Study on laser ignition and combustion characteristics of micron-sized aluminum and Al-Mg alloys particles

In response to the problems of easy sintering and long ignition delay time of micron aluminum in the combustion of aluminum-containing propellants, choose the way to add magnesium to metal aluminum to construct an alloy system, through boiling, micro-explosions are generated during the ignition and combustion process to weaken the sintering behavior and shorten the ignition delay time of aluminum. Selecting aluminum and Al-Mg alloy powder fuel with a particle diameter of about 10 μm as the research object, a set of individual-particle fuel laser ignition and microscopic high-speed imaging experimental devices was built that can observe the whole process of ignition and combustion of micron-sized fuel. Thermal analysis was used to detect and characterize the thermal decomposition process of micron-sized aluminum and Al-Mg alloy powders; combined with the results of scanning electron microscopy, the difference in ignition performance of micron-sized individual particle aluminum and Al-Mg alloys was studied. Experiments have found that, compared with aluminum, the initial oxidation temperature of Al-Mg alloys is lower and the combustion is more complete. However, the effect of adding magnesium to aluminum is only reflected before 900 °C. The ignition and combustion images and flame propagation laws of micron-sized single-particle aluminum and Al-Mg alloys were obtained. It was found that adding magnesium shortened the ignition delay time, and the combustion produced less residual.


Introduction
Aluminum is widely used in the field of energetic materials owe to its low oxygen consumption, high energy density and low cost.Many scholars have researched aluminum's oxidation mechanism, ignition and combustion properties.Micronized aluminum has disadvantages such as easy agglomeration during combustion and a long ignition delay time.The improvement of aluminum performance and combustion characteristics is not only of fundamental significance for exploring its combustion mechanism but also very important for the practical application of aluminum.Magnesium is inexpensive and simple to obtain.In particular, it has an extremely lower boiling point than aluminum and burns with explosive qualities.Because the oxide is porous and loose, it can be used to improve aluminum's application flaws.
Studies have shown that using Al-Mg alloys will exhibit different combustion properties [1][2][3][4][5].Compared with pure aluminum, the Al-Mg alloy has a lower ignition temperature, a shorter ignition delay, and a higher burning rate.On the one hand, magnesium oxide has better permeability than aluminum oxide; that is, oxygen has better diffusion properties in magnesium oxide; on the other hand, compared with aluminum, magnesium is easier to catch fire, and the heat released during magnesium combustion will accelerate the melting and ignition of aluminum.
For research on the combustion process of Al-Mg alloy powder, EI Popov et al. [6] studied the combustion characteristics of Al-Mg alloy with a particle size range of 100-600μm when heated by a heating wire and proposed that the combustion of Al-Mg alloy is divided into two stages.When the magnesium content is greater than 30%, the two stages of Al-Mg particle combustion are continuous.At less than 30% magnesium, at the end of the first stage, the size of the flame zone is reduced to the size of the particle itself, gas phase combustion ceases, and combustion of aluminum occurs only after secondary ignition of the particle.Behnejad [7] believed that the cracking of Al-Mg alloy particles was caused by the huge difference in the boiling temperature of magnesium and aluminum.When the particles were in a high-temperature environment, the boiling magnesium exploded, micro-exploding to cause the cracking of aluminum.Shoshin et al. [8] used a laminar premixed flame burner to study the combustion of Al-Mg alloys in air.It is found that the ignition temperature of alloy powder is about 1000K, which is lower than 2300K for pure aluminum, and the flame propagation velocity (airflow velocity for igniting aerosol) of Al-Mg alloy is faster.The combustion products were examined by XRD, and it was found that in addition to Al2O3 and MgO, there were also components such as Al2MgO4.Breiter et al. [9] showed that the oxide film plays an important role in the ignition and combustion of particles.The volatility of magnesium is much greater than that of aluminum, and the vapor pressure of magnesium at 1000℃ is six orders of magnitude higher than that of aluminum.Xiao [10] studied the application effect of Al-Mg alloys in fuel-rich solid propellants by comparing the oxidation processes of aluminum and Al-Mg alloys.He said that the ignition temperature of the Al-Mg alloy is low, the combustion is rapid, and there is no agglomeration phenomenon on the burning surface.Therefore, Al-Mg alloys can improve combustion efficiency in fuel-rich applications and effectively reduce the probability of nozzle clogging.Differences in the boiling points of elements in binary alloys may lead to intraparticle boiling, which facilitates the efficient breakup and atomization of alloy droplets.Blackman et al. [11] reported the micro-explosion of Al-Mg and Al-Li particles heated by a 213 W/cm 2 CO2 laser in the air and showed that this micro-explosion can improve the combustion efficiency of metal fuel in rocket engines and reduce the two-phase flow loss.Breiter et al. [9] proposed that the micro-explosion phenomenon of Al-Mg alloy particles in the oxidant-fuel mixture flame is related to the eutectic composition.Wainwright et al. [12] reported micro-explosions of Al-Zr composite particles and found that in air, the presence of N2 induced micro-explosions by acting as a source of rapid bubble formation and growth.Huang et al. [13] studied the ignition and combustion characteristics of single-particle Al-Mg alloy and Al-Mg alloy powder pile, respectively, and explained the reason for the phased combustion of Al-Mg alloy from the mechanism.
In this paper, a series of experiments on laser ignition and combustion of micron-sized Al-Mg alloy particles were carried out by means of laser ignition, and thermal analysis was used to detect and characterize the thermal decomposition process of micron-sized aluminum and Al-Mg alloy powders.There are differences in the ignition performance of single-grain aluminum and Al-Mg alloys.The ignition and combustion images, flame propagation laws, etc. of micron-sized single-particle aluminum and Al-Mg alloys were obtained.It was found that the addition of magnesium greatly shortened the ignition delay time, and the combustion produced less residue.This research is of great significance for improving the ignition and combustion properties of aluminum and revealing the combustion mechanism of Al-Mg alloys in depth, laying a foundation for further promoting the practical application of aluminum.

Experimental setup
The experimental setup is shown in Figure 1.After the laser is redirected and expanded, it is reflected to the microscopic objective through the coated beam splitter and then focused on the sample particles to ignite the sample particles.The Phantom VEO710 series high-speed camera produced by VRI Company in the United States is used to record combustion flame images.The camera uses a 12-bit color CMOS sensor with a maximum shooting speed of 680000 frames per second.The infrared band light emitted by the infrared laser (Beijing Laserwave, 1064nm, 2W tunable continuous output, stability < 1%, waist diameter ≤ 2mm, divergence angle ≤ 1.5, spot mode TEM00) transfers heat to the sample particles through energy radiation, achieving continuous and stable heating.The DaHeng CCI-08 series thermoelectric power meter (DHC, GCI-080205) was used to measure the laser power applied to the sample particles in each experiment, and the laser power density value for each experiment was obtained by combining the measured spot area.

The scanning electron microscope of aluminum and aluminum-magnesium alloy
The microscopic morphology of the sample was observed by a scanning electron microscope, Figure 2 and Figure 3 are scanning electron microscope image of Al and Al-Mg alloy particles.It can be seen from the image that the surface of the Al powder is smooth and covered with a dense oxide film.The surface of the Al-Mg alloy is relatively rough, and many small nano-spherical particles are attached.The reason is that Al-Mg alloy is produced by high-temperature atomization method, superalloy powder undergoes rapid solidification during the solidification process, the powder is mainly spherical, and the surface solidification structure is dendrite and cellular crystal.The mixed solidification structure of the composition, as the powder particle size decreases, the solidification structure on the powder surface gradually changes from dendrites to cellular crystal structure.This is mainly caused by the different cooling speeds of the powder during the atomization process.The powder with a larger particle size cools relatively slowly and tends to form a dendrite structure; while the powder with a smaller particle size cools faster and tends to form a cellular crystal structure.[14].As the particle size of the powder decreases, a mixed structure consisting of cellular crystals and dendrites is formed, which reduces dendrite segregation and is beneficial to the uniformity of the internal structure of the powder.
The energy spectrum scanning of the Al-Mg alloy powder was carried out, and the ratio of Al:Mg was measured to be 76:24.In the mapping image, yellow represents the Al element, and red represents the Mg element.It can be seen from the results in the figure that the size and distance of the grain boundary are nanoscale, the two metals are evenly distributed in the sample, and there is obvious diffusion or intermetallic formation between the components, which ensures the relative uniformity of the physical properties of the sample particles.

The thermogravimetry of aluminum and aluminum-magnesium alloy
The thermal decomposition characteristics of Al and Al-Mg alloys in air atmosphere were investigated by thermogravimetry.The temperature range is 25-1400°C, the air flow rate is 50 mL/min, the heating rate is 10°C/min, and the sample volume is 15 mg.The TG-DSC-DTG curve of the thermal decomposition process of micron aluminum in air is shown in Figure 6.There are three obvious stages of weight gain in the sample, and the increase in sample mass indicates that the sample undergoes oxidation reaction in air.The three-stage weight gain was 1.31%, 10.68%, and 10.33%, respectively.
When the temperature reaches 660 °C, the DSC curve records an obvious endothermic peak, which is consistent with the melting point of aluminum.At this temperature, the inner core of the sample aluminum particles begins to melt.When the temperature exceeds 660°C, the boiling point of aluminum, the sample undergoes a secondary accelerated weight gain process.Trunov [15] attributed this result to the crystal transformation of alumina, so the weight gain process showed a step change.Zhou [16] of Zhejiang University studied the morphology and crystal transformation of aluminum particles during the thermal oxidation process.According to the SEM and XRD results at 25°C, 550°C, 700°C, and 1100°C, it was confirmed that the particles of aluminum particles in the thermal reaction process The fragmentation is caused by the expansion of the internal molten aluminum.When the temperature is lower than 550°C, the Al element exists in the form of Al single substance and amorphous alumina.When the temperature rises to 700°C, the crystal forms appear as γ-Al2O3 and α-Al2O3 oxide layer, when the temperature reaches 1100°C, the Al elemental substance in the sample reacts almost completely, and the γ-Al2O3 in the sample disappears, replaced by a large amount of α-Al2O3.
Al-Mg alloy also has obvious three-stage weight gain, and the three-stage weight increases are 17.15%, 29.49%, and 37.91%, respectively.The weight gain ratio of the Al-Mg alloy reaches 84.55% compared with the initial state at about 1100 °C, which is basically consistent with the theoretical value, indicating that the alloy particles have completely burned at about 1100 °C.Al-Mg alloy has a eutectic melting peak at around 447°C, and the DSC curve from 900°C to 1400°C is consistent with that of pure aluminum, indicating that the Mg element has completely reacted before 900°C.
According to the research of Xiao [5], the Al-Mg alloy starts to burn at about 550°C, producing β-Al3Mg2 and γ-Al12Mg17, and the reaction product after 650°C appears α-Al2O3.
The ignition temperature of the material can be judged from the TG-DTG curve.That is, draw a vertical line to the X axis through the first peak value of the DTG curve, intersect the TG curve at point a, draw a tangent line through point a on the TG curve, intersect with the baseline of TG at point b, and the temperature corresponding to point b is the material.The ignition temperature, also known as the epitaxy initiation temperature, is shown by the green auxiliary line in the figure.It can be seen from the figure that the ignition temperature of Al is 886°C and that of Al-Mg alloy is about 520°C.Compared with the initial oxidation temperature of Al's TG curve, the initial oxidation temperature of the Al-Mg alloy is decreased.

Analysis of laser ignition process of aluminum particles
In this case, the particle diameter was ~10.2 μm, and its ignition power and power density were 200 mW and 5.34 ×10 5 W/cm 2 , respectively.The Al particle was ignited by laser with a delay time of 2.5 ms.With the continuous action of the laser, the particles first expand and become larger due to heat, and the outer layer turns red.At 9.9 ms, a flame appeared on one side of the particle.As the combustion progressed, the flame continued to spread to the entire surface of the particle, and the combustion ended at 12 ms.After burning, a large amount of black residue can be seen scattered around the original location.

Analysis of laser ignition process of Al-Mg alloy
As show in Figure 9 ,in this case, the particle diameter was ~12.7 μm, and its ignition power and power density were 200 mW and 5.34 ×10 5 w/cm 2 , respectively.The Al-Mg alloy particle was ignited by laser with a delay time of 0.392 ms.After ignition, the combustion lasted 0.224 ms.The second ignition followed the first combustion after 0.924 ms, and the second combustion lasted 2.240 ms.A short interval was found between the two combustion stages.The flame radiation intensity during stage II was obviously stronger than stage I.During combustion, the expansion, gaseous emission, and micro-explosion of individual Al-Mg alloy particles in air can be clearly identified in a sequence.The micro-explosion occurred before ignition.It can be inferred that the two burning stages were corresponding to the combustion of Mg composition and Al composition in Al-Mg alloy, which kept agreement with the literatures [12][13][14].It can be clearly seen from the picture that there is basically no residue after burning.

Flame structure and propagation
The combustion of single Al-Mg alloy particles in Figure 9 clearly demonstrated the flame structures, which were generally symmetrical and circular.A tiny flame structure presented a spherical crown.Moreover, the flame sizes were much larger than the particle sizes.These suggest that the combustion of the Al-Mg alloy belonged to homogeneous gas-phase combustion.During combustion, the flame front propagated forward and backward over time, which depended on the reaction rate and diffusive rate of oxygen.In air, the oxygen is diffused by natural convection.After ignition, the reaction rate depended on the temperature in the reaction zone.Figure 10 demonstrates the flame propagation velocity of three Al-Mg alloy particles after ignition.
Figure 10.Flame propagation velocities of individual Al-Mg alloy particles.It can be found that generally, the larger the particle size, the longer the burning time of the particles is.Although the three particle sizes were different, the tendency of their flame propagation was similar.The flame propagation can be divided into two stages, corresponding to the combustion of elemental Mg and Al in alloys.After ignition, the flame front propagated quickly to a maximum velocity and then propagated at a low velocity.It indicates the gasification combustion of Mg and its consumption.Following stage I, during stage II, the propagation of the flame front presented a similar pattern of first increase then decrease, suggesting the combustion of Al and its consumption.During combustion, the combustion was first controlled by diffusion and then by reaction kinetics.Zero propagation velocity suggests that the combustion is dynamically stabilized.

Conclusion
The following conclusions can be obtained through the thermal physical property analysis and laser ignition and combustion experiments of micron-sized single-particle aluminum and Al-Mg alloys.Compared with aluminum, the initial oxidation temperature of the Al-Mg alloy is lower and the combustion is more complete, but the effect of the addition of Al-Mg is only reflected before 900 ℃.Compared with pure aluminum, the ignition delay time of an Al-Mg alloy with similar particle size is shorter, and the combustion produces less residue.The Al-Mg alloy particles burn in two stages in sequence.The first stage is mainly the combustion of the element Mg, and the second stage is mainly the combustion of the element Al.The combustion intensity of the second stage is obviously stronger than that of the first stage.

Figure 1 .
Figure 1.Experimental setup with laser ignition and microscopic imaging

Figure 2 .Figure 3 .
Figure 2. The SEM image of aluminum

Figure 4 .Figure 5 .
Figure 4.The energy spectrum scanning of the Al-Mg alloy

Figure 7
are the TG, DSC, and DTG curves of Al and Al-Mg alloys, respectively.The black line in the figure is the TG data, which represents the mass change curve of the sample particles with temperature, and the weight gain or weight loss at different temperatures can be obtained.The blue line is the DSC curve, which is the endothermic state of the analyzed sample.The upper convex peak in the figure indicates the endothermic reaction, and the lower concave peak indicates the exothermic reaction.The red dotted line is the data of DTG, which is the derivative of TG, and the temperature point with the fastest decomposition rate of the sample can be obtained.

Figure 8 .Figure 9 .
Figure 8. Ignition and combustion process of individual aluminum particle

Table 1 .
Melting and boiling points of Al, Mg and their oxides