Minimal Effort for Maximum Result: How an Optimisation Approach can Help Improve Manufacturing in the Aerospace Industry.

Achieving a lightweight aircraft without compromising on strength is a major goal in the aerospace and defence industries. While strength is essential for safety, the weight of an aircraft directly impacts its fuel efficiency. Weight reduction in metallic components like fuselage skins is typically achieved by reducing the thickness of the metal. But physical methods using hard tools can create stress, burrs, and distortions on the metallic surface, which affects performance and compromises safety. To overcome this, industrial manufacturers typically use a process called chemical milling.

Chemical milling, also known as chemical etching, is a subtractive machining technique that uses chemical etching baths with control conditions to selectively remove any unwanted sections from a wide range of materials and create the preferred shape. The process can also be used to create blind features, which are features such as rivets that can be installed only from one side of the workpiece. Chemical milling is key to obtaining high-precision, lightweight, thin metal parts, with suitable surface finishing.

Identifying the optimal conditions—such as the choice of etching agent, etch rate, and surface finish precision—is key to an effective chemical milling procedure. In a new study published in the Journal of the Indian Chemical Society, researchers from the Indian Institute of Petroleum and Energy (IIPE) have ascertained the optimal chemical milling conditions for two commonly used metals in aircraft, aluminium 2024-T3 clad and 7075-T6 bare. “Traditionally, one variable at a time (OVAT) method

2 employs for the process optimization. But our study outlines a method to optimise multiple conditions (variables) at a time, while accounting for the interactions between them,” says Dr. Ravi Kumar Sonwani, Assistant Professor at the Department of Chemical Engineering, IIPE, who led the study.

For their study, the researchers first performed chemical milling on several samples under different process conditions. The main parameters they focused on were the process temperature and the composition of the etchant bath, namely, concentration of sodium hydroxide, sodium sulphide, triethanolamine, and dissolved aluminium. During the chemical milling process, the metal surface was first pre-treated to remove oil, grease, rust, dust, and other substances. After this, maskant was sprayed on the portions of the metal surface that were to be protected from dissolution. The surface was then dipped in etchant solution. De-smutting and cleaning were performed after that to remove residual materials on the surface. The etched parts were then analysed using scanning electron microscopy (SEM), x-ray diffraction (XRD), and x-ray fluorescence to understand their surface characteristics, such as surface roughness and depth of the etched cut.

The data obtained from the chemical milling of the different samples was then put into an optimisation tool, Taguchi orthogonal array. Taguchi method is a set of statistical methods that are commonly used in engineering and manufacturing to improve the quality of produced goods. Taguchi method is employed to design the process parameters (here, composition of etchant and temperature) and study the correlations between different process variables and their outcomes (here, morphology and composition). Importantly, Taguchi method can provide the optimal conditions under which the highest possible yield can be obtained with minimal effort. The researchers used the Minitab Statistical Software tool to perform the Taguchi optimisation.

The team used experimental values from a total of 27 experiments as input for the Taguchi orthogonal array to evaluate the depth of cut and surface roughness. “We also analysed the experimental results of surface roughness values using a statistical formula known as analysis of variance (ANOVA),” explains Dr. Sonwani. “It compares the statistical variance across the mean values of different groups. We used ANOVA to identify which process factors significantly affect surface roughness.”

They found that 50 g/L of triethanolamine, 125 g/L of sodium hydroxide, 20 g/L of sodium sulphide, and 20 g/L of dissolved aluminium was the optimal etchant composition. The optimal process temperature was found to be 95 °C. The researchers then performed chemical milling experiments under optimal conditions to justify the predictions of the optimisation model. Under optimal conditions, it was possible to achieve a minimum surface roughness of 30.0 μ inch for aluminium 2024- T3 clad and 29.0 μ inch for 7075-T6 bare. The optimised conditions presented in the study can be directly employed in the aerospace and defence industries to produce metallic components made of aluminium. “Additionally, our study shows that the Taguchi method can provide the optimal conditions for chemical milling in fewer trials than the traditionally employed OVAT approach,” adds Dr. Sonwani. “It is our hope that the experiment design and approach laid out in our study can be used in industries to obtain optimal chemical milling conditions for other materials as well.” In addition to the aerospace industry, the chemical milling process is also used in the automobile, construction, electronics, and medical industries to manufacture lightweight metallic components with appropriate weight-to-strength ratio. With such wide applicability, the approach detailed in this study could find extensive industrial use in the future.

Congratulations to the authors on their new publication!