**3. Results and discussion**

#### **3.1 Stage I**

Results of secondary dendrite arm spacing (SDAS—**Figure 6**) measurements show that the average SDAS for the block casting is 60 μm which is reflected by the large grain size observed in **Figure 7(a)**. Due to the high solidification rate obtained

**Figure 6.** *Backscattered electron images of as cast B319.1 (a) block mold casting. (b) L-shaped mold casting.*

**Figure 7.** *Macrographs of B319.1 alloy for (a) block casting and (b) L-shaped casting.*

with the L-shaped casting, the average SDAS was found to be 25 μm, as is also confirmed by the small grain size noted in **Figure 7(b)**.

From **Figure 8**, it will be observed that the strength values for the as-cast alloy samples of B319.1 exhibit UTS values of about 319 MPa. The B319.1 alloy contains Al, Si and Cu, and Mg and Fe as strengthening elements. The primary strengthening phases for B319.1 are the θ Al2Cu phase and eutectic silicon. The alloying elements added to B319.1 enhanced both yield and tensile strength. The T6 temper allows for increased strength where it develops more stable mechanical properties with a corresponding loss of ductility. Aging at 170°C for 10 hours hardens the alloy, due to the formation of Guinier-Preston zones and coherent θ' Al2Cu phase particles [17–20]. Overaging can be done either at high temperatures or prolonged exposure at an intermediate temperature, and results in the simultaneous formation of relatively large, non-coherent θ' Al2Cu plates which act as hard non-shearable obstacles to dislocations. Such non-shearable particles lead to lower UTS but with high strain-hardening rate, due to the accumulation of Orowan loops around the strengthening particles. As the strain is increased, the buildup of primary shear loops generates intense stress fields around the strengthening precipitates. [21–23].

As coarsening occurs, the inter-particle spacing is widened which will have a direct effect on the dislocation motion. According to the Orowan relationship (**Figure 9**), larger inter-particle spacing results in a decrease in the resistance to

**Figure 8.** *Variation in YS, UTS and %El at different quenching rates and different aging parameters for B319.1 alloy.*

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

**Figure 9.**

*Ostwald ripening mechanism: larger particles grow at the expense of the smaller particles [28].*

dislocation motion thereby facilitating the occurrence of Orowan looping. The increased deformability of the matrix via the easy dislocation motions leads to reduced strength and subsequently diminished quality index values in the castings [24–30]. Aging at lower temperature results in formation of precipitates; with fine sizes, high density and lower inter-particle spacing. In this case, the precipitates provide strong resistance to dislocation motion and the occurrence of Orowan looping becomes difficult leading to a hardening of the materials and an increase in the overall strength, as shown in **Figure 8** [28].

Residual stresses (RS) are elastic accommodation of non-uniform plastic strains generated either thermally or by phase transformation. Generally, the hardness is inversely proportional to the square root of grain size (Hall-Petch equation). Greater the hardness, greater will be the residual stresses. Thus, it could be concluded that grain size has an inverse effect on residual stresses. In general, it is observed that the residual stresses measured are compressive in nature, and are generated due to the steep thermal gradient between core and outer layer at the start of the quenching/cooling process [10] as well the precipitation of complex phases such as α Al15(Mn,Fe)3Si2, β-Al5FeSi and CuAl2 in the B319.1 alloy [16, 31–33].

Generally, stress relief involves uniform heating of a part to a suitable temperature, holding at this temperature for a period of time, followed by slow cooling to prevent the reintroduction of thermal stresses, as stress relieving is highly dependent on the temperature. At high temperatures, such as those used in solution heat treatment, the material yield strength is remarkably reduced, causing plasticity mechanisms to relieve the elastic strain through rapid thermal activation of dislocations. It should be noted that at high temperature, major reduction in residual stresses can be encountered with major decrease in the properties of the material as the precipitates get coarser and lose their hardening capabilities during annealing at high temperatures [34–36]. In other words, heat-treatable aluminum alloys cannot be stress relieved by annealing as the temperature required to encourage stress relief will coincide with that which promotes the precipitation of the second phase constituents, so that stress relieving must be attained at a lower temperature (i.e. during aging).

The amount of residual stresses relieved through T6 treatment provides only modest reduction in residual stresses; while aging at 250°C causes at least 75% residual stress relaxation and can annihilate most locked-in residual stresses with increasing time. This behavior could be attributed to the fact that dislocation glide or climb occurs more readily at higher temperatures. Specimens with large SDAS (60 μm) were also found to be more prone to residual stress relief. In general, the increase in SDAS is found to reduce the amount of residual stresses that originate and facilitate residual stress relaxation which is related to the reduction of

#### **Figure 10.**

*Variation of tensile stresses and residual stresses in B319.1, as a function of different working parameters.*

mechanical strength at lower solidification rates. Finally, the levels of residual stress are markedly reduced because of stress dissipation through the dislocation glide mechanism.

**Figure 10** summarizes the ultimate tensile stress (UTS) and residual stress (RS) values obtained for the B319.1 alloy, as a function of different working parameters and quenching media. The figure demonstrates that material with higher strength, as in the case of B319.1 alloy, produces higher residual stresses (compared to 356 alloy under same heat treatment conditions [24]). It also shows that there is direct proportionality between UTS and RS with quenching rate. The relaxation of residual stresses is significantly dependent on aging temperature and proceeds smoothly with the increase in aging time. A significant increase in the residual stresses is observed in specimens with low SDAS, as in the L-shaped casting, while lower residual stresses are measured in specimens obtained from the block casting, with high SDAS.

#### **3.2 Stage II: influence of working parameters on the development of stresses in I4 and V-6 engine blocks**

Microstructural analysis was carried out using optical microscopy to observe the dendrite structure in both I4 and V6 engine blocks at different locations. Optical microscopy revealed a variation in the dendritic structure along the length of the cylinder bridge region of both I-4 and V-6 engine blocks. It was observed that the top of the cylinder bridge contained relatively coarse dendrites, while the bottom of the cylinder contained finer dendrites, **Figure 11**.

The secondary dendrite arm spacing (SDAS) was measured at the top and bottom regions of the cylinder bridges. The average SDAS was found to decrease from 57 to 40 μm for the I-4 engine block, and from 41 to 21 μm in the case of the V6 engine block. For both types of engine blocks, the SDAS results for the bottom region of the cylinder bridge indicate a shorter solidification time, i.e. a higher cooling rate compared to the top region of the cylinder bridge [37].

**Figure 12(a)** and **(b)** illustrate partial spheroidization of Si eutectic phase after the application of solution heat treatment. However, full modification of the Al-Si eutectic was not observed since the modified B319.1 alloy used in engine block

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

#### **Figure 11.**

*Optical micrographs showing the dendrite structure of (a) top region of I-4 engine and (b) top region of V-6 engine.*

**Figure 12.** *Optical micrographs showing Si morphology in I4-engine blocks: (a) as received and (b) 8 h solution treatment at 500°C condition.*

production, contained a larger amount of Si than the standard 319 alloy [38]. To reach full modification of the Al-Si eutectic, larger additions of Sr., longer heat treatment times, and higher cooling rates would be required. **Figure 13** demonstrated the actual size and density of the precipitates obtained after T6 and T7 aging treatments, for aging times of 10 and 100 hours. As may be seen, at the T7 aging temperature of 250°C, the precipitates are coarser, rod-like in shape, and spread further apart after 100 hours aging time, compared to what is observed at the T6 aging temperature of 170°C.

#### **3.3 Distortion of an engine block is inevitable with time due to the presence of residual stresses**

The distortion may either be a product of thermal growth or the product of tensile residual stresses that exceed the yield stress of the block material or alloy. Thermal growth means changes in volume related to phase transformation during heat treatment of the alloy. In case of thermal growth, it is found that the T7 treatment offers the best dimensional stability over T4 and T6 treatments as it produces the stable θ (Al2Cu), phase which has a lower specific volume when compared to θ' (Al2Cu) neglecting the effect of thermal growth distortion [39]. Such distortion may occur through the introduction of excessive residual stresses. When theses residual stresses exceed the yield stress of the material, distortion occurs [40].

The residual stresses in the I-4 engine blocks in the as-cast, air cooled, and air cooled + freezing conditions (30°C) were 100, 70, and 50 MPa, respectively. These results indicate that the SHT process partially relieved some of the tensile

**Figure 13.**

*Backscattered electron images of the size and density of the precipitates in I-4 engine block: (a) after aging at 170°C for 10 hours; (b) after aging at 170°C for 100 hours (c) after aging at 250°C for 10 hours; (d) after aging at 250°C for 100 hours.*

residual stresses which evolved in the Al-cylinder bridge region, with a subsequent reduction when freezing was performed through the operation. **Figure 14** reveals that there is significant relieving of residual stresses ongoing from the as-cast and to the SHT condition where these residual stresses are relieved by 25, 75, and 65%, respectively, when subjected to air cooling, warm water quenching and cold-water quenching. This trend indicates that SHT plays an important role in the relieving of residual stresses. Previous research studies [10, 41] concluded that residual stresses can be relieved thermally either instantaneously, when locked-in stresses exceed the yield strength or gradually through creep mechanisms.

At slow cooling rates, there is no significant difference in cooling rates between aluminum (Al) and cast iron (CI) liners and since the aluminum contracts to a greater extent with decreasing temperature, large residual stresses are developed due to the thermo-mechanical mismatch between the two materials resulting from the hindrance of free contraction of the aluminum. On the other hand, at high cooling rates such as when the blocks are quenched in water, the CI liners cool at very high rates similar to the surrounding aluminum. This leads to the contraction

**Figure 14.** *Residual stress development at different quenching/cooling rates.*

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

#### **Figure 15.**

*Effect of stable vs. cyclic freezing on the development of residual stresses.*

of both Al and CI liners at similar rates, reducing the thermo-mechanical mismatch between them, resulting in much lower stresses inside the engine blocks.

Freezing after quenching is considered one of the techniques which can be used to further reduce the amount of residual stresses by reversing the pattern of thermal gradient imposed during solution heat treatment. Despite the benefits of cryogenic treatment on both mechanical properties and the residual stresses developed in ferrous alloys, there are few reports in the literature related to the freezing treatment of nonferrous materials and the consequent effect on residual stress and mechanical properties [32, 34].

**Figure 15** illustrates the effect of freezing on the development of residual stresses. At least 20% reduction in residual stresses after the implementation of the freezing process is noted, which supports the effectiveness of the freezing

#### **Figure 16.**

*Effect of freezing and aging on the development of residual stresses in two- and four-cylinder engine blocks: (a) T6 at 170°C and (b) T7 at 250°C.*
