May/June_AMP_Digital

FEATURE A D V A N C E D M A T E R I A L S & P R O C E S S E S | M A Y / J U N E 2 0 1 8 3 8 Q uenching is a critical step in metal heat treatment in the manufacture of high performance compo- nents. While the quenching process produces parts with good mechanical properties, an undesirable effect of quenching is induced thermal residual stress—a leading cause for quality issues associated with high-cycle fatigue. Therefore, a common approach during product develop- ment involves trying different quench media (air or water) and part orientations to minimize residual stresses from quenching (Fig. 1). However, the choice of quench medium and part orientation during quenching is often determined by intuitive engineering judgement at best and a trial-and- error approach at worst. Digital verification using finite element analysis (FEA) has gained popularity due to its efficiency. The computation- al method for predicting residual stress involves calculating the temperature history and using temperature data as the thermal load to structure analysis for stress and deforma- tion calculations. However, before the industry adopts the computational fluid dynamics (CFD) method for heat trans- fer analysis, it must prove to be reliable compared with the popular method of temperature calculation using the heat transfer coefficient (HTC) method. The biggest drawback of the HTC method is that it relies on thermocouple measure- ment for calibration and the calibrated HTC might not apply to different part designs and quenching processes. Now, with the advancement in CFD technologies, tem- perature history for quenching can be accurately calculated. Because thermal residual stress is directly linked to nonuni- form temperature distribution in the metal, the spatial tem- perature gradient canbe evaluated to study theperformance of different quench media and quenching configurations. AIR QUENCH PROCESS FOR CYLINDER HEADS The main heat extraction mechanism in air quenching is forced convection. In the CFD model investigated here, it is assumed that the buoyancy effect and radiation heat transfer have a negligible effect on accuracy. Plots of CFD simulation results and thermocouple readings show excel- lent agreement in overlapping curves, validating the model (Fig. 2). CFD is used to study four different air quenching con- figurations (Fig. 3). An advantage of CFD simulation over physical testing is the ability to visualize flow patterns and identify low heat-transfer regions under stagnant air pock- ets. Quenching configurations in Fig. 3(a), (b), and (c) repre- sent a conveyer style quenching environment, while Fig. 3(d) represents a basket style environment. Cooling curves (Fig. 4) show that the cylinder head quenched in a basket, Fig. 3(d), cools faster than those quenched on a conveyer, Fig. 3(a), (b), and (c). In addition, USING VIRTUAL TOOLS FOR QUENCHING PROCESS DESIGN Using virtual tools to study aluminum cylinder head quenching processes delivers valuable information for process design and optimization. James Jan and Madhusudhan Nannapuraju Ford Motor Co., Livonia, Mich. Fig. 1 — Left, heat treatment process for aluminum cylinder heads; right, quality concern associated with quenching process. 6