Transient analysis of residual strain during heat treatment of multi-material engine blocks using in-situ neutron diffraction A. Lombardi a,n , D. Sediako b , A. Machin a , C. Ravindran a , R. MacKay c a Centre for Near-net-shape Processing of Materials, Ryerson University, Toronto, Ontario, Canada b Canadian Neutron Beam Centre, Chalk River, Ontario, Canada c Nemak of Canada Corporation, Windsor, Ontario, Canada article info Article history: Received 21 April 2015 Accepted 21 May 2015 Available online 22 May 2015 Keywords: 319 Al Engine Blocks Casting Solution heat treatment Residual stress In-situ neutron diffraction abstract Aluminum engine blocks with gray iron cylinder liners are prone to tensile residual stresses along the cylinder bores, which may cause distortion and reduce engine efficiency. This study pioneered the ap- plication of in-situ neutron diffraction to measure the residual strain in the cylinder bores of Al engine blocks as a function of time during solution heat treatment, simulated at the spectrometer. This involved designing and incorporating a unique heating system for the engine blocks. The results indicated gradual relief in residual strain with time due to creep until strain was completely relieved. Furthermore, post- treatment cooling led to the development of tensile residual strain due to thermo-mechanical mismatch between the gray iron liners and the surrounding Al alloy. Crown Copyright & 2015 Published by Elsevier B.V. All rights reserved. 1. Introduction In recent years, lightweight Al alloys have been used ex- tensively in automotive applications, such as engine blocks, to improve vehicle fuel efficiency and comply with stringent gov- ernment legislation. However, gray cast iron cylinder liners are necessary to improve wear resistance in the cylinder region, making these engines prone to the development of high tensile residual stress along the cylinder bores during production [1–3]. The large tensile residual stress, if excessive, may induce perma- nent dimensional distortion in the engine block [4] and reduce operating efficiency, resulting in environmental (increased carbon emissions) and economic (expensive automotive recalls) ramifications. Previous studies have illustrated that heat treatment is an ef- fective method to reducing tensile residual stress in Al engine block castings containing gray iron cylinder liners [1,3,5]. How- ever, these studies were performed following heat treatment, and consequently did not observe how the residual stresses and strains relax as a function of time during heat treatment. Therefore, there is a need to determine how residual stresses/strains relieve during heat treatment of complex components containing dissimilar materials. The data from this unique study is vital in the devel- opment of optimal heat treatment schedules for multi-material components such as engine blocks. 2. Experimental methodology The current study utilized 319 Al alloy (composition in Table 1) engine blocks containing gray iron cylinder liners, which were precision sand cast using the Cosworth process. Initially as-cast engine blocks were heated to the production solutionizing temperature (470 °C for 1 h heating time) while mounted to the neutron spectrometer table. Heating of the engine blocks was accomplished using an array of tubular coil heaters inserted into the cylinder bores and strip heaters placed around the cylinder bridge of interest, as illustrated by the rectangle in Fig. 1(a). In addition, the engine blocks were wrapped in 50 mm thick Kaowool insulating blankets to mitigate heat loss. Thermo- couples were inserted into various locations of the engine blocks to serve as a feedback for PID temperature controllers, which maintained the set point temperature throughout the experiment. A transient strain relief profile was developed using in-situ neutron diffraction at a cylinder depth of 50 mm (Fig. 1(b)). Fol- lowing this experiment, the engine blocks were cooled while wrapped in Kaowool. A complete scan was performed at ambient temperature from top (x ¼ 0) to bottom (x ¼ 120 mm) of the cy- linder to quantify the strain developed during cooling. The Al (331) reflection and a monochromatic neutron beam with a wavelength of 1.55 Å were used for all experiments. In addition, the engine blocks were oscillated by 8° to mitigate the impact of the coarse grain structure found in the engine block [3,4]. Residual strain in the axial (ε A ) orientation was determined along the cylinder bridge using the “peak shift” method (Eq. 1) which related the interplanar spacing of the specimen (d hkl ) Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/matlet Materials Letters http://dx.doi.org/10.1016/j.matlet.2015.05.094 0167-577X/Crown Copyright & 2015 Published by Elsevier B.V. All rights reserved. n Corresponding author. E-mail address: a2lombar@ryerson.ca (A. Lombardi). Materials Letters 157 (2015) 50–52