Investigation of additive manufactured micro-lattice structures for defence applications

Three types of steel used in defence applications for AFVs are rolled homogeneous armour (RHA), modified homogeneous armour (MHA), and high non-magnetic steel (HNS). RHA is a standard high-strength low alloy steel with a tensile strength of 900–1050 MPa and a hardness of around 300 VHN. MHA, an stainless steel, offers improved ballistic protection with a tensile strength of 1300–1450 MPa and a hardness of about 450 VHN. HNS is designed for special grade armour applications, boasting a tensile strength of 940–1075 MPa and a hardness of approximately 270 VHN. HNS is used particularly in AFV floor plates due to its nonmagnetic nature and cost-effectiveness, being 30% to 40% cheaper than RHA. The integration of these materials into AFVs, however, results in weight, manoeuvrability, and stability issues. Researchers are exploring micro-lattice structures, for which we propose honeycomb and gyroid, additively manufactured by laser powder bed fusion (LPBF) to address these concerns and reduce weight and thickness without compromising mechanical properties and blast mitigation capabilities.

The honeycomb and gyroid test specimens’ final designs are achieved through collaboration with defence research labs (CVRDE and DMRL) and an end-to-end study. Table 1 showcases the designs obtained after multiple trial-and-error iterations. Figure 1 illustrates the individual designs of the honeycomb and gyroid structures to ensure optimal weight reduction is achieved without compromising integrity elements. The design iteration follows a similar approach to the sandwich panel, incorporating a 2-mm outer thickness to meet the testing requirements and facilitate attachment to AFV underbody structures.

Fig. 1.

Designing armour for the AFV66 as a parameter for evaluation for the design module specimens against explosive attacks requires considering surface roughness and micro-Vickers hardness. Surface roughness impacts blast loading performance, while micro-Vickers hardness determines strength and resistance to deformation. LPBF-manufactured specimens undergo analysis for different conditions to enhance protection [11]. The microhardness of the micro-lattice structure test specimen was carried out as per the ISO 14577-1 standard, using Tukon/Wilson 1102 - Vickers (scale: microns) illustrated in Figure 2d, with dead weight loads of 0.01 to 1-kg and 2-kg options. The number of measurements were six per sample in order to obtain average values on the same surface after multiple indentations owing to the intricate features of the lattice structures. In the roughness measurement process, a total of four different locations on the front view of the lattice structure were selected for measurement. The measurements were taken using the MFT-5000 Tribometer (RTEC Instruments) with an R-20x lens and white light as indicated in Figure 2e. The software Gwyddion (64 bit) was employed for post-processing of the acquired data. The measurements were conducted to assess the surface roughness of the lattice structures. These measurements provided insights into the average surface roughness values at the specified locations, contributing to our understanding of the surface quality and characteristics of the lattice structures.

The objective of this investigation is to improve the protective capacity of the armour against explosive attacks. Tables 5 and 6 present the response variables related to the design criteria for honeycomb and gyroid micro-lattice structures, respectively. The gyroid is a surface of minimal dimensions featuring a profoundly intricate geometric structure that presents a repetitive arrangement of interlinked struts, thereby limiting consideration to only one type of design. It is distinguished by its elaborate and sponge-like visual aesthetics. As a result of the high volume of specimens, unique codes have been assigned to each one. The specimens were divided equally into ‘As Printed’ or ‘Non-Stress Relieved’ (designated as ‘A’), ‘Stress Relieved’ (designated as ‘B’), and the final ‘Heat Treated’ (designated as ‘C’, ‘D’, and ‘E’). Specimens were distributed in sets of three to facilitate diverse analyses. The six specimens of each section were further subdivided into structures of gyroid or honeycomb.

The two parameters under consideration were surface roughness and micro-Vickers hardness. The desired output was minimal surface roughness and maximum micro-Vickers hardness. These parameters are preferred, as they lead to a reduction in localised stress concentrations and improved resistance against deformation and penetration, respectively. The vector plane that meets this criterion will be deemed a suitable test coupon dimension, as it closely approximates the optimal solution for withstanding blast loading, thereby enhancing its efficacy in safeguarding both the vehicle and its occupants. A statistical approach has been employed to identify the vector plane that best meets the specified criteria.

2.4.1.

The statistical method of evolution for micro-lattice structures: MCDM TOPSIS

Multi-criteria decision-making (MCDM) involves evaluating alternatives based on multiple attributes. Statistical techniques like analytic hierarchy process (AHP), technique for order of preference by similarity to ideal solution (TOPSIS), and preference ranking organisation method for enrichment evaluations (PROMETHEE) [12] are commonly used in MCDM. TOPSIS is favoured for its simplicity, for accommodating both quantitative and qualitative criteria, and for providing a distinct ranking of alternatives. It is widely applied in material selection, design, and in the aerospace and defence industries. The method employed to determine a solution that closely approximates the ideal is a straightforward and clear process as follows:

Normalization of performance:

Normalized performance of alternative x_i on criterion c_j is determined by: 1 rij=aij∑k=1nakj2 $${r_{ij}} = {{{a_{ij}}} \over {\sqrt {\mathop \sum \limits_{k = 1}^n a_{kj}^2} }}$$

Weight allocation:

Weight w_j allocated to criterion c_j, where w_j ≥ 0 and ∑j=1mwj=1$\mathop \sum \nolimits_{j = 1}^m {w_j} = 1$.

Decision matrix construction:

Construct a matrix R = [r_ij], where rows represent alternatives and columns represent criteria.

Ideal and negative ideal solutions:

Ideal solution A+=(a1+,a2+,…,am+)${A^ + } = (a_1^ + ,a_2^ + , \ldots ,a_m^ + )$, where aj+=max(rij⋅wj)$a_j^ + = \max ({r_{ij}} \cdot {w_j})$, where n and m are the number of rows and columns, respectively.

Negative ideal solution A⁻ is determined by aj−=min(rij⋅wj)$a_j^ - = \min ({r_{ij}} \cdot {w_j})$ for all alternatives i = 1, 2, …, n.

Euclidean distance:

Distance of x_i from ideal solution: 2 Si+=∑j=1mwj⋅(rij−aj+)2 $$S_i^ + = \sqrt {\mathop \sum \limits_{j = 1}^m {w_j} \cdot {{({r_{ij}} - a_j^ + )}^2}} $$

Distance of x_i from negative ideal solution: 3 Si−=∑j=1mwj⋅(rij−aj−)2 $$S_i^ - = \sqrt {\mathop \sum \limits_{j = 1}^m {w_j} \cdot {{({r_{ij}} - a_j^ - )}^2}} $$

Relative proximity:

Relative closeness of x_i to ideal solution: 4 Ci= S i − S i ++ S i − $${C_i} = {{S_i^ - } \over {S_i^ + + S_i^ - }}$$

The alternatives should be ranked according to their proximity to the ideal solution.

The alternative that exhibits the greatest degree of relative closeness is deemed to be the optimal alternative.

The utilisation of a particular analytical approach has yielded a representation of the TOPSIS analysis pertaining to honeycomb structures in the absence of stress relief, with stress relief, and following heat treatment. Table 7 displays the findings.

Table 7.

TOPSIS analysis for honeycomb micro-lattice structure for LPBF-generated specimens

Non-stress-relieved condition
Normalized residual matrix			Weighted residual matrix		Distance to ideal
Coupon no.	NRa	NVHN	WRa	WVHN	Si+	Si-	Ci	Rank
2A	0.369	0.419	0.185	0.209	0.039	0.007	0.154	4
3A	0.325	0.421	0.163	0.210	0.017	0.029	0.633	2
4A	0.365	0.440	0.183	0.220	0.037	0.010	0.204	3
5A	0.291	0.408	0.146	0.204	0.000	0.046	0.994	1
6A	0.382	0.371	0.191	0.186	0.047	0.001	0.002	5
Stress-relieved condition
Normalized residual matrix			Weighted residual matrix		Distance to ideal
Coupon no.	NRa	NVHN	WRa	WVHN	Si+	Si-	Ci	Rank
2B	0.477	0.446	0.239	0.223	0.038	0.094	0.713	3
3B	0.484	0.409	0.242	0.205	0.042	0.090	0.682	4
4B	0.450	0.369	0.225	0.185	0.026	0.106	0.803	2
5B	0.401	0.425	0.201	0.212	0.000	0.131	0.999	1
6B	0.662	0.421	0.331	0.210	0.131	0.001	0.005	5
Heat-treated condition
Normalized residual matrix			Weighted residual matrix		Distance to ideal
Coupon no.	NRa	NVHN	WRa	WVHN	Si+	Si-	Ci	Rank
2C,2D,2E	0.379	0.880	0.190	0.440	0.064	0.007	0.099	4
3C,3D.3E	0.254	0.800	0.127	0.400	0.003	0.068	0.954	2
4C,4D,4E	0.251	0.809	0.125	0.405	0.001	0.070	0.982	1
5C,5D,5E	0.289	0.880	0.144	0.440	0.019	0.052	0.736	3
6C.6D,6E	0.390	0.871	0.195	0.435	0.070	0.001	0.018	5

Using the TOPSIS method, the optimal solution for the honeycomb A286 micro-lattice structures under different conditions as determined for non-stress-relieved and stress-relieved specimens, is the solution with dimensions of 24mm × 20mm × 30 mm (L × W × H) with a wall thickness of 2 mm, hole diameter of 0.8 mm, and infill thickness of 0.8 mm. For heat-treated (final) specimens, the dimensions are the same except for a hole diameter of 1 mm and an infill thickness of 1 mm.

Furthermore, this research involves an analysis and comparison of nearly optimal solutions with RHA, MHA, and HNS materials by examining their surface properties. To examine microlattice structures effectively, the process begins with the creation of small specimen samples that represent these intricate structures. These specimens are then cut into precise sections using wire-electrical discharge machining (W-EDM) to ensure accuracy. Each of the samples are further subjected to stress relieving and heat treatment. Any subsequent thermal residual stress caused due to W-EDM, would be compensated for in the final specimen, thereby displaying the true nature of the lattice structures. Subsequently, each specimen was meticulously ground and polished for morphological characterization, a crucial step for accurate X-ray diffraction (XRD) analysis. To eliminate any contaminants, the samples undergo thorough cleaning, first with acetone, then with either methanol or ethanol. The final stage before optical microscopy (OM) or scanning electron microscopy (SEM) analysis involves immersing the samples in an etchant solution for approximately 5 min. This process reveals crucial microstructural details, making the samples ready for in-depth analysis as indicated in Figure 3. The residual stress analysis of nearly optimal solutions was conducted on honeycomb and gyroid micro-lattice structures and compared with RHA, MHA, and HNS materials.

Fig. 3.

To evaluate corrosion resistance, accelerated laboratory corrosion tests were conducted within a controlled salt-spray chamber. This approach simulated and expedited corrosive conditions, offering insights into how well materials endure corrosion challenges. Test specimens representing various materials and structures were placed in the chamber, where a saline mist emulated harsh conditions.

Salt-spray test parameters:

Number of specimens: For each type of structure and material, specific specimen quantities were utilized. Honeycomb structures in both ideal and non-ideal conditions, including as-printed, stress-relieved, and heat-treated specimens, were represented by six samples each. Gyroid structures had three samples for as-printed, stress-relieved, and heat-treated conditions. Additionally, one sample each was used for RHA, MHA, and HNS materials.

These comprehensive test parameters ensure the accuracy and reliability of our corrosion analysis, allowing for a thorough assessment of corrosion resistance characteristics in the context of blast mitigation for AFVs.

The investigation employed XRD, using a Bruker D8 XRD machine, to analyse residual stresses in studied materials, revealing insights into the internal stresses impacting mechanical properties. Evaluating residual stresses in protective structures made from RHA/MHA/HNS steels versus bulk forms must account for additive manufacturing effects. Layered printing introduces varied thermal histories and structural changes, causing distinct stress distributions. Thus, protection structures may differ from bulk materials in residual stresses due to manufacturing. Residual stress distributions significantly affect material behaviour, especially in blast mitigation. Therefore, comprehending how geometry and manufacturing impact stress patterns is vital for optimizing structural performance.

Our study focused on A286 honeycomb and gyroid micro-lattice structures fabricated via additive manufacturing. Analysing residual stress aimed to uncover links between lattice geometry, manufacturing, and stress patterns. This exploration enhances understanding of blast mitigation and structural integrity by unravelling these connections.

The powder specimens underwent examination, and the EDS assessment detailing the distribution of elements and the SEM analysis of the powdered specimen for the LPBF procedure are presented in Figure 4.

Fig. 4.

A286 steel powder exhibits exceptional properties due to the synergistic interaction of its elements. Iron provides robustness, toughness, and thermal stability, while nickel enhances high-temperature strength and ductility. Chromium improves corrosion resistance, and titanium contributes to high-temperature performance. Molybdenum enhances strength, hardness, and corrosion resistance at elevated temperatures [13]. These qualities make A286 steel powder ideal for additive manufacturing applications, including blast mitigation. Spherical powder (Figure 4d) production in LPBF ensures uniformity and efficiency in the printing process. Microlattice test specimens of honeycomb and gyroid structures were produced for comparison with RHA, MHA, and HNS materials, aiming to provide an alternative underbody protection solution for AFVs.

The present study investigates the correlation and comparison of A286 honeycomb and gyroid micro-lattice structures with RHA, MHA, and HNS materials. Figure 5 showcases comprehensive optical microscopy images of the RHA, MHA, and HNS materials.

Fig. 5.

Figure 5 depicts the microstructures of RHA, MHA, and HNS steels observed through OM and SEM images. Coarse grain structures in RHA steel offer advantages such as increased ductility and toughness, as well as enhanced energy absorption during explosive blasts [14], but they may act as stress concentrators leading to premature steel failure and reduced blast mitigation efficacy. Conversely, fine grain structures in MHA steel can improve strength and hardness but may increase brittleness, making it less effective in blast mitigation. HNS steel strikes a balance with its dual microstructure of fine and coarse grains. Fine grains enhance strength, toughness, and energy absorption during blasts, while coarse grains may still pose stress concentration risks with visible formation of pits present in the material. To achieve optimal blast mitigation, it is essential to optimize processing parameters and employ post-processing techniques like heat treatment, mechanical deformation, and surface treatment. These approaches can improve the microstructure of RHA, MHA, and HNS steels, enhancing their mechanical properties and overall effectiveness in minimizing the impacts of explosive blasts. Additionally, SEM images of the steel materials provide further insights. The RHA exhibits a uniform microstructure with elongated, well-defined grains, likely due to the rolling process. Surface porosity is evident, however, and can be addressed using additively manufactured A286 steel. The MHA displays a mixture of fine and coarse grains, indicating a lack of potential grain refinement processes. The HNS material exhibits fine and coarse grains, along with additional nitrogen-containing phases that can improve mechanical and corrosion properties [15].

As a result, while the presence of both fine and coarse grains in armour steel can have an impact on its capacity to mitigate blasts, careful optimisation of the processing conditions and the use of additional post-processing techniques are needed, with a restriction of weight addition, which, for the current conventional material, results in a deadlock situation. Producing honeycomb and gyroid microlattice structures aids in the understanding of its evolution in blast operation as an alternative modular solution, providing an alternative for weight reduction while effectively mitigating the effects of an explosive blast [16].

Figures 6 and 7 depict OM and SEM images of honeycomb and gyroid metallic micro-lattice structures under three conditions: as-printed (non-stress relieved), stress-relieved, and heat-treated. The as-printed condition reveals voids and pores, indicating potential detrimental effects on blast mitigation capabilities. These imperfections can reduce energy absorption potential, increase vulnerability to failure upon impact, and compromise structural soundness [17]. For honeycomb structures, voids and pores compromise compression strength and energy absorption during blasts, while in gyroid structures, they can lead to stress concentration and reduced strength and stiffness. However, optimization of powder deposition parameters, inter-layer fusion, and post-processing treatments like stress relief can achieve predominantly fine grain structures under both conditions. To mitigate this issue, shot peening is employed, inducing compressive residual stress and promoting grain refinement, while reducing tensile residual stresses.

Fig. 6.

Fig. 7.

The stress-relieved honeycomb and gyroid metallic micro-lattice structures exhibit fine-grained compositions, enhancing their blast mitigation capabilities. These structures demonstrate notable strength and toughness, making them less susceptible to deformation and failure upon impact [18]. The presence of grain boundaries between adjoining cells contributes to improved mechanical properties, while gyroid structures’ refined microstructure enhances their strength, stiffness, and fatigue resistance under impact loading [19]. To further optimize the material’s properties, the test specimens undergo heat treatment.

Heat treatment enhances mechanical properties, including strength, hardness, and ductility, by refining the microstructure. In the SEM image of honeycomb structures, distinct boundary lines indicate the presence of grain boundaries, reinforcing mechanical properties and blast mitigation capabilities. The refined gyroid structure exhibits increased strength, stiffness, and fatigue resistance under impact loading, although its SEM images lack well-defined boundaries due to its microstructural attributes. Overall, heat treatment effectively improves the microstructure of both structures, resulting in fine-grained materials with enhanced mechanical characteristics and blast mitigation capabilities, making honeycomb structures suitable for diverse applications, including blast mitigation [20].

Corrosion analysis of honeycomb and gyroid micro-lattice structures is crucial for optimizing their blast mitigation effectiveness [21]. Corrosion can weaken metallic materials, compromising their mechanical properties and reducing their ability to withstand blasts. Accelerated lab corrosion testing was performed using a salt spray chamber at a DRDO facility illustrated in Figure 8. Results for the corrosion rate are presented in Table 8, and the correlation between honeycomb, gyroid A286 micro-lattice structures, RHA, MHA, and HNS is shown in Table 9, providing valuable insights for enhancing their performance in blast mitigation applications.

Fig. 8.

Honeycomb and gyroid micro-lattice structures exhibit significantly lower corrosion rates compared to RHA, MHA, and HNS materials, as shown in Table 10. The corrosion resistance attenuation for honeycomb and gyroid A286 micro-lattice structures in correlation to RHA, MHA, and HNS before compression tests is 57.23%, 34.83%, 47.36%, and 41.31%, 10.6%, 27.75%, respectively. After compression tests, the attenuation in corrosion resistance is 37.61%, 4.92%, 23.21% for honeycomb and 17.88%, 25.16%, 1.08% for gyroid in correlation to RHA, MHA, and HNS, respectively. The higher surface area-to-volume ratio of honeycomb and gyroid micro-lattice structures, along with their unique interconnected cellular geometry, contributes to their superior corrosion resistance and enhanced durability.

The interconnected cellular configuration of the honeycomb micro-lattice ensures a homogeneous stress distribution, reducing localized stress concentrations that can trigger corrosion. This feature mitigates the risk of localized corrosion and material deterioration. Additionally, the expanded surface area and distinctive microstructure contribute to its superior corrosion resistance compared to the gyroid micro-lattice. The innate robustness and rigidity of honeycomb and gyroid micro-lattice structures make them highly effective in blast mitigation, with a longer lifecycle compared to traditional materials. In contrast, conventional materials like RHA, MHA, and HNS display higher corrosion rates, compromising their mechanical properties and resilience, making them less reliable for blast mitigation applications, especially in underbody AFV use across different terrains.

Residual stress analysis is crucial for enhancing blast mitigation in armoured fighting vehicles, particularly in the underbody region. It provides insights for armour design improvement, identifies vulnerable areas for strategic reinforcement, and aids in refining manufacturing processes [22, 23]. The central XRD facility offers residual stress data for A286 honeycomb and gyroid microlattice structures in correlation with RHA, MHA, and HNS materials, as depicted in Figure 9 and Table 11.

Fig. 9.

Table 11 reveals a distinct contrast in residual stress measurements between micro-lattice structures and conventional monolithic materials. Table 12 reveals that the compressive residual stress attenuation in honeycomb A286 micro-metallic lattice structures is 47.30%, 40.12%, and 25.70% in relation to RHA, MHA, and HNS material, respectively. The study reveals that gyroid A286 micro-metallic lattice structures exhibit attenuation percentages of 36.6%, 27.96%, and 10.61% in relation to RHA, MHA, and HNS materials, respectively. In relation to micro-lattice structures such as honeycomb and gyroid, it has been observed that the compressive residual stress attenuation is 16.88% higher in honeycomb as compared to the gyroid lattice structure.

Metallic micro-lattice structures, particularly honeycomb ones, exhibit high compressive residual stress, making them promising for blast mitigation (16.88% attenuation compared to gyroid lattice). During a blast event, the lattice structure efficiently absorbs and disperses the energy of the blast wave while maintaining its structural integrity, preventing catastrophic failure. In contrast, conventional materials like RHA, MHA, and HNS lack equivalent compressive residual stress and are susceptible to brittle fracture under blast-loading circumstances. The distinctive geometry of honeycomb micro-lattice structures with interconnected hexagonal cells generates high compressive residual stress, enhancing endurance against fatigue and crack propagation. On the other hand, gyroid lattice structures, with a recurring cubic unit cell, possess favourable mechanical properties but lack comparable compressive residual stress as honeycomb structures [6]. In a blast event, the honeycomb micro-lattice structure undergoes controlled deformation, efficiently absorbing and redistributing the blast wave’s energy, preserving structural integrity, and avoiding catastrophic failure due to elevated compressive residual stress [7]. Gyroid lattice structures may lack equivalent compressive residual stress, making them susceptible to brittle fracture under blastloading conditions [8]. In comparison, honeycomb structures exhibit significant strength, making them more suitable for blast mitigation applications.