Experimental and computational study on thermo‐mechanical fatigue life of aluminium alloy piston

Abstract

To assess the life of a new diesel aluminium alloy piston under thermal shock loads, thermo-mechanical fatigue (TMF) testing was conducted to characterise the TMF properties of the piston alloy, and an empirical model based on the constraint ratio concept was proposed to predict the TMF life of the piston. Considering that the empirical model required expensive experimental support, a platform with high-frequency induction heating was established to simulate the force on the piston under thermal shock loads to calculate the piston life using the thermal shock test. Additionally, a finite element method was developed to compute the distributions of temperature, strain, and stress during this process. The characteristics of crack initiation and propagation in TMF test rods and piston mock-ups were also investigated. The results showed that the TMF test rod suffered brittle fracture with brittle quasi-cleavage features. The microcracks mainly occurred in primary Si particles due to stress concentration around the primary Si particles induced by the difference between the thermal expansion coefficients of Si and Al. From a macro perspective, the piston initially cracked at the rim above the pinhole, where the stress is larger than that along other directions. From a micro perspective, the protrusions of various sizes on the piston rim were induced by the compression stresses at high temperature. The piston cracks usually initiate around primary Si particles, propagate along the edge of primary Si in a straight line, bifurcate and then stop at a certain depth. If the piston was only heated, cracks or plastic deformations were not produced. The piston life can be assessed using the proposed empirical model based on the constraint ratio concept or thermal shock testing based on the developed platform. The difference between the predicted and experimental life was not greater than 7%.

Evaluation of plastic deformation and prediction of thermal mechanical fatigue life of an Al–Si alloy piston for diesel engines

“…During thermal analysis, to simulate the heat input from the induction heater used in the thermal shock testing of the piston, a piecewise surface heat source depicted by Equation 1 11 was adopted. where q is the heat flux, η is the heating efficiency, Q is the rated power with a value of 80 kW, S is the surface area of the induction coil with a value of 14,800 mm 2 , h 1 is the distance between the coordinate origin and the bottom of heat zone with a value of 47.22 mm, h 2 is the distance between the coordinate origin and the top of heat zone with a value of 49.75 mm, and x and z are the radial and axial coordinates in the cylindrical coordinate system, respectively.…”

“…The heat conduction in the model obeyed Fourier's law and the heat output induced by convection and radiation can be depicted by Equation 2 11,22 where q out is the total heat output of the model, h is the film coefficient with a value of 0.033 mW·mm −2 ·°C −1 , 11 T b is the room temperature with a value of 25°C, and T a is the surface temperature of the model.…”

“…where q out is the total heat output of the model, h is the film coefficient with a value of 0.033 mWÁmm À2 Á C À1 , 11 T b is the room temperature with a value of 25 C, and T a is the surface temperature of the model. During mechanical analysis, the finite element mesh used in the thermal analysis was remained, and the adopted boundary condition only prevented the rigid body displacement of the model.…”

“…When the diesel engine operates, the temperature of the piston ranges from 100 C to 400 C. 11 In this study, four thermal shock tests of the piston were separately carried out in this temperature range, the detailed conditions are presented in Table S1. Before the testing, a thermocouple was installed at T3 or R3, as shown in Figure 3.…”

“…In experimental approaches, the durability testing of engines is a mainstream way to obtain the piston life, but it has a shortcoming of long test period. 7 Considering the piston experiences severe thermal and mechanical loads especially thermal shock load when the engine starts or changes from idle to full load, the thermal shock testing of pistons using laser heating 8,9 or induction heating 10,11 has been used to estimate the life of piston under thermal shock load. Although the thermal shock testing of pistons is of an accelerated method, it is still extremely timeconsuming.…”