Keywords

1 Introduction

ASTM [1] defines Additive Manufacturing (AM) as a process of joining materials to make objects from 3D models in a layer-by-layer strategy. The design freedom of AM allows engineers to make complex parts that are difficult to produce with traditional methods [2]. As a result, the technology has found application in the aerospace, automotive and medical industries [3]. Compared to conventional manufacturing, AM boasts of freedom of design, flexibility, cost and time effectiveness [4]. Among the different AM systems, the Fused Deposition Modelling (FDM) is the most common cost-effective and safe technology [5].

The FDM is an AM process involving the use of a filament of a thermoplastic material that is fed through the extruder followed by material deposition and curing in successive layers [6]. The FDM process involves melting a polymeric material, which is extruded and deposited layer by layer to produce the desired component [7]. The manufacturing process of FDM makes the technology easily accessible for small and medium scale enterprises to use it in the manufacture of local products. Existing literature has reported that the use of the FDM technology for the fabrication of hydraulic pump impeller boasts of material, time and cost effectiveness with similar performance to the impeller manufactured conventionally [8, 9]. Although the surface roughness of the impeller poses, a constraint but this challenge can be addressed via post-processing. Much of the studies in literature are focused on the use of experiments to determine optimum parameters for producing parts using FDM. There is limited emphasis on the initial numerical analysis. Hence the study aims to investigate the structural integrity and suitability of FDM-produced pump impellers using numerical analysis. This work contributes in both method and theory to the investigation of the stress, deformation and strain response of the impeller and each blade at different blade position, pressure and rotational speeds. The material considered for the application is tough PLA due to its superior mechanical properties. The study provides an insight into the behaviour of the performance of pump impeller under the required service condition. It is useful in providing guidance application of FDM to produce functional parts. Through the study, the capability of AM as a suitable approach for enabling local sustainable production of spare parts is demonstrated. Furthermore, this study can promote sustainability of the manufacturing process in terms of material, energy, time and cost effectiveness as well as environmental friendliness without compromising the performance of the manufactured impeller.

2 Materials and Methods

The component considered in this study for modelling and simulation is the automobile pump impeller. Pumps are vital components of many industrial plants and find applications in a wide range of endeavours from power plants to oil and gas plants to manufacturing plants and water treatment and distribution plants. Thus, the usage, maintenance, and replacement of pump impellers are common operations within these industries which influence their day-to-day running in terms of cost and service-delivery. FDM additive manufacturing process comes with the advantage of ease of manufacturing and replacing impellers in-situ, with less downtime, while staying in touch with the demands of meeting industry targets. However, according to Salifu et al. [10], the usage, maintenance and replacement of low-end, low-life components like impellers are based on operation life assumptions and quick-fix ends, which sometimes have detrimental consequences. The advent of advanced computing and computing power over the years for investigating the performance of additively manufactured components have been highlighted [11,12,13]. Fameso and Desai [14], and Fameso et al. [15] encouraged predictive study of the mechanical integrity of additively manufactured components using, commercial finite element analysis (FEA) codes in modelling and simulating their working operation and condition even before production. The thermomechanical behaviour of the FDM impeller under typical pumping operation condition was determined using ABAQUS CAE/2020 commercial software. The stress and strain distribution across the 3D modelled impeller were evaluated as developed under operating conditions. Since pumps are built to work under different conditions and working temperatures, stresses developed can be as a result of any or a combination of cyclic, centrifugal and thermal loads which must not exceed the limits of the material’s mechanical and thermal properties, otherwise increased deterioration rate and eventual failure will be imminent if not inevitable.

2.1 Model Development

The mechanical properties of the impeller’s material are presented in Table 1. The model part was created as single solid homogenous part with no nodal separations or assembly ties/constraints between the base of the impeller and its blades, in relation to its as-cast manufacturing state.

Table 1. Mechanical Properties of PLA
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3D hexagonal-shaped controlled mesh with advancing front algorithm was used in discretizing the modelled part into finite elements and the convergence in size of the elements was measured based on the thermos-mechanical stresses developed in the part. This was used in determining the mesh size of 5 mm as the most suitable mesh which strikes a balance between computational cost and accuracy. The meshed impeller model had about 7,000 linear continuum hexahedral 8-node brick elements.

2.2 Analysis Steps and Loading Conditions

In the analysis of sequential interactions of thermal and mechanical forces on the impeller as carried out in this study, static general and steady state heat transfer steps were created and sequentially invoked. The selection of the thermal and mechanical conditions (temperature, operating speed and pressure) were guided by the existing literature within the context of common applications of the impeller. The novelty of the modelling approach lies in the fact that the thermal analysis was first carried out to compute the heat transfer and all thermal effects of the operating environment on the impeller. The output is stored in a database, which includes the nodal stresses due to heating and changes in temperature. This output database was subsequently invoked as initial conditions for the analysis of mechanical forces in the static general analysis step. This approach is aimed at improving the precision of investigating the stress, deformation and strain of the impeller and each blade at different blade positions, pressures and rotational speeds.

Thermal interactions were created between the part and its environment. A sink upper design limit temperature of 420 K with film coefficient of 10 kW/m2K was applied to the anterior façade of the impeller where the blades interact with the fluid it supplies energy to, while a sink temperature of 298 K with film coefficient of 18 W/m2K was applied to the posterior façade which adjourns the external working environment of the impeller. Both temperatures correspond to the operating temperature gradient between the impeller and the fluid on one hand, and the impeller and its outside surroundings on the other hand. The film coefficients also represent the convective coefficients of heat transfer between the impeller and the fluid and air streams respectively.

3 Results

The results and superimposed graphical plot of deformation and strains on the impeller at different rotating speeds and a constant operating pressure of 0.11 MPa and at different operating pressures but constant rotational speed are presented in Fig. 1 and Fig. 2.

Fig. 1.
figure 1

Deformation and strain of impeller at different rotational speed

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Fig. 2.
figure 2

Deformation and strain of impeller at different pressure

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It can be observed from Fig. 1, that the impeller experiences increasing strain as its operating speed increases, with the rate of increase in strain maintaining a gentle near-linear profile at speeds of between 200 and 750 rad/s, corresponding to 1,900 rpm to 7,125 rpm, which is within the operating range of most motor vehicles. However, tested at rotational speeds higher than these, the strains experience exponential increase, with a sharp jump in the profile from a gentle slope to a rather steep one, suggesting that operating in this condition will be detrimental to the integrity of the impeller. The result of these increasing strains with operating speed is a continuous increase in the maximum possible deformation that the impeller will experience as a result of a combinations of the rotational body forces and other physical agents associated with its operation in such conditions.

Figure 2 similarly presents increased deformation resulting from the corresponding increase in strains dues to rising operating pressures. The results and contour plot for the stress, deformation and strains on the impeller at the different rotating speeds and operating pressures are presented in Fig. 3.

Fig. 3.
figure 3

Stress, deformation and strains on the impeller at rotating speed of 100 rad/sec and operating pressure of 0.11 MPa

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Figure 4 shows that low operating speeds and the relatively lower 0.11 MPa operating pressure, the maximum induced stresses on the blades are going to be below yield, with zero plastic strain experienced. The strains in this region can be managed by mounting dampers that will absorb and reduce these effects.

Fig. 4.
figure 4

Stress, deformation and strains on the impeller at rotating speed of 300 rad/sec and operating pressure of 0.11 MPa

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The bulk of the strains experienced will be elastic strains which will not cause any permanent distortions or damage. All the deformations especially at the leading edges will be recoverable. As expected, the only region of concern will be the groove where the impeller mounts the shaft. The effects of cycling stresses. The distribution profiles at a slightly higher speed of 300 rad/s corresponding to 2850 rpm. However, the gradual commencement of build-up of stresses in beginning to manifest, with a possibility of plastic strain inducing yield at some location on the load bearing leading edge of blade 2. Deformations induced at these conditions are still largely recoverable making it safe still, to be operated under such condition. The effects of ramping up the operating speed to the limits at 1000 rad/s corresponding to 9500 rpm maintained first at 0.11 MPa and then at an upper limit of 0.22 MPa are presented respectively in Figs. 5 and 6.

Fig. 5.
figure 5

Stress, deformation and strains on the impeller at rotating speed of 1000 rad/sec and operating pressure of 0.11 MPa

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Fig. 6.
figure 6

Stress, deformation and strains on the impeller at rotating speed of 1000 rad/sec and operating pressure of 32 MPa

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The deformation and strain response of each blade to the conditions highlighted thus far are graphically presented in Fig. 7. In addition, these results have corroborated results presented in earlier, which has revealed that extreme operating speeds of up to 1000 rad/s or pressures of 0.22 MPa are not suitable conditions under which the impeller will operate without compromising its efficiency and structural integrity.

Fig. 7.
figure 7

The deformation and strain response of each blade at different blade position, pressure and rotational speed

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4 Conclusion

The aim of this study is to investigate the performance of FDM in the manufacture of pump impellers at different operating speeds and pressures. This was achieved with the aid of modelling and simulation in the Abaqus CAE. 3D hexagonal-shaped controlled mesh with advancing front algorithm was used in discretizing the modelled part into finite elements and the convergence in size of the elements was measured based on the thermos-mechanical stresses developed in the part. It was found that extreme operating speeds of up to 1000 rad/s or pressures of 0.22 MPa were not suitable conditions under which the impeller will operate without compromising its efficiency and structural integrity. The study provides an insight into the behaviour of the performance of pump impeller under the required service condition. It is useful in providing guidance application of FDM to produce functional parts. In addition, this study can promote sustainability of the manufacturing process in terms of material, energy, time and cost effectiveness as well as environmental friendliness without compromising the performance of the impeller manufactured conventionally. Future work can consider the comparative analysis between the numerical results and the actual experimental results, optimisation of the process parameters and the prototyping of the modelled component.