specimen at low magnification. Based on microstructural observations, it appears that the reacting zone restores most of the information about the combustion reaction.
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Fig. 9. Low-magnification micrograph of a typical quenched specimen.
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The XRD results are shown in Fig. 10. It can be observed that no oxidation
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occurred during the cold-spray process and the product is NiAl, which corresponds to the atomic ratio of the initial materials. In the reacting zone, Al3Ni and Al3Ni2 are also
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observed. Quenched specimens of the CR6 and CR15 also show the same feature.
Fig. 10. XRD patterns of cold-sprayed specimens after quenching.
The liquid associated with the melted Al has a very important effect on the self-propagating reaction [21,40]. As shown in Fig. 11(a), what took place first in the solid-liquid reaction was the formation of eutectic Al3Ni-Al owing to the melting of 19
ACCEPTED MANUSCRIPT the Al matrix and the dissolution of Ni particles in it. As the Ni content in the Al liquid increased due to the continuous dissolution of Ni particles, polygonal particles in the Al liquid were observed, and the EDX result (24.36 at.% Ni) corresponds to the crystallization of Al3Ni during the quenching, as shown in Fig. 11(b). With the
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continuous dissolution of Ni, more and more Al3Ni was transformed into Al3Ni2 (39.62 at.% Ni). Finally, NiAl gradually precipitated out in the saturated Al-Ni liquid,
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as shown in Fig. 11(d).
Fig. 11. (a) Ni particles immersed in the liquid Al, and (b) show the appearance of a large amount of regular Al3Ni, indicating an increase in Ni content in the Al-Ni liquid; (c) more Al3Ni2 precipitated out from the Al-Ni liquid; (d) NiAl precipitated, meaning that saturation was beginning to be reached.
From the microstructural evolution results, as mentioned above, the existence of a common dissolution-precipitation reaction mechanism as reported by Fan et al. [43] 20
ACCEPTED MANUSCRIPT can be suggested. In self-propagating mode, the combustion reaction was controlled by the solid-liquid reaction, and a similar reaction scheme can be expressed as follows [21]: Ni(s ) + Al(m ) → Ni(s ) + AlNi x (m ) → NiAl( s ) ,
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where s denotes solid and m melt, and x varies from 0.3 to 0.67. According to previous studies [44,45], it is important that the heat of Ni dissolution into the liquid Al can reach 60% of the overall heat of reaction. A self-sustained reaction wave can
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propagate due solely to the heat of dissolution. Therefore, the critical factor for determining the reaction velocity may be the dissolution of Ni. For the materials
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produced by cold pressing, the ignition process may involve the melt spreading of liquid Al over Ni particles due to the Ni particle being in partial contact with the Al droplet only at the Ni/Al interface [43,46–48]. In this work, all samples were first fabricated by cold spraying. According to the features of the microstructure (Fig. 1),
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the Ni particles are surrounded by the Al matrix, and the low porosity of the samples and increased contact area between the two reactants makes the Ni particle more easily wholly immersed and dissolved in the Al liquid. Hence, the model
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corresponding to the reaction mechanism in this work can be drawn schematically as
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in Fig. 12, which can be corroborated by the analysis proposed by Baras and Kondepudi[49].
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Fig. 12. Schematic of reaction mechanism of self-propagating reaction for cold-sprayed material.
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In addition, from both the SEM observation of the microstructure and the XRD pattern of the product region, it can be seen that the combustion reaction is complete,
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since in the product zone no other phase except NiAl was detected by XRD. In previous works, multiple phases, such as Al3Ni, Al3Ni2, and even unreacted Ni, were
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observed in the final product zone by Biswas and Roy [46] and Li et al. [48] under conditions using coarser Ni particles and a cold-press mixture. The large size of the Ni powders and the high porosity increases the time needed for the dissolution of Ni, and consequently, the combustion wave velocity of the cold spray is higher than that of cold-press pellets. Compared with the as-sprayed materials, the reaction mechanism treated by cold rolling was essentially the same. The enhancement in combustion velocity for these cold-rolled materials may be attributed to the increased velocity of Ni dissolution. For 22
ACCEPTED MANUSCRIPT all the samples, in which Ni particles were surrounded by the Al matrix, when the Ni particles were immersed in the Al liquid the dissolution was determined to occur in the contact area between reactants. The increase in the Al-Ni interfacial area due to cold rolling leads to more eutectic reactions during the dissolution period, allowing
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the liquid to be consumed much faster. As expected, Fig. 13 shows the difference in the reacting zone of forming liquid Al in the three quenched samples. As the rolling passes increased, more pores and Al3Ni were observed in the reacting zone adjacent to
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the initial zone, indicating that more liquid Al was consumed, which is also in agreement with the results of Naiborodenko and Itin [50], namely that the particle
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sizes of both reactants are important in determining the combustion velocity.
Fig. 13. Reacting zone of forming liquid Al in (a) CS, (b) CR6, and (c) CR15
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quenched materials.
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4.3. Modeling reaction velocity prediction The reaction velocities for these materials show a clear dependence on Sv
between Ni particles and the Al matrix, as plotted in Fig. 14,
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Fig. 14. Predictions and measurements of combustion reaction velocities versus
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contact area per unit volume.
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To predict the reaction velocity, modeling studies have been carried out to relate the velocity of the reaction wave to the thickness of the layers or the particle size. According to the analyses of Hardt et al. [51–53] and of Armstrong and Koszykowski [54], the rate shows an inverse dependence on the thickness of the reactant layers and
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by implication, on the particle size of the powders.
In the present work, the initial Ni particle size of these materials was all the same, and the Al reactants existed in the form of a matrix, showing the fact that most Ni
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particles appear flat geometry(Fig.3) after the spraying impact and rolling. Based on the analysis of Hardt et al.[51-53], a layered model was used for the derivation of
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velocity expression. Fig.15(a) showed the layered cell, in which dAl and dNi were the thickness of Al and Ni reactants, respectively. For the 1:1 Ni-Al stoichiometric system, the value of dAl:dNi is 3:2. δ in Fig.15(b) was represented as the penetration depth of heat flux going into the unreacted material by Goodman[55].
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Fig. 15. (a) Model of a layered cell, (b) schematic representation of reaction in the cell Firstly, the derivation of combustion rate was based on the Fourier’s
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one-dimensional heat transfer equation[56], and the simple form is
(1)
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∂T κ ∂ 2T Q ∂F = ⋅ + ⋅ ∂t cρ ∂x 2 c ∂t
Where κ is the thermal conductivity(J·cm−1·s−1·K−1), c is the heat capacity(J·g−1·K−1), and ρ is the density(g·cm−3), Q is the heat of reaction(J·g−1),F is the reacted fraction.
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The second term on the right-hand side of the Eq.(1) was associated with the heat generation of the reaction[57], and has been derived by Hardt and Phung[51] (2)
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∂F 1 D0 exp(− E ( RTa )) = ⋅ ∂t Wd Ni2 F
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Where the W is defined as 1 + d Al d Ni , for the 1:1 Ni-Al stoichiometric system its value is 2.5. The Wd Ni is the total thickness of a cell. Wd Ni = S V
(3)
Where S represents the contact area between Ni and Al reactants in the
cell(Fig.15(a)) and V is the volume.Therefore, the Wd Ni can be substituted by the statistic results Sv (µm-1) as mentioned above which can be obtained directly by the statistic measurements. The Eq.(2) becomes 25
ACCEPTED MANUSCRIPT ∂F D exp(− E ( RTa )) = S v2W ⋅ 0 ∂t F
(4)
D0 is the pre-exponential factor of the diffusion coefficient(cm2·s−1), E is the reaction activation energy, R is the universal gas constant(J·mol−1·K−1), Ta is the
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maximum reaction temperature (1911 K). In the present work, the combustion propagation processes demonstrate a macroscopically constant wave velocity(Fig.8). For a simple treatment, we assumed
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that the reaction occurs at a steady state to describe the general trend in one dimension.
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The temperature profile(Fig.16) in the steady state is constant with time and moves with a constant velocities[53]:
(5)
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x = u + vt
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Fig. 16. Temperature profile
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Using Eq.(5), Eq. (1) can be expressed as
∂T κ ∂ 2T Q ∂F −v⋅ = ⋅ + ⋅ ∂u cρ ∂u 2 c ∂t
(6)
Where Q/c = Ta-T0, and a quadratic polynomial to the temperature profile can be
expressed as T = Ta −
2(Ta − T0 )
δ
u+
Ta − T0
δ
2
u2
(7)
The δ mentioned above is
26
ACCEPTED MANUSCRIPT δ=
2κ cρ v
(8)
Using Eqs. (4), (7) and (8),on can write Eq. (6) at µ=0 as
κ
κ c 2 ρ 2 v 2 (Ta − T0 ) ∂F = ⋅ + (Ta − T0 ) ⋅ ( ) 2 cρ 2κ ∂t
(9)
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v⋅
cρv(Ta − T0 )
Finally, we derived the following expression for the relation between reaction velocity and contact area per unit volume(Sv)
2κWD0 exp − E RTa Cpρ
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v = Sv
(10)
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The values of the parameters used and their sources are listed in Table 2.
Table 2. Parameters of the combustion reaction. Units
Value
Reference
κ
J·cm−1·s−1·K−1
0.65
[53]
D0
cm2·s−1
1.0
[53]
Cp
J·g−1·K−1
0.753
[53]
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Parameter
The predicted velocities are shown in Fig. 14. The experimental observations of
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the dependence of velocity on Sv give an activation energy of 129 kJ/mol, which is reasonably close to the commonly accepted value of 125 kJ/mol for the Ni-Al system, a value that appears to reflect the rate-determining nature of the liquid-diffusion step [27,50]. The assumed values were chosen to give general agreement with the experimental trend. The variations in Sv are capable of explaining the corresponding variations in reaction velocities within the context of the model.
27
ACCEPTED MANUSCRIPT 5. Conclusions (1) Cold spraying has been found to be a promising technique for generating near-fully-dense reactive bulk materials with the porosity less than 1%. Using a combination of cold-pack rolling and cold spraying, a crack-free sheet with large
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deformation was achieved, the largest total thickness reduction could reach 95%. A more uniform microstructure and increased contact area between the reactant powders were produced by more gradual rolling passes.
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(2) In thermal explosion mode, the solid-solid reaction temperature threshold of cold-sprayed samples after cold-pack rolling treatment decreased. With increasing
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rolling number, the onset of solid diffusion reaction shifted to a lower temperature range by 30-40°C.
(3) In self-propagating mode, the combustion wave velocity of cold-sprayed samples increased after rolling. The combustion wave velocities for the materials with
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fifteen gradual rolling passes could reach 130-140 mm/s.
(4) The statistic results of contact area per unit volume between Ni and Al reactants (Sv) can be used to substitute the initial size of reactants and expression
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kJ/mol.
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showed a liner dependence of velocity on Sv, giving an activation energy of 129
Acknowledgement
The authors would like to acknowledge the Central Iron & Steel Research
Institute for its technical support in cold rolling. We also thank LetPub (www.letpub.com) for its linguistic assistance during the preparation of this manuscript.
28
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Funding This work was supported by the National Natural Science Foundation of China
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(51671031).
Conflicts of interest
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None.
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Highlights
Reactive bulk materials were deformed without cracks by gradual cold-pack rolling.
The contact area per unit volume (Sv) between the Ni and Al increased after
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rolling. The rolling leads to a decrease in the solid-solid reaction temperature threshold.
An enhanced combustion velocity after rolling were demonstrated.
The combustion reaction velocity showed a linear dependence on the Sv.
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S0925-8388(18)30171-3
DOI:
10.1016/j.jallcom.2018.01.170
Reference:
JALCOM 44634
To appear in:
Journal of Alloys and Compounds
Received Date: 23 October 2017 Revised Date:
10 January 2018
Accepted Date: 12 January 2018
Please cite this article as: H. Zhao, C. Tan, X. Yu, X. Ning, Z. Nie, H. Cai, F. Wang, Y. Cui, Enhanced reactivity of Ni-Al reactive material formed by cold spraying combined with cold-pack rolling, Journal of Alloys and Compounds (2018), doi: 10.1016/j.jallcom.2018.01.170. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT Enhanced reactivity of Ni-Al reactive material formed by cold spraying combined with cold-pack rolling
Huilin Zhaoa,b, Chengwen Tana,b,c,*, Xiaodong Yua,b,c, Xianjin Ninga,b, Zhihua Niea,b,
a
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Hongnian Caia,b, Fuchi Wanga,b, Yan Cuid
School of Materials Science and Engineering, Beijing Institute of Technology,
b
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Beijing 100081, China
National Key Laboratory of Science and Technology on Materials under Shock and
c
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Impact, Beijing Institute of Technology, Beijing 100081, PR China
Laboratory of Advanced Materials Behavior Characteristics, Beijing Institute of
Technology and Institute of Space Medico-Engineering, Beijing 100081, PR China Central Iron& Steel Research Institute, Beijing 100081, PR China
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d
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*Corresponding author.
E-mail address: [email protected] (C. Tan).
1
ACCEPTED MANUSCRIPT Abstract Near-fully-dense (porosity<1%) Ni-Al reactive bulk materials were obtained by cold spraying and large deformation without crack was successfully achieved by the subsequent gradual cold-pack rolling, and the largest total thickness reduction could
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reach 95%. The influences of cold-pack rolling on the reactivity of the cold-sprayed Ni-Al reactive materials were investigated. The material reactivity was addressed through the solid-state reaction threshold temperature in thermal explosion mode
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conducted by differential scanning calorimetry and combustion wave velocity in self-propagating mode measured by a high-speed camera. For the cold-sprayed
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materials with rolling treatment, the onset of solid diffusion reaction shifted to a lower temperature range by 30-40°C in the DSC experiment. An obvious increase in the combustion reaction reactivity of the cold-sprayed samples after treatment with cold-pack rolling was demonstrated. The combustion wave velocities for the materials
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with fifteen gradual rolling passes could reach 130-140 mm/s. The combustion reaction velocity showed a linear dependence on the statistic results of contact area per unit volume(Sv) between Ni and Al reactants and an expression was proposed,
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giving an activation energy of 129 kJ/mol.
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Keywords: Nickel aluminum; Cold spraying; Pack rolling; Reaction velocity; Microstructural evolution
2
ACCEPTED MANUSCRIPT 1. Introduction An aluminum- (Al-) metal mixture is one class of reactive materials in which a combustion reaction can be ignited using different methods, such as shock loading [1,2] and local or global heating [3,4]. The highly exothermic nature of Al-metal
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mixtures, associated with combustion reactivity, is sufficient for military applications, such as reactive fragments [5], shaped-charge liners [6], or penetrating warheads [7], and supplies additional energy and enhances damage effects. Of all the Al-metal
with
the
following
characteristics:
a
high
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mixtures, nickel-aluminum (Ni-Al) is an optimal exothermic mixture combination heat
of
reaction
per
unit
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mass/volume(1381J/g), adiabatic reaction temperature (1910K), and low cost [8]. Ni-Al reactive materials can be fabricated through a top-down mechanical process or a bottom-up deposition method. The mechanical processes include cold-isostatic pressing [9.10],cold rolling[11-14], and swaging[15,16]. Mechanical
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processing usually has the advantages of low cost and simple processing. Meanwhile, some disadvantages which would limit the application in reactive fragment are inevitable. For the cold-isostatic pressing, the fraction of the theoretical maximum
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density (TMD) of samples produced in this way is usually low, e.g., 80% [9]. High
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porosity and will decrease the reactivity and reduce the energy density. In the cold rolling, the severe plastic deformation can bonds the alternating Al and Ni sheets together and reduce the average bilayer thickness in the laminate. However, the two materials will deform at the different rate due to their different hardness. Cracks can be generated and become more prevalent as the extent of rolling increases,leading to lower materials utilization compared to the cold pressing. Prof. Timothy Weihs and co-workers[16] explored the rotary swaging of elemental powders for fabricating reactive rods. This mechanical method works by rapidly reducing the diameter of the 3
ACCEPTED MANUSCRIPT tube and increases the total packing density of the composite with little fracture. Like other mechanical methods, the compacted samples obtained by swaging are also limited to relatively simple geometry. Physical vapor deposition (PVD), especially magnetron sputtering, is a most
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method for multilayer films with the average reactant spacing on a nanometer scale [17-27]. The Ni-Al nanofoils have been studied extensively by Rogachev[19-22] and Weihs’ group[23-27]. Vapor deposition can be used to form dense multilayer nanofoils
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with unique reactive characteristics (e.g., a combustion velocities in the range of 10-100 m s−1 ,according to the recent review by Rogachev [19]). However, the
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geometry of sputtered materials is limited to foils.The high capital costs and the lower throughputs limit the application of reactive fragment in which the efficiency of bulk production is required.
The cold-gas dynamic spray process, or simply cold spraying, is an alternative
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deposition technique for rapidly manufacturing large quantities of sensitive energetic materials with high reactivity and low porosity [28,29].When the powders impinge on the base material, bonding will take place entirely through the expensive localized
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deformation due to the dynamic impact, which is similar to the mechanical
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interlocking[30,31].Compared to the PVD or the mechanical methods, an attractive advantage is the possibility to obtain complex shapes[32]. Meanwhile, the use of an inert carrier gas also limits oxidation of deposited materials [33]. The potential makes it an ideal candidate for the bulk production of reactive materials used in the military. Normally, considering the application for the reactive fragments, reaction threshold temperature and self-propagating reaction speed under static ignition conditions are important measurements for the reactivity in thermal explosion mode and self-propagating reaction mode, respectively. The diffusion distance and 4
ACCEPTED MANUSCRIPT surface-to-volume ratio of the contact area between reactants play an important role in the reactivity. The powders used in the cold-spray process are usually on the micrometer scale. A micrometer-scale microstructure means a relatively lower contact area compared with the nanoscale. Therefore, the cold-sprayed blanks should be
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further refined through severe plastic deformation to increase the contact area between reactants. However, the mechanical combination of the powders makes the room-temperature ductility of Ni-Al mixtures poor [34], limiting their further
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deformation. Pack rolling, as the most efficient way to fabricate brittle sheets, is a highly efficient means of obtaining product [35]. Pack rolling, which involves the
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rolling of “difficult-to-form” materials covered on all sides by a material of similar strength, has been widely used in fabricating magnesium alloys [36] or TiAl-based alloys [37], which exhibit poor formability at room temperature. The cold-sprayed blank will be in a three-dimensional compressive stress state provided by the pack,
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which could minimize fracture within the sample and thus leads to higher materials utilization compared to cold rolling. Therefore, the methods using cold spray combined with pack rolling will have many advantages. One is through the high
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velocity impact of spray processing breaking surface oxides of Al powders and
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enhancing bonding between powders. The second is through the adding of the pack leading to large deformation without crack and control the degree of microstructural refinement. There can be no doubt that the technique of High energy Ball Milling could enhance the reactivity and ignition sensitivity, which have been shown by Mukasyan and co-workers[38, 39]. However, owing to the cold welding, the size of the particles will increase and the powders should be sieved to isolate the particles 5-45µm size range for the cold spraying, reducing the powder utilization ratio and productivity. On the other hand, the process of intensive interaction between the 5
ACCEPTED MANUSCRIPT particles and milling media will cause the powder to flatten, and the irregular flaky powders with low fluidity will reduce the spray efficiency. In the present work, the influences of cold-pack rolling on the reactivity of the cold-sprayed Ni-Al reactive materials were investigated to obtain basic data for the
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future application as reactive fragment. The Ni-Al composites were fabricated by cold spraying combined with cold-pack rolling. The reactivity of these materials is addressed through the two basic modes of a combustion reaction. In thermal
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explosion mode, the threshold of the reaction is measured by differential scanning calorimetry (DSC). In self-propagating reaction mode, combustion wave velocity was
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measured by a high-speed camera. A combustion front quenching method was used to arrest the self-propagating combustion reaction. The phase analysis was performed using X-ray diffraction (XRD), and the microstructural evolution in the specimen after the combustion front quenching was observed and analyzed using scanning
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electron microscopy (SEM) in backscattered electron mode (BSE) and by energy-dispersive X-ray (EDX) spectrometry.
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2. Experimental procedures
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2.1. Materials manufacturing
High-purity spherical powders of Al and Ni measuring 5−30 µm in diameter
were deposited by cold spraying (KM-CDS3.2). The operating parameters used are presented in Table 1. The cold-sprayed bulk material is shown in Fig. 1(a). A near fully dense structure, in which porosity is less than 1%, was obtained , with the density, using the Archimedes method, measuring 5.17 g/cm3; the stoichiometric atomic ratio of Ni-Al approached 1:1. Fig. 1(b) shows a SEM image in BSE mode of the composites prior to cold-pack rolling, with the Ni particles (bright areas) 6
ACCEPTED MANUSCRIPT surrounded in a matrix of deformed Al. It should be noted that some of the Ni powders have agglomerated in the Al matrix (indicated by arrows) during spraying, resulting in a non-uniform microstructure of the cold-sprayed material, as shown in
Table 1 Cold spray process operating parameters.
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regions A and B.
Working gas
Stagnation temperature T(℃)
Pressure(MPa)
Substrate
Standoff distance(mm)
Ni+Al
He
150
0.6
6061-Al
15
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Sample
Fig. 1. (a) Bulk Ni-Al reactive material produced by cold spray; (b) cross-section
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view of cold-sprayed sample.
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The cold-sprayed (CS) sample was removed from the substrate on which it was deposited and Ni-Al composite sheets measuring 40×30×3 mm3 were cut from it and encapsulated in a 6061-Al alloy can. The pack design is shown in Fig. 2. The CS plate with the pack was rolled on a mill with a roller dimension of Φ250×400 mm2 for 3, 6, 9, 12, and 15 gradual 20% thickness reductions, respectively. We refer to the cold-rolled sheets using the number of thickness reductions; that is, CR3, CR6, CR9, CR12, and CR15.
7
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Fig. 2. (a) Schematic and (b) photograph of pack design.
Under the precise control of rolling parameters, including speed, reduction for
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each pass, and so on, a crack-free Ni-Al composite sheet can be fabricated through different gradual cold-rolling passes. The cold-pack-rolled Ni-Al sheets are shown in
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Figs. 3(a)–3(d). Owing to the characteristics of pack rolling, the two ends of the cold-sprayed bulk were extended into the 6061Al-alloy packing materials, and a crack-free sheet with large deformation was achieved. The total thickness reduction were in the range of 50%-95%. During compression loading, the rigid Ni particles
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move in the Al matrix [Figs. 3(e)–3(h)]. Such gradual 20% rolling reductions appear to produce a more uniform microstructure, which agrees with the results reported
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previously by Stover et al. [12] in Ni/Al reactive foils.
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Fig. 3. (a)–(d) macro-photographs of CR3, CR6, CR9, and CR15, respectively; (e)
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and (f) corresponding microstructure of rolling cross-section.
Contact area per unit volume (Sv) between Ni particles and the Al matrix was
characterized on the scanning images, using Image-Pro plus 6.0 software (Media Cybernetics, Inc., Rockville, MD, USA), to measure the extent of contact between the two reactants; Sv is expressed in µm−1. The measurements were made on each of 20 different cross-section microstructure images (200×) obtained by uniform random 9
ACCEPTED MANUSCRIPT sampling. The measurements for all the sheets are shown in Fig. 4. The statistical results showed an increase in Sv between the Ni particles and the Al matrix with more rolling numbers. Given the micrometer-sized reactants studied here, and the relatively low remperature increase during cold rolling, the effect of pre-reaction should be
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negligible.
Fig. 4. Dependence of contact area per unit volume on the number of rolling passes.
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2.2. Ignition threshold temperature
The material reactivity in thermal explosion mode was verified through DSC experiments carried out for the CS, CR6, and CR15 materials in a temperature range
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of 25–1000°C at a heating rate of 10°C/min under an Ar flow of 20 mL/min with a
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10-mg sample filling an Al2O3 DSC pan. Further analyses in the phase evolution were studied by controlled annealing method under the same heating conditions as in the DSC process.
2.3. Combustion wave velocity In the case of self-propagating reaction mode, the material reactivity of CS, CR6, and CR15 was addressed through measurements of the combustion wave velocity. 10
ACCEPTED MANUSCRIPT Samples were cut into rectangular strips along the rolling direction with dimensions of 40mm×10mm×0.1mm, which were then placed in a free-hanging form. Combustion reactions were ignited with flame at one end of the strips in the air atmosphere. The reaction was observed using a Phantom v.125 high-speed camera (Vision Research,
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Inc., Wayne, NJ, USA) set at 3200 fps. The video was then analyzed to measure the combustion wave propagation velocity based on the speed at which luminous front
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moved through the sample.
2.4. Combustion front quenching test
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Another sheet was sandwiched between two steel blocks, which function as heat sinks, keeping it partially exposed, and the reaction was initiated at the bare end, as shown in Fig. 5(a). A sample with an extinguished reaction front was obtained [Fig. 5(b)]. Phase analysis was performed by XRD, the microstructures were observed by
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SEM (the microscope was equipped with a backscattered-electron detector) and
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chemical analysis was carried out using EDX spectrometry.
Fig. 5. (a) Schematic of the quenching experiment; (b) photograph of quenched sample.
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ACCEPTED MANUSCRIPT 3. Experimental results 3.1. Ignition threshold temperature Multiple DSC scans were run on each of materials with a heating rate of 10°C/min,
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as shown in Fig. 6. There are three separated peaks in all of the curves (marked by letters). All three materials show two broad exotherms between 400 and 600°C, and the double-bottomed exotherm in the CS materials gradually diminishes. In this
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temperature range, these reactions were controlled by solid-state diffusion, and with increasing rolling number, the peaks of the first exotherm shifted to a lower
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temperature range by 30-40°C. The sharp exotherm at 640°C corresponding to the solid-liquid reaction, namely the thermal explosion reaction, is associated with the melting reaction of Al. In the DSC experiments, the relative magnitudes of the
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solid-state and solid-liquid reaction exotherms will correspond to the extent of the reaction. Considering to this, it is evident that the CS materials exhibit mostly a liquid-state reaction, and a small amount of reaction in the solid state. While, CR15
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materials exhibit the significantly largest solid-state reaction and only limited
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liquid-state reaction. The results indicated that the solid-state diffusion reaction were more prominent in the materials with rolling treatment. To identify the reaction products associated with each exothermic peak, the three
samples were heated up to 450°C, 500°C, 550°C, and 580°C respectively, and then quenched, using the same heating rate as in the previous DSC runs. The XRD patterns and cross-section of these quenched materials are shown in Figure 7. The as-sprayed sample annealed to 450°C only contained Ni and Al (Fig. 7a). In contrast, in the CR6 and CR15 materials that was annealed to 450°C, Al3Ni layers can be observed in the 12
ACCEPTED MANUSCRIPT SEM images(EDS analysis suggests that the Ni/Al atomic ratio is approximately 1/3). All the materials quenched at 500°C (peak A) consist of three phases: Ni, Al and Al3Ni (Fig. 7b). This indicates that the first exothermic peak observed in DSC (peak A)
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corresponds to the formation of Al3Ni. It has been shown that materials annealed to just before and after peak B (570°C) consist of the phases Ni, Al3Ni2, Al3Ni and Al. Due to the small amount of Al in the CR6 and CR15, it could not be identified from
SC
the XRD scan. Therefore, DSC peak positions A and B in the curve of the samples are
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associated with the exothermic formation of Al3Ni and Al3Ni2, respectively. This is also agreement with the previous reports [12]. The intensities of the intermediate product Al3Ni (Fig. 7b) and Al3Ni2 (Fig. 7d) peaks in the XRD scans may correspond to the extent of solid-state diffusion reaction after heating up to the same temperature
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number.
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within same time frame, and the degree was higher in the materials with more rolling
Fig. 6. DSC curves of the three materials.
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Fig.7. XRD patterns and SEM images of the three materials subjected to DSC scans up to (a) 450°C, (b) 500°C, (c) 550 and (d) 580°C.
3.2. Wave rate of combustion reaction
Fig. 8 illustrates the combustion wave propagation as a function of time. In this study, the materials were ignited with flame, and a pre-combustion region will appear
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at the end of ignition. After the self-propagating reaction occurred, the combustion wave propagates forward and then the propagating velocity tends to a macroscopically steady-state value at the distance of about 15-20mm from the
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ignition end. The combustion velocity was measured using the combustion wave in
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the steady-state stage, regardless of the pre-combustion stage. We refer to the beginning time of the steady-state self-propagating reaction as the zero point. The samples were hung to reduce conductive heat losses from the sample to the sample holder by decreasing the contact area between the two. Obviously, for the CR15 material, the largest extent of changes in shape and the tense flame indicated the enhanced reactivity achieved through cold rolling. A linear fit was made to the combustion wave growth distance versus time plots. The region of ignition and changes in shape were not taken into account when calculating the velocity. 15
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Fig. 8. Image sequence of combustion wave propagation and wave growth distance
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versus time for (a) CS, (b) CR6, and (c) CR15.
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The measured wave velocities of the combustion reaction for the CR6 and CR15 materials range from 100 to 110 mm/s and from 130 to 140 mm/s, respectively. For the cold-sprayed sheets, the velocities vary between 60 and 100 mm/s. The experimental results for the cold-sprayed materials are more dispersed than those of the cold-rolled samples. The velocities of all samples in this study are higher than those of the cold-pressed samples reported by Munir and Anselmi-Tamburini [40] in which the velocities were usually lower than 50 mm/s.
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ACCEPTED MANUSCRIPT 4. Discussion
4.1. Enhanced reactivity in Solid-state reaction
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In the thermal explosion mode, the influences of cold rolling on the enhanced reactivity were illustrated by the decreased onset temperature of the solid-state diffusion and the more extent within the same time frames of heating in the DSC.
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These results would be attributed to the accumulation of the mechanical energy and increased contact surface produced by the rolling, respectively. In the process, rolling
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treatment would play a similar role as mechanical activation produced by ball milling[41] or other mechanical methods such as swaging[16]. When the samples were heated under the given heating rate, the reaction were
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controlled by the solid diffusion below the eutectic temperature(640°C), and Al3Ni will be the first intermediate product, forming a layer between the reactants(Fig.7b). In the process, the reserve of the mechanical energy after cold rolling will act as the
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driving force, and result in a lower onset temperature of the interaction between the
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initial reactants, showing the shift of the first peak in DSC curves (Fig.6). Then, the solid-state reaction continues as the Al diffuses through to the intermediate product interface, and the Al3Ni2 will also form in a layer-by-layer manner. However, the Al3Ni has one semi-coherent interface with Al and this interface has a relatively high energy [42]. The intermediate product layer may act as a barrier and the stored mechanical energy may have been realized in the formation of Al3Ni. Therefore, the shift of second peak cannot be distinguished.
17
ACCEPTED MANUSCRIPT In addition, there can be approximate square relationships between the time required for solid-state diffusion reaction and the diameter of initial particles at the same temperature adopted by Thadhani[1].The particles size also corresponds the
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diffusion distance, which is inversely proportional to the contact surface at a certain volume. Severe plastic deformation of the cold-sprayed materials occurred during the rolling (Fig.3), and as a result, the contact surface increases, as shown by the statistic
SC
measurements (Fig.4). Therefore, after rolling treatment, more intimate mixing made
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the solid-state reaction favored in the DSC, showing the weakness of the subsequent liquid-state reaction at eutectic temperature. 4.2. Reaction mechanism
It has been shown that the use of cold spraying combined with a pack rolling
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process allows fabrication of bulk samples with nearly full density and is proven to be beneficial for enhancing the reactivity via decreasing the reaction threshold
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temperature or increasing the combustion reaction velocity, for example. To obtain complementary information about the reaction, SEM and XRD were
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performed on the samples after quenching. Microstructural evolution is illustrated in Fig. 9 by showing a CS specimen at low magnification. Based on microstructural observations, it appears that the reacting zone restores most of the information about the combustion reaction.
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Fig. 9. Low-magnification micrograph of a typical quenched specimen.
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The XRD results are shown in Fig. 10. It can be observed that no oxidation
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occurred during the cold-spray process and the product is NiAl, which corresponds to the atomic ratio of the initial materials. In the reacting zone, Al3Ni and Al3Ni2 are also
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observed. Quenched specimens of the CR6 and CR15 also show the same feature.
Fig. 10. XRD patterns of cold-sprayed specimens after quenching.
The liquid associated with the melted Al has a very important effect on the self-propagating reaction [21,40]. As shown in Fig. 11(a), what took place first in the solid-liquid reaction was the formation of eutectic Al3Ni-Al owing to the melting of 19
ACCEPTED MANUSCRIPT the Al matrix and the dissolution of Ni particles in it. As the Ni content in the Al liquid increased due to the continuous dissolution of Ni particles, polygonal particles in the Al liquid were observed, and the EDX result (24.36 at.% Ni) corresponds to the crystallization of Al3Ni during the quenching, as shown in Fig. 11(b). With the
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continuous dissolution of Ni, more and more Al3Ni was transformed into Al3Ni2 (39.62 at.% Ni). Finally, NiAl gradually precipitated out in the saturated Al-Ni liquid,
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as shown in Fig. 11(d).
Fig. 11. (a) Ni particles immersed in the liquid Al, and (b) show the appearance of a large amount of regular Al3Ni, indicating an increase in Ni content in the Al-Ni liquid; (c) more Al3Ni2 precipitated out from the Al-Ni liquid; (d) NiAl precipitated, meaning that saturation was beginning to be reached.
From the microstructural evolution results, as mentioned above, the existence of a common dissolution-precipitation reaction mechanism as reported by Fan et al. [43] 20
ACCEPTED MANUSCRIPT can be suggested. In self-propagating mode, the combustion reaction was controlled by the solid-liquid reaction, and a similar reaction scheme can be expressed as follows [21]: Ni(s ) + Al(m ) → Ni(s ) + AlNi x (m ) → NiAl( s ) ,
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where s denotes solid and m melt, and x varies from 0.3 to 0.67. According to previous studies [44,45], it is important that the heat of Ni dissolution into the liquid Al can reach 60% of the overall heat of reaction. A self-sustained reaction wave can
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propagate due solely to the heat of dissolution. Therefore, the critical factor for determining the reaction velocity may be the dissolution of Ni. For the materials
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produced by cold pressing, the ignition process may involve the melt spreading of liquid Al over Ni particles due to the Ni particle being in partial contact with the Al droplet only at the Ni/Al interface [43,46–48]. In this work, all samples were first fabricated by cold spraying. According to the features of the microstructure (Fig. 1),
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the Ni particles are surrounded by the Al matrix, and the low porosity of the samples and increased contact area between the two reactants makes the Ni particle more easily wholly immersed and dissolved in the Al liquid. Hence, the model
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corresponding to the reaction mechanism in this work can be drawn schematically as
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in Fig. 12, which can be corroborated by the analysis proposed by Baras and Kondepudi[49].
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Fig. 12. Schematic of reaction mechanism of self-propagating reaction for cold-sprayed material.
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In addition, from both the SEM observation of the microstructure and the XRD pattern of the product region, it can be seen that the combustion reaction is complete,
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since in the product zone no other phase except NiAl was detected by XRD. In previous works, multiple phases, such as Al3Ni, Al3Ni2, and even unreacted Ni, were
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observed in the final product zone by Biswas and Roy [46] and Li et al. [48] under conditions using coarser Ni particles and a cold-press mixture. The large size of the Ni powders and the high porosity increases the time needed for the dissolution of Ni, and consequently, the combustion wave velocity of the cold spray is higher than that of cold-press pellets. Compared with the as-sprayed materials, the reaction mechanism treated by cold rolling was essentially the same. The enhancement in combustion velocity for these cold-rolled materials may be attributed to the increased velocity of Ni dissolution. For 22
ACCEPTED MANUSCRIPT all the samples, in which Ni particles were surrounded by the Al matrix, when the Ni particles were immersed in the Al liquid the dissolution was determined to occur in the contact area between reactants. The increase in the Al-Ni interfacial area due to cold rolling leads to more eutectic reactions during the dissolution period, allowing
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the liquid to be consumed much faster. As expected, Fig. 13 shows the difference in the reacting zone of forming liquid Al in the three quenched samples. As the rolling passes increased, more pores and Al3Ni were observed in the reacting zone adjacent to
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the initial zone, indicating that more liquid Al was consumed, which is also in agreement with the results of Naiborodenko and Itin [50], namely that the particle
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sizes of both reactants are important in determining the combustion velocity.
Fig. 13. Reacting zone of forming liquid Al in (a) CS, (b) CR6, and (c) CR15
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quenched materials.
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4.3. Modeling reaction velocity prediction The reaction velocities for these materials show a clear dependence on Sv
between Ni particles and the Al matrix, as plotted in Fig. 14,
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Fig. 14. Predictions and measurements of combustion reaction velocities versus
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contact area per unit volume.
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To predict the reaction velocity, modeling studies have been carried out to relate the velocity of the reaction wave to the thickness of the layers or the particle size. According to the analyses of Hardt et al. [51–53] and of Armstrong and Koszykowski [54], the rate shows an inverse dependence on the thickness of the reactant layers and
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by implication, on the particle size of the powders.
In the present work, the initial Ni particle size of these materials was all the same, and the Al reactants existed in the form of a matrix, showing the fact that most Ni
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particles appear flat geometry(Fig.3) after the spraying impact and rolling. Based on the analysis of Hardt et al.[51-53], a layered model was used for the derivation of
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velocity expression. Fig.15(a) showed the layered cell, in which dAl and dNi were the thickness of Al and Ni reactants, respectively. For the 1:1 Ni-Al stoichiometric system, the value of dAl:dNi is 3:2. δ in Fig.15(b) was represented as the penetration depth of heat flux going into the unreacted material by Goodman[55].
24
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Fig. 15. (a) Model of a layered cell, (b) schematic representation of reaction in the cell Firstly, the derivation of combustion rate was based on the Fourier’s
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one-dimensional heat transfer equation[56], and the simple form is
(1)
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∂T κ ∂ 2T Q ∂F = ⋅ + ⋅ ∂t cρ ∂x 2 c ∂t
Where κ is the thermal conductivity(J·cm−1·s−1·K−1), c is the heat capacity(J·g−1·K−1), and ρ is the density(g·cm−3), Q is the heat of reaction(J·g−1),F is the reacted fraction.
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The second term on the right-hand side of the Eq.(1) was associated with the heat generation of the reaction[57], and has been derived by Hardt and Phung[51] (2)
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∂F 1 D0 exp(− E ( RTa )) = ⋅ ∂t Wd Ni2 F
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Where the W is defined as 1 + d Al d Ni , for the 1:1 Ni-Al stoichiometric system its value is 2.5. The Wd Ni is the total thickness of a cell. Wd Ni = S V
(3)
Where S represents the contact area between Ni and Al reactants in the
cell(Fig.15(a)) and V is the volume.Therefore, the Wd Ni can be substituted by the statistic results Sv (µm-1) as mentioned above which can be obtained directly by the statistic measurements. The Eq.(2) becomes 25
ACCEPTED MANUSCRIPT ∂F D exp(− E ( RTa )) = S v2W ⋅ 0 ∂t F
(4)
D0 is the pre-exponential factor of the diffusion coefficient(cm2·s−1), E is the reaction activation energy, R is the universal gas constant(J·mol−1·K−1), Ta is the
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maximum reaction temperature (1911 K). In the present work, the combustion propagation processes demonstrate a macroscopically constant wave velocity(Fig.8). For a simple treatment, we assumed
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that the reaction occurs at a steady state to describe the general trend in one dimension.
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The temperature profile(Fig.16) in the steady state is constant with time and moves with a constant velocities[53]:
(5)
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x = u + vt
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Fig. 16. Temperature profile
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Using Eq.(5), Eq. (1) can be expressed as
∂T κ ∂ 2T Q ∂F −v⋅ = ⋅ + ⋅ ∂u cρ ∂u 2 c ∂t
(6)
Where Q/c = Ta-T0, and a quadratic polynomial to the temperature profile can be
expressed as T = Ta −
2(Ta − T0 )
δ
u+
Ta − T0
δ
2
u2
(7)
The δ mentioned above is
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2κ cρ v
(8)
Using Eqs. (4), (7) and (8),on can write Eq. (6) at µ=0 as
κ
κ c 2 ρ 2 v 2 (Ta − T0 ) ∂F = ⋅ + (Ta − T0 ) ⋅ ( ) 2 cρ 2κ ∂t
(9)
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v⋅
cρv(Ta − T0 )
Finally, we derived the following expression for the relation between reaction velocity and contact area per unit volume(Sv)
2κWD0 exp − E RTa Cpρ
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v = Sv
(10)
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The values of the parameters used and their sources are listed in Table 2.
Table 2. Parameters of the combustion reaction. Units
Value
Reference
κ
J·cm−1·s−1·K−1
0.65
[53]
D0
cm2·s−1
1.0
[53]
Cp
J·g−1·K−1
0.753
[53]
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Parameter
The predicted velocities are shown in Fig. 14. The experimental observations of
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the dependence of velocity on Sv give an activation energy of 129 kJ/mol, which is reasonably close to the commonly accepted value of 125 kJ/mol for the Ni-Al system, a value that appears to reflect the rate-determining nature of the liquid-diffusion step [27,50]. The assumed values were chosen to give general agreement with the experimental trend. The variations in Sv are capable of explaining the corresponding variations in reaction velocities within the context of the model.
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ACCEPTED MANUSCRIPT 5. Conclusions (1) Cold spraying has been found to be a promising technique for generating near-fully-dense reactive bulk materials with the porosity less than 1%. Using a combination of cold-pack rolling and cold spraying, a crack-free sheet with large
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deformation was achieved, the largest total thickness reduction could reach 95%. A more uniform microstructure and increased contact area between the reactant powders were produced by more gradual rolling passes.
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(2) In thermal explosion mode, the solid-solid reaction temperature threshold of cold-sprayed samples after cold-pack rolling treatment decreased. With increasing
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rolling number, the onset of solid diffusion reaction shifted to a lower temperature range by 30-40°C.
(3) In self-propagating mode, the combustion wave velocity of cold-sprayed samples increased after rolling. The combustion wave velocities for the materials with
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fifteen gradual rolling passes could reach 130-140 mm/s.
(4) The statistic results of contact area per unit volume between Ni and Al reactants (Sv) can be used to substitute the initial size of reactants and expression
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kJ/mol.
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showed a liner dependence of velocity on Sv, giving an activation energy of 129
Acknowledgement
The authors would like to acknowledge the Central Iron & Steel Research
Institute for its technical support in cold rolling. We also thank LetPub (www.letpub.com) for its linguistic assistance during the preparation of this manuscript.
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Funding This work was supported by the National Natural Science Foundation of China
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(51671031).
Conflicts of interest
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None.
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Highlights
Reactive bulk materials were deformed without cracks by gradual cold-pack rolling.
The contact area per unit volume (Sv) between the Ni and Al increased after
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rolling. The rolling leads to a decrease in the solid-solid reaction temperature threshold.
An enhanced combustion velocity after rolling were demonstrated.
The combustion reaction velocity showed a linear dependence on the Sv.
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