**Abstract**

There is direct proportionality between ultimate tensile stress (UTS) and residual stresses (RS). Residual stresses gradually decrease with decreasing cooling/ quenching rates. Quenching in cold water develops highest, whereas air cooling produces lowest, residual stresses. Significant increase in RS is observed in specimens with low dendrite arm spacing (high solidification rate), while lower residual stresses are measured in specimens with high dendrite arm spacing (low solidification rate). For I-4 and V-6 engine blocks, there is refinement in microstructure due to the increase in solidification rate along the cylinder length. The developed residual stresses are normally tensile in both engine types. Air cooling following solution heat treatment produces higher RS compared to warm water and cold water quenching. Solution heat treatment and freezing lead to maximum RS relaxation where 50% of the stresses are reduced after the solution heat treatment step. Aging time and temperature are directly proportional to the residual stresses relaxation. Relaxation of RS also depends on the geometry and size of the workpiece. It should be mentioned here that the I-4 and V-6 cylinder blocks were provided by Nemak-Canada (Windsor-Ontario-Canada). Residual stress measurements technique and procedure are typical of those used by the automotive industry in order to provide reliable data for industrial applications supported by intensive experiments.

**Keywords:** residual stresses, Al cylinder blocks, stress relaxation, effect of microstructure, heat treatment, quenching conditions

## **1. Introduction**

Residual stress is generally referred as an internal stress, which exists in equilibrium inside a component in the absence of any external forces or constraints, temperature gradients, or any other external influences [1]. Any existing residual stresses are considered as elastic stresses that are kept under static equilibrium. Elastic limit is the maximum value that can be reached by any residual stresses. Any stresses higher than the value of elastic limit with no opposing forces will be relieved by plastic deformation until it reaches the value of the yield stress [2].

Excessive residual stresses may be generated due to the large difference in thermal expansion coefficient between the aluminum alloy (2.4 <sup>10</sup><sup>5</sup> <sup>K</sup><sup>1</sup> ) and cast iron (1.5 <sup>10</sup><sup>5</sup> <sup>K</sup><sup>1</sup> ) [3]. The presence of these residual stresses renders engine blocks prone to either distortion or failure. Distortion of the cylinder bores results in a loss in compression of the air-fuel mixture due to improper sealing between the cylinder wall and the piston. This loss of sealing causes a portion of the compressed air-fuel mixture to leak out of the combustion chamber by a process known as "blow-by" [4] which reduces the engine efficiency. In conclusion, aluminum engine blocks with gray iron cylinder liners are prone to tensile residual stresses along the cylinder bores, which results in distortion, cracks, and a reduction in engine efficiency. Several ideas have been introduced in order to change the cast iron liners with another suitable replacement but due to technical and economic problems cast in liners are considered the most effective option in engine block manufacturing [5].

Many automobile parts are made of aluminum alloys such as engine blocks, cylinder heads, and suspension parts, and to perform efficiently and eliminate premature failure, residual stresses must be minimized. During service, these parts undergo heating and cooling cycles which promote residual stresses. Presence of residual stresses in the casting deteriorates fatigue life and dimensional stability of the part [6]. Tensile residual stresses can result in distortion and cracking of the component during quenching or machining and if this occurs during service, it can cause a reduction in efficiency or failure of the part [7]. The presence of residual stresses and/ or distortion in a structural component, such as an aluminum casting, has a negative influence on the component's dimensional tolerance, performance and fatigue life [6].

Dynamic simulation was conducted on two crankshafts, cast iron and forged steel, from similar single cylinder four stroke engines [8]. Finite element analysis was done for different engine speeds and as a result, critical engine speed and critical region on the crankshafts were obtained. Stress variation over the engine cycle and the effect of torsional load in the analysis were investigated. Results from FE analysis were verified by strain gages attached to several locations on the forged steel crankshaft. Modeling of residual stresses in quenched cast aluminum components was carried out by Wang et al. [9]. To simulate residual stress and distortion induced during quenching, a finite element based approach was developed by coupling an iterative zone-based transient heat transfer algorithm with material thermo-viscoplastic constitutive model. With the integrated models, the numeric predictions of residual stresses and distortion in the quenched aluminum castings are in a good agreement with experimental measurements.

The automotive industry is the largest consumer of Al-Si cast alloys, where these alloys have replaced steel for the sake of greater fuel efficiency and higher performance, attributed to their much lighter weight and high thermal conductivity. Thus, Al-Si castings have gradually replaced automobile parts such as transmission cases, intake manifolds, engine blocks and cylinder heads that were formerly manufactured using steel and cast iron. The most common aluminum casting alloys that are used in the automotive industry are 319.0 (Al-6Si-3.5Cu), 332.0 (Al-9.5Si-3Cu-l.0Mg), 355.0 (Al-5Si-l.3Cu-0.5Mg), A356.0 (Al-7Si-0.3Mg), A357.0 (Al-7Si-0.5Mg), 380.0 (Al-8.5Si-3.5Cu), 390.0 (Al-17.0Si-4.5Cu-0.6Mg), 413.0 (Al-12Si) and 443.0 (Al-5.2Si) alloys. Amounts of alloying elements differ between different series, leading to changes in the final properties [5, 8].

Residual stresses can be classified into two groups according to their origin: the first one is macroscopic residual stresses which correspond to the residual stresses originating from heat treatment, machining, and mechanical processing, while the second group is microscopic residual stresses which often originate from lattice defects such as vacancies, dislocation pile-ups and thermal expansion/contraction mismatch between phases and constituents, or from phase transformations [6, 9].

The magnitude of residual stress depends on the stress-strain behavior and the degree of the temperature gradient attained during the quenching operation, which produces strain mismatch. It is found that the magnitude of the residual stresses is

#### *Generation and Relaxation of Residual Stresses in Automotive Cylinder Blocks DOI: http://dx.doi.org/10.5772/intechopen.93664*

directly proportional to the yield stress and Young's modulus (E). Furthermore, the stress-strain behavior at elevated temperature is an important factor in determining the amount of residual stresses [2]. Certain physical properties also increase the amount of strain mismatch (residual stresses) such as low thermal conductivity (k), high specific heat (c), high coefficient of thermal expansion (α) and high density (ρ) [2].

Residual stresses are often regarded as undesirable and harmful. Prolonging service life of any product can be achieved if such harmful residual stresses are eliminated or reduced. Several methods have been introduced in order to reduce these residual stresses. Annealing is one of these methods which involves exposing the material to very slow rates of cooling and heating with the aim of relieving stresses without altering the microstructure. If the temperature is too high then recrystallization might happen, leading to change in properties such as the yield stress which may not be desirable. The residual stresses relaxation by annealing occurs by one of two main mechanisms. The first is plasticity caused by reduced yield strength at an elevated temperature where instantaneous relief of stress occurs as the temperature is increased. The second mechanism is a creep based mechanism, which allows stress relief to occur over time [1].

Residual stresses can be quantified by many techniques. There are mechanical techniques such as sectioning, hole-drilling, curvature measurements, and crack compliance methods. These techniques correlate the measured residual stresses in components to the distortion. Diffraction techniques cover electron diffraction, X-ray diffraction, and neutron diffraction, which quantify the residual stresses by measuring the elastic strains in components. Other techniques, including magnetic and ultrasonic techniques, and piezo spectroscopy are also used to measure the residual stresses developed [6]. The mechanical techniques are considered destructive tests while the others are non-destructive tests but their accuracy is dependent on the microstructural variation and geometric complexity of the component structure.

An engine block as shown in **Figure 1** is the largest metal component in a car and is the most intricate. It holds and supports all other engine components such as cylinders and pistons and contains passages for coolant. The engine block is where combustion converts into mechanical energy that drives transmission propelling the car. Engine blocks used to be made of iron but today most of them are made of

**Figure 1.** *Cast iron V-cylinder block (closed deck type) including a crankcase [10].*

aluminum alloy for fuel efficiency. It is the largest and most complex single piece component in the car to which all other parts are attached. It represents from 3 to 4% of the total weight of the car. The block is typically arranged in a "V," inline, or I-4 horizontally-opposed (also referred to as flat) configuration and the number of cylinders may range from 3 to as much as 16 [10].

Carrera et al. [11] conducted a series of experimental tests to measure the residual stresses using strain gauges attached to different automotive engine blocks **Figure 2**. They discovered the development of tensile stresses higher than 150 MPa when the engine block contained the cast iron liners, while the engine blocks without cast iron liners exhibited 20 MPa compressive stresses in the cylinder bridge (**Table 1**). Furthermore, it has been observed that the residual stresses are affected by the dimension of the block and the wall thickness of the cylinder bridge where residual stresses decrease as the thickness increases. It was also found that V-8 engine blocks develop higher residual stresses than I-4 blocks with equivalent walls thickness [11]. These observations match the results for residual stresses obtained from the finite element model made by Su et al. [12].

Carrera et al. [13] and Colas et al. [14] analyzed residual stresses in complex aluminum castings. Measurements of residual stresses were carried out by extensometric means in automotive engine blocks. The results indicate that tensile stresses are caused during cooling of the aluminum alloy restricted by iron liners. Such observation is confirmed by measurements carried out in engine blocks cast without liners that develop compressive stresses in their cylinder bridges. The residual stresses are affected by the dimension of the block and the wall thickness of the interliner bridge, implying that a bigger block, such as a V-8 will develop higher

**Figure 2.**

*Identification of the cylinder bridges in an I-4 block. Glued strain gauges are indicated: (a) eight cylinders and (b) four cylinder blocks.*


#### **Table 1.** *Characteristics of the studied blocks.*

*Generation and Relaxation of Residual Stresses in Automotive Cylinder Blocks DOI: http://dx.doi.org/10.5772/intechopen.93664*

residual stresses than an I-4 that has walls of equivalent dimensions. Heat treating contribute to reduce the residual stresses.

A practice of shaking the blocks at an early stage contributes in reducing the residual stresses. Measurements of residual stresses in automotive blocks can be used as an early warning of changes taking place during processing of the material, and can be an aid when changes in design have to be made. According to Elmquist et al. [15], the feeders, which act as extra heat sources, affect residual stresses locally and helps to differences in stresses beneath the feeders, compared to corresponding areas between the feeders. **Table 2** lists the most frequently used alloys in the production of automotive components. Residual stresses as shown in **Figure 3** can be classified into two groups according to their origin: the first one is macroscopic residual stresses originating from heat treatment, machining, and mechanical processing. The second group is microscopic residual stresses which often originate from lattice defects.
