**11. Industrial application potentials of Al-Cu-Mn-Zr alloys**

Industrial application of Al-Cu-Mn-Zr alloys require certain additional considerations on and above their excellent thermal stability and associated improvement in most of the high temperature mechanical properties as mentioned above. These include, but certainly not restricted to the ease of casting and defect formation, assessment of mechanical properties e.g. fatigue testing at larger component scale, possibility to adopt alternate component fabrication methodologies like additive manufacturing, wrought processing etc. [89]. Some of these aspects are mentioned below:

#### **11.1 Hot tearing resistance**

Hot tearing is a crucial casting defect that can affect the structural stability and properties of as-cast components [90, 91]. During solidification, molten metal usually remains in semi-solid state (mushy zone) for considerable duration. It also undergoes severe volume contraction and associated thermal stresses within the semi-solid metal regions. Under this condition, cracks form in the solidified component if there is an inadequate supply of molten mass to fill up the shrinkage volume. Controlling such defects in castings is difficult but extremely important for improving the fatigue life per se [91].

Sabau et al. [92] studied the hot tearing resistance of cast Al-Cu-Mn-Zr alloys with and without grain refiners in comparison to base Al-Cu and Al-Si alloys. The base Al-Cu alloy with >7 wt% Cu exhibits a grain refined microstructure in the casting. It also shows a decrease in the length of columnar to equiaxed transition zone that in turn improves the hot tearing resistance. In addition, simultaneous presence of Si and Fe (>0.2 wt%) increases the hot tearing resistance for this alloy. The low Cu containing alloys, on the other hand, possess coarse columnar grains within the cast microstructures which facilitates hot tearing for them. For the Al-Cu-Mn-Zr alloys, when Cu is added above 7 wt%, significant grain refinement occurs which further contributes to their excellent hot tearing resistance. In addition, when 0.1 wt% Ti is added as additional grain refiner, resultant Al-Cu-Mn-Zr alloy exhibits the finest microstructure and correspondingly, the best hot tearing resistance [89]. Ti added Al-Cu-Mn-Zr alloy was therefore speculated suitable for industrial applications [92].

In the measurement of hot tearing resistance, Sabau et al. [92] used an in-house multi-arm casting setup with varying arm length in a permanent mold. In this

six-armed mold, the shortest arms were free from any visible cracks for all the alloys having varying amount of Cu and Ti, while the longest arm had severe cracking for these alloys. As per the visible inspection, a cracking index (*Ci)* was assigned to each of the arms where 0 corresponds to no crack condition and 2.5, 5, 7.5, and 10 was assigned to small, moderate, severe and completely fractured arms, respectively. Hot tearing index (HTI), *Ms* was then defined as the simple average of *Ci* over these six arms i.e.

$$M\_t = \frac{1}{6} \sum\_{i=1}^{6} C\_i \tag{7}$$

The length of the arms, however, also plays a crucial role in the formation of these cracks. The longer arms are more susceptible to cracking compared to the shorter ones so that the weighted average was preferred for the calculation of HTI. The weighted average *(MJ-K)* was defined as:

$$\mathbf{M}\_{l-K} = \frac{\sum\_{i=l}^{K} w\_i \mathbf{C}\_i}{\sum\_{i=l}^{K} w\_i} \tag{8}$$

where, *J* and *K* corresponds to the shortest and longest arms and *wi* is the weight factor which is inversely proportional to the length of the arm; *J* was considered as 2 and *K* as 5. As per the industrial standard, hot tearing index (HTI) equal or less than 3.3 is considered acceptable when *Ci* is in the range of 0–10. The HTI for various Al-Cu-Mn-Zr alloys with promising industrial application potential are listed in **Table 1**. Important to note that the fluidity of Al-Cu-Mn-Zr alloys with respect to temperature, especially under solidification conditions is also of immense importance for successful industrial application standpoint. However, no such data reporting the fluidity of Al-Cu-Mn-Zr alloys is available yet.

#### **11.2 Additive manufacturing of Al-Cu-Mn-Zr alloy**

In recent times, additive manufacturing (AM) is proven to be an extremely useful and alternate technique for shaping intricate parts in industrially relevant scales with excellent property combinations compared to cast counterparts [93]. For Al alloys however, additive manufacturing is a rather complicated and challenging process due to several factors like poor powder flowability, high thermal conductivity, laser reflectivity etc. [94]. In this regard, Shyam et al. [95] successfully fabricated AM parts from Al-Cu-Mn-Zr alloys by selective laser melting (SLM)


#### **Table 1.**

*Hot tearing index (HTI) of different Al-Cu-Mn-Zr alloys with varying Ti content.*

*New-Age Al-Cu-Mn-Zr (ACMZ) Alloy for High Temperature-High Strength Applications… DOI: http://dx.doi.org/10.5772/intechopen.104533*

without any hot tearing using optimized processing parameters. The substrate temperature for SLM was kept 200°C which was sufficient for in-situ formation of strengthening θ<sup>0</sup> precipitates.

Due to laser melting, AM Al-Cu-Mn-Zr alloys form typical "peacock tail" microstructure having overlapping melt pools. In addition, the AM microstructure consists of long columnar grains at the top and fine equiaxed grains at the bottom of the melt pools. Such refined AM microstructure yields comparatively higher strength up to 300°C compared to the cast Al-Cu-Mn-Zr alloys. The bimodal grain size distribution and refined grain boundary intermetallic precipitates further enhance the tensile elongation for AM Al-Cu-Mn-Zr alloys. However, creep properties for these AM alloys are somewhat compromised compared to the cast counterparts due to high proportion of grain boundaries in the refined AM microstructure. Overall, AM Al-Cu-Mn-Zr alloys are envisioned having potential in complex component manufacturing for high temperature applications owing to the simultaneous positive effects of refined microstructure and in situ formation of thermally stable strengthening θ<sup>0</sup> precipitates [95].

#### **11.3 Environmental impact from Al-Cu-Mn-Zr alloy**

In the current global scenario, any new alloy development must help in reducing environmental impact e.g. carbon footprint and green house emissions [96]. The primary target area for Al-Cu-Mn-Zr alloys is automotive industry, which also formed the early motivation of their inception and further development [97]. The aim was to develop Al alloys for engine components that experience high working temperature (�300°C) e.g. cylinder heads in combustion engines. The use of Al-Cu-Mn-Zr alloys for making such components can effectively raise the working temperature and increase the fuel efficiency of next generation passenger vehicles, thereby proving environment friendly in terms of fuel consumption.

## **12. Conclusion**

The present book chapter elucidates a comprehensive review about the development as well as the science and technology behind the new-age Al-Cu-Mn-Zr (ACMZ) alloys. The major observations are summarized below.


effectively restricts θ<sup>0</sup> precipitates from transforming to stable θ precipitates up to a much higher temperature.

