1 Introduction

Fragmentation warheads, used in military and defense applications, have been broadly classified into two types: preformed and casing (natural) fragmentation warheads. Preformed fragmentation warheads are designed with fragments’ mass and geometry specifically tailored for a particular application (Zecevic et al. 2015). However, they tend to be expensive due to the additional handling required and often have a lower structural strength as a warhead. On the other hand, casing fragmentation (natural fragmentation) warheads are more cost-effective and structurally robust, making them suitable for situations where structural integrity and cost reduction are important considerations (Beetle and Schwartz 1976; Zecevic et al. 2015). Casing fragmentation warheads produce natural, albeit uncontrolled fragment distribution, i.e., various fragment sizes according to Mott's distribution, and hence are less lethal as compared to preformed fragments (Grady 2006; Helte et al. 2019). In addition, brittle material tends to fragment into smaller fragments as opposed to ductile one, even if the first usually possess a higher strength (Zecevic et al. 2004; Arnold and Rottenkolber 2008; Tanapornraweekit and Kulsirikasem 2011).

To overcome the respective limitations of both preformed and natural fragmentation warheads, controlled fragmentation warheads have been developed. Controlled fragmentation warheads utilize a casing (as in the case of casing fragmentation) with additional treatments to induce a specific fragmentation pattern, thus combining the advantages of both preformed and natural fragmentation warheads (Arnold 2002; Villano and Galliccia 2013). Several methods exist for achieving controlled fragmentation, such as internal and external grooves, double casing, localized chemical surface treatments, laser micro drilling, and localized melting (Beetle and Schwartz 1976; Villano and Galliccia 2013). While some methods, such as grooving, involve material removal and thus may weaken the casing (reduced thickness, stress concentrations), others may overcome this limitation by altering the material’s properties locally without removal, e.g., by deep laser melting (Zhang et al. 2008).

Fragmentation warheads have their limitations, as penetrating through thick concrete walls for instance. For such scenarios, completely different warheads are being used, known as projectiles (also known as penetrator and hence the words penetrator and projectile are used interchangeably herein). Projectiles typically consist of an ogive or conical steel nose connected to a thick walled steel tube (casing) filled with high explosives. Projectiles are designed to penetrate hard targets while maintaining structural integrity (Hansson 2011), with lethality usually achieved through overpressure (blast) rather than fragmentation (Mahoney et al. 2005). However, fragmentation can also be advantageous in certain scenarios, such as providing additional damage in open spaces or multi-purpose warheads that need to penetrate hard targets combined with fragmentation capabilities (Mahoney et al. 2005; Hansson 2011; Zecevic et al. 2015). Hence, controlled fragmentation is a natural choice for multi-purpose projectiles.

Prior research on controlled fragmentation using localized melting methods mostly studied the fragmentation behavior as a function of melting process parameters (Zhang et al. 2008; Gulzar et al. 2009; Villano and Galliccia 2013; Shen et al. 2017; Liu et al. 2021). The fragmentation occurred in the melted area due to the local brittleness (despite the higher hardness) of that area (Zhang et al. 2008; Shen et al. 2017; Liu et al. 2021). Liu et al. (2021) reported that in some cases, microcracks were found in the melted region which contributed to the fragmentation process. To avoid those thermal and cold cracks, the material's contaminants levels should be reduced. In addition, they (Liu et al. 2021) reported that high laser energy levels (240 J/mm) should be used to avoid cracking while increasing the laser penetration depth, keeping almost the same ratio between the melted layer depth and width as for lower laser energies. Those microcracks might have a negative influence when material’s integrity is important as in the case of projectiles compared to pure fragmentation warheads. One of the few researchers who conducted penetration research on small caliber projectiles with controlled fragmentation by localized laser melting was Shen et al. (2019), validating the projectiles’ sustainability against a steel armor plate. However, fragmentation behavior was not tested in their study and a small scale projectile was used.

A comprehensive investigation of the combined performance of a full-sized multi-purpose projectiles (penetrator) with controlled fragmentation achieved through localized deep melting by laser beam was studied herein. It is shown that a set of parameters can be found to combine both structural integrity and controlled fragmentation using that method, without compromission. The projectile's material, the selection of deep laser melting process parameters, field tests arena for fragmentation performance evaluation, and penetration tests arena are described in Sect. 2. The results of the tests are presented in Sect. 3, followed by a discussion in Sect. 4, and concluding remarks in the conclusions section.

2 Materials and methods

2.1 The projectile and melting pattern

The projectile consisted of AISI 4340 steel that had undergone hardening, resulting in a static yield stress of about 1200 MPa and an ultimate stress of 1350 MPa, with a hardness of approximately 420 HV. To mitigate the risk of thermal and cold cracking along the melted layers, premium grade material in accordance with AMS 6414 was utilized, thereby reducing the presence of contaminants such as sulfur (S) and phosphorus (P). The projectile was designed as a long thick-walled cylinder with a wall thickness of 17.8 mm, having a short conical front nose part, as shown in Fig. 1. The total weight of the projectile was 73.2 kg, which consisted of 14 kg of PX-289 (PBXN-109 like) high explosive containing 64 wt% RDX, 20 wt% Aluminum powder and 16 wt% HTPB binder. For the melting pattern, left and right helix paths were used. This melting pattern resulted in a diamond shape fragments with a sharp angle of 60 degrees, 35.4 mm width and 62.7 mm length as shown in Fig. 1.

Fig. 1
figure 1

The geometry of the tested projectile. Section A-A presents the cross-section dimensions. The blue enlarged diamond shape presents the desired fragmentation pattern achieved by the melting pattern

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2.2 Deep laser melting parameters

The study began by establishing the deep laser parameters using a laser welding machine. Short cylinders made of the same hardened AISI 4340 material and with the same wall thickness (17.8 mm) as the projectile were used as dummies to test various welding parameters. A total of 48 different combinations of laser parameters (beam power, focus and torch velocity) were tested using the dummy cylinders, and specified in Table 1 of the appendix section. The incidence angle of the laser beam with the casing was left perpendicular during all melting parameter tests, as the influence of that angle was shown to be small (even negligible) on fragmentation behavior (Qiu et al. 2021). Figure 2A illustrates the dummy cylinder with melted paths used for the parameter study.

Fig. 2
figure 2

A Dummy cylinder for laser parameters study. The melted lines represent a set of laser parameters. B Cross section of a melted line presenting the different regions of the melted material (BM, HAZ, WM), penetration depth and width, and a cooling crack

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A full-depth penetration of the laser melting was difficult to achieve due to the large wall thickness and required effort, hence it was decided that laser penetration above 50% (in thickness) would be sufficient in resemblance to Shen et al. (2017). All 48 melted paths (set of melting parameters) were sectioned and visually inspected for their penetration depth and width. The best eight results were further inspected to evaluate more accurately the laser's penetration depth and width, alongside the microstructure and hardness as described below. At the end, the melting parameters combining the deepest welding penetration with thinnest melted zone were selected as the working parameters for the fragmentation and penetration tests. Figure 2B describes a cross section and the different regions of the melted path, i.e., the base mental (BM), heat affected zone (HAZ), the melted (or welded) material (WM), the measurement of penetration width and depth are also presented.

The maximum available beam power of 5 kW was used to achieve maximum depth penetration. The beam focus was varied within a range of − 20 to + 12 mm relative to the outer surface of the cylinder, while the beam velocity was varied within a range of 0.1–1.0 m/min. The welding process was carried out on both cold cylinders and preliminarily heated cylinders to avoid cracking. To evaluate the quality of the welds and check for any potential defects, a fluorescent penetration inspection was carried out, on the best eight melted lines (sets of melting parameters), in accordance with ASME Sect. 5 recommendations. The microstructures of those best eight melting lines were analyzed using an optical microscope after being polished and etched with Nital reagent (98 ml of ethanol and 2 ml of HNO3), in accordance with ASTM E3-17. Hardness tests on those melting lines were performed on INNOVATEST model NOVA 360 hardness tester, using Vickers method with 10 kg load according to ASTM E92-17 standard on polished transverse section through the melted area.

No additional heat treatment was done after melting for two reasons: first, the difference in hardness is beneficial for good fragmentation (although might add stress concentration regions), and additional heat treatment would probably homogenize this difference; second, the base metal, quenched and tempered 4340 steel, needed to maintain its strength for structural integrity upon penetrating the target.

2.3 Fragmentation arena

To evaluate the fragmentation quality achieved by the selected melting parameters, a fragmentation arena was built (Reynolds and Huntington-Thresher 2016), as shown in Fig. 3. Water tanks were strategically placed to capture the fragments for post-mortem examination of geometry, fragmentation pattern, and weights after the explosion. Common practice in fragmentation arena tests is to use evidence sheets. Those evidence sheets are used for some reasons as target imitation, spatial distribution study, hide the explosion’s fireball to avoid blinding cameras, and to access the fragments velocity. In addition, the evidence sheets present an imprint of each fragment, thus presenting its approximate size upon impact with the sheet. That imprint represents the fragments wholeness, if they arrived as a whole or broke to pieces upon explosion, or if large portion of the casing didn’t fragment at all upon explosion. It was decided for the tests herein to use steel evidence sheets with 3 mm thickness. Those sheets did not mimic any specific target but enabled the study of the fragments' spectral distribution and their appropriate fragmentation pattern upon explosion. The evidence sheets were also used for hiding the fireball for the high-speed recording, which was carried out by Photron 7512, and was later used to estimate the fragments’ velocity.

Fig. 3
figure 3

Fragmentation arena setup, containing water tanks to capture fragments, evidence sheet to visualize the fragment's penetration and the warhead before explosion

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For the fragmentation arena, a warhead section was used due to safety concerns (high-explosive limitations). It was placed 4.5 m from the evidence sheets and 6 m from the water tanks (Fig. 3), and about 1.2 m above the ground, those distances were chosen to avoid any unnecessary damage from the blast. The high explosive weight of the warhead section was reduced to 4.5 kg while keeping the wall thickness and outer diameter at 17.8 mm and 148 mm, respectively. The length of the section was shortened to 265 mm, as shown in Fig. 4. Bottom and top lids were added in addition to a booster, to ensure proper detonation and reducing end effects.

Fig. 4
figure 4

Warhead section for fragmentation testing. Models of the warhead's section and cross section are presented (left), with the section itself (right) before testing

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2.4 Penetration test

Qualitative 3-point bending tests were carried out prior to the penetration test to validate the materials integrity upon bending after applying the melted patters, the methods and results are summarized in the appendix section. After that basic verification of the materials integrity, a Davis gun (Davis 1914) was employed to launch the projectile at a speed of 320 m/s towards an 80 cm in thickness concrete wall with 1% volumetric reinforcement. To eliminate boundary effects, the height and width of the target were both set to 3 m. To introduce bending forces acting on the projectile upon real scenario of penetration (Hansson 2011), the target was positioned at 70-degrees relative to the projectile's body. To evaluate the velocity and track the trajectory of the projectile, three high-speed Photron cameras were used: ~ 45° to the flight direction by Photron 2010 (camera 1), perpendicular to the flight vector by Photron 640 (camera 2) and after penetrating the target by Photron 2512 (camera 3). The experimental configuration of the penetration test is shown in Fig. 5.

Fig. 5
figure 5

Penetration test experimental setup, the projectile's impact at 70˚ to the target is presented alongside the high-speed cameras' position and their numbering. The targets’ dimensions are also specified

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3 Results

3.1 Deep laser melting parameters

All laser melting parameters used on cold (at room temperature) cylinders resulted in cracked melted areas (Fig. 2A). However, this process provided valuable insight into the laser penetration depth and HAZ which helped in calibrating the exact welding parameters. Summary of the best results are given in Fig. 6, where it is shown that the lower the torch velocity the deeper the laser penetration. The Deepest laser penetration was achieved around 0 mm from the focal point of the laser beam (Focus), negative and positive values resulted in reduced laser penetration depths. It is evident from Fig. 6 that the deepest penetration (as requires) was achieved for 0.3 m/min torch velocity and 0 mm focus as opposed to 0.5 m/min torch velocity or other focus distances. Those parameters, 0.3 m/min velocity and 0 mm focus, were further used for the fragmentation and penetration tests.

Fig. 6
figure 6

Laser penetration depth as a function of the laser focal distance for two torch velocities, presenting the best laser parameters (deepest laser penetration)

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The same parameters were then used on preheated cylinders to 250 °C before laser melting started, resulting in nearly identical penetration depth but without any cracks. To detect cracks, a fluorescent dye penetration inspection was carried out on the best eight laser melting paths, i.e., deepest penetration with moderate HAZ, and no cracks were found after pre-heating. The best combination of melting parameters resulted in 12.6 mm penetration depth (about 70% of the wall thickness) with a 10 mm width, including the heat affected zone, as shown in Fig. 7. In addition, Fig. 7 presents the microstructure of the base metal (BM), heat affected zone (HAZ), and melted (or welded) material (WM) for the selected melting parameters. As expected for hardened 4340 steel, the BM which was hardened prior to the melting procedure presented a relieved martensitic microstructure with an average hardness of 424 HV. The HAZ presented a combination of bainitic and martensitic structure, with a hardness of 350 HV. The WM consisted of unrelieved martensite, corresponding to the needle-like structure, with a hardness of 602 HV.

Fig. 7
figure 7

The best melted line inspection presenting its cross section with no cracks after preheating of the material. A presents the base metal (BM) relieved martensitic microstructure, B presents the heat affected zone (HAZ) with a combination of bainitic and martensitic microstructure, and C presents the melted material (WM) unrelieved martensite microstructure

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3.2 Fragmentation arena tests

After selecting the melting parameters, a fragmentation test was conducted on the warhead section (Fig. 4) to evaluate its fragmentation characteristics. The water tanks captured about two rows of the warhead according to their position, i.e., 7 diamond-shaped fragments were recovered. Inspection of the fragments revealed three types of fragmentation patterns as shown in Fig. 8. Those patterns included diamond-shaped fragments (Fig. 8B) according to the melted pattern, triangular fragments (Fig. 8A) situated at the charge boundaries and sheared areas (Fig. 8C) between each two diamonds (fragments) as commonly observed in controlled fragmentation warheads (Liang et al. 2017). Those fragmentation patterns are schematically shown in Fig. 9, alongside a side view of a diamond-shaped fragment presenting the fracture planes. It is emphasized that Fig. 9 represents only schematically the fracture path, which does not have to start at the melted region root. It can be seen from the contour of the diamond-like fragments in Fig. 8B and 9, that they failed (fragmented) in ~ 45°, i.e. by shear. Figure 8 also illustrates a typical penetration hole of the fragments in the evidence plates. The average fragments' velocity was found to be 1400 m/s.

Fig. 8
figure 8

Fragments’ shapes and penetration hole shapes in the evidence plate. A Triangular fragments from the boundaries of the tested warhead section, B diamond-like fragments according to the melted pattern, and C fragments from shear planes between adjacent diamond-like fragments

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Fig. 9
figure 9

Schematical representation for the shear area fragmentation between adjacent diamond-like fragments. It should be emphasized that the fracture doesn’t have to start exactly at the root of the melted area. The red ellipse indicates the fracture path, which is represented by the fracture planes of one of the specimens

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The three groups of fragment geometries, as shown in Fig. 8A–C (diamond, triangle, and sheared area), were weighed and categorized into three weight groups. Figure 10 shows the cumulative number of fragments found in the water tanks, distributed by their geometry and mass groups. While the sheared areas had a wide range of fragments’ mass distribution, both the triangle and diamond fragments resulted in more precise weights. However, the diamond fragments resulted in half of their designed theoretical mass (compared to their size as shown in Fig. 1). This mass reduction is a direct result of the sheared area fragments, i.e., part of the theoretical weight of the diamond fragments was divided between the diamond and sheared are fragments.

Fig. 10
figure 10

The cumulative number of fragments caught it the water tanks, divided by their geometry (diamond, shear, and triangle—according to the legend) and mass groups

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3.3 Penetration test

Once the optimal melting parameters for fragmentation were established, and the qualitative 3-point bending tests (see appendix) presented satisfactory results for the material’s properties after deep laser melting, a dynamic penetration test was conducted to verify the projectile’s structural integrity under real penetration loading. A projectile with the selected melting parameters (as shown in Fig. 1) was fired into a concrete target, penetrating it as would be expected from a projectile without melting patterns. A rear piston and front ring were used to fit the projectile's diameter to the Davis gun's bore, and to seal the gases during acceleration in the gun. The projectile was fired at 316 m/s and exited the concrete target at 178 m/s, the velocities were measured by cameras number 2 and 3 (Fig. 5), respectively. Eye examination of the retrieved projectile after penetration of the concrete target showed no damage to the projectile, such as cracks, fracture, or permanent deformation. Snapshots from the penetration process are depicted in Fig. 11, while the projectile before and after penetration is illustrated in Fig. 12.

Fig. 11
figure 11

Snapshots of the projectile penetrating the target. The numbers of cameras are according to Fig. 5. From left to right, the figures present the moment of impact, penetration process and the intact projectile after penetrating the target

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Fig. 12
figure 12

Projectile before and after penetration, including the rear piston and front ring to fit between the diameters of the bore and projectile, and for sealing the pushing gases inside the Davis Gun. Intact projectile can be seen after penetration

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The undeformed projectile after penetration, its velocity behind the target and the general penetration behaviour resembled those of a projectile without melted pattern and according to prior estimations.

4 Discussion

The development and testing of a full-sized, multi-purpose warhead capable of improved fragmentation while keeping its penetration abilities was studied. Controlled fragmentation methos using deep laser melting technique was used to achieve the pre-designed fragmentation pattern. The same full sized projectile (1090 mm length, 147.8 mm outer diameter, and 17.8 wall thickness) was used for both tasks (fragmentation and penetration), unlike previous works where those applications were separated, and thin walled warheads were used.

It was found that using the laser with its maximal power (5000 W), with torch velocity of 0.3 m/min, while keeping the focal point of the laser beam on the outer diameter of the casing (Focus = 0 mm), resulted in the deepest laser penetration. The resulted melted area depth (laser penetration) was 12.6 mm which is about 70% of the wall thickness. It was shown that although full penetration was not achieved due to the laser power limitation, partial penetration was enough for well controlled fragmentation pattern. Hence, justifying the preliminary assumption of at least 50% laser penetration in resemblance to milling grooves on the outer surface of a projectile (Shen et al. 2017).

The fragmentation pattern was consistent with the melted pattern, demonstrating the benefit of this procedure for fragmentation warheads. As expected, a shear region was detached between each pair of fragments due to the shear forces acting on the warhead's casing while inflating and fragmenting (Liang et al. 2017). The fragments' velocity was similar to the one measured in previous tests for natural fragmentation of the same warhead (without melted areas, not reported herein). Keeping the fragments’ high velocity is an advantage for the lethality compared to pre-formed fragmentation warheads, where gases can leak between fragments resulting in lower fragments' velocity and hence reduced lethality.

It should be mentioned that the fragments mass was about half of their designed mass, an average of about 70 g as compared to 150 g (according to their theoretical geometry). This mass loss was due to the large wall thickness which enabled large sheared areas to be detached during the fragmentation process. However, those sheared area fragments are not “wasted” since they also might contribute to the total lethality of the warhead once their distribution is known. Having said that, the small dispersion of the diamond fragments’ mass presented good control over the fragmentation process, unlike natural fragmentation, implying that one could take into account their possible mass reduction during the design process.

Using the deep laser melting method on cold metal resulted in cracks in the melted area. To avoid cold and heat cracking, preliminary heating to 250 °C was used for the hardened 4340 premium steel used. Preventing cracks was important to maintain the structural integrity and thus the penetration ability of the projectile.

Penetration into a concrete target revealed that structural integrity was maintained without damage, as expected from a regular projectile and regardless of the built-in stress concentrations due to localized melting. Using 3-point bending tests (appendix section) as a preliminary integrity test presented the right tendency for the material's integrity, although projectiles will experience different stress configurations and much higher strain rates than in static tests. In such a way, a simple test could be carried out for preliminary examination of the tested material after applying deep laser melting, thus giving quantitative indication of the expected integrity of the projectile upon penetration test.

Since the melted areas were found to be much harder than the base metal, and projectiles are usually designed to be loaded up to their elastic limit during penetration, an interesting point can be raised which is out of scope for this study. Is it possible that those melted areas could be used as strengthening mechanisms for projectiles thus allowing increased loading as compared to regular ones? This is in resemblance to strengthening by ribs in regular design, despite the inherent microscopic stress raisers due to the unrelieved martensite found in the melted area and some softening of the HAZ.

In conclusion, deep laser melting provides a good combination for multi-purpose warheads, improving fragmentation pattern and hence its lethality, without impairing penetration capabilities when the melting parameters are properly selected.

5 Conclusions

The development of a full-sized, multi-purpose projectile, with 17.8 mm wall thickness, using a controlled fragmentation with deep laser melting method was studied. The tested projectile resulted in an improved fragmentation pattern while maintaining its penetration ability. Even with localized microscopic stress raisers, the examination of the full-sized product showed no negative impact on its penetration performance. In addition, it was shown that despite the low strain rate of simple 3-point bending test as opposed to penetration, the test showed good quantitative correlation to the projectile’s integrity upon penetration by using a specimen with the melted pattern. Using this methodology, a straightforward effective combination of the fragmentation and penetration capabilities, without sacrificing either, was achieved.