Introducing Metal Matrix Nano composites in to shipbuilding

Few thoughts on Introducing Metal Matrix Nano composites in to shipbuilding

 (Complete literature may be found on "The Naval Architect" July/August 2012 Edition)

Reducing structural weight is one of the most important ways of reducing fuel consumption and improving the performance of ships and other types of transportation equipment. An estimated 75% of the average motor vehicle's fuel consumption directly related to factors associated with vehicle weight.

My approach is to develop materials that combine relatively low mass (weight) with the requisite strength, flexibility, and other performance criteria. The aircraft industry was the first to introduce lightweight materials (e.g., aluminum alloys) on a widespread scale beginning in the 1920s. This continues today with the adoption of lightweight composite materials in other industries as well.

Less weight, consistent with other performance and safety requirements, means more useful work can be extracted from a unit of fuel or other energy source. Weight-reducing technologies are critical to the success of new, highly efficient energy technologies such as hybrid vehicles.

Shipbuilding is the second largest consumer of lightweight materials. There is a growing demand for the lightweight metal; Aluminum represents the second largest metals market in the world.  Light, strong, and corrosion-resistant aluminum is the ideal metal for shipbuilding.

History of Aluminum in shipbuilding: It was first used for building a steam passenger boat in 1891. The boat named Le Migron was designed in Switzerland and was intended to carry 8 passengers. This was the first boat partially made of aluminum, which confirmed the very opportunity of using aluminum in shipbuilding.

Later, The Scottish shipbuilding yard Yarrow & Co presented a 58-m motor torpedo boat made of aluminum. This boat named ‘Falcon’ was manufactured for the navy of the Russian Empire. The boat reached a speed of 32 knots, a record for those times.

But in late 1890s the cost of aluminum was 35 higher than the cost of steel, which hampered active use of the ‘light metal’. Another shortcoming was discovered later on: corrosion. Although it sounds strange today, it turned out that the yachts made of aluminum at the beginning of the century were exposed to severe corrosion in salt water. The service life of all these vessels turned out to be significantly less than that of similar vessels made of steel. Imperfect manufacturing processes and a lack of understanding of all aluminum properties and its capabilities hampered wide dissemination of this metal in shipbuilding. Engineers faced a complex problem which they managed to solve only a few decades later.

After a lot of research from the early 1900s, Aluminum Alloy 5083 is considered the base alloy of the shipbuilders; it was registered by the Aluminum Association in 1954. Although this alloy is often called the ‘shipbuilding’ alloy, it is also widely used in many other industries. Alloy 5083 initially won popularity in shipbuilding thanks to its properties, such as high strength, corrosion resistance, good mouldability, and excellent welding characteristics.

By the 1960s, improvements in the technology, as well as reduction of the cost of aluminum led to extensive use of the ‘light metal’ in shipbuilding. Aluminum was used in manufacturing the shells of yachts, superstructure, masts, and port infrastructure. In the 1970s, high-speed passenger vessels first appeared in Scandinavia - catamarans made of aluminum. Being light and quick, they proved their profitability and speed advantage, and became standard for passenger transportation for many years.

Until recently, alloy 5083 virtually had no competitors among other aluminum alloys. In 1995, aluminum alloy 5383 was registered, which is an improved version of alloy 5083. The corrosion was increased, and its impact strength and yield point of welded constructions were increased by 10% and 15% respectively. These improvements potentially allow for a considerable reduction in the mass of welded vessels. 

In 1999, aluminum-based alloy 5059 was registered with the American Aluminum Association, which was called Alustar. This new alloy proved that aluminum can be stronger than steel. The alloy has the values of ultimate strength and yield point comparable with the corresponding values of low-alloy steel S235, AlCu4SiMg (AA2014). This alloy developed for the shipbuilding industry also has considerably improved strength characteristics compared to the traditional alloy 5083. The yield point before welding is increased by 26% and by 28% after welding.

Studies continue, and probably, very soon the scientists will present us even lighter and stronger aluminum alloys, which will allow manufacturers to create vessels and structures of the new generation.

Why should we consider Aluminum for ship building?

ü  High Strength to weight ratio

ü  Density one-third that of steel

ü  Excellent corrosion resistance

ü  Weldable

ü  Ease of forming, bending and machining

ü  Availability and diversity of functional semi-finished products

ü  High thermal and electrical conductivity

ü  Recyclable

ü  Non-magnetic

The results of which would be the following:

ü  Fuel Savings

ü  Increased Range

ü  Increased Payload

ü  Higher Speeds

ü  Maneuverability

ü  Stability

ü  Less maintenance

ü  Lower total ownership cost

New building price breakdown are generally composed as below for 3 Major Shipbuilding countries:

	Countries
	China	Korea	Japan
Labor	10 %	19 %	22 %
Steel	30 %	27 %	26 %
Equipment	60 %	54 %	52 %

If Aluminum is used:

ü  Diverse product forms such as sheet/plate, extrusions, castings and forgings enable parts consolidation & design simplification

ü  Joining technologies: automation and mechanization

ü  Lighter structure results in fuel savings.

Nanotechnology is all about being small.  After all, the prefix “nano” means one billionth of something.  For example, a nanometer is a billionth of a meter.

Nanocomposites are lighter, stiffer, less brittle, and more dent- and scratch-resistant than conventional plastics.  Some nanocomposites are also more recyclable, more flame retardant, less porous, better conductors of electricity, and can be painted more easily.

 The Concept: Use of Carbon Nanotube reinforced Aluminum as a base shipbuilding Material. My goal is to Design a Ship with a view of building her using Aluminum Nanocomposite. In this literature we shall concentrate how the composite is fabricated and advantages of its application in shipbuilding. 

Why reinforcing? What are the objectives of reinforcing?

The reinforcement of metals can have many different objectives. The reinforcement of light metals opens up the possibility of application of these materials in areas where weight reduction has first priority. The development objectives for light metal composite materials are to improve those properties that are avoiding Aluminum to be used in commercial shipbuilding. 

·         Increase in yield strength and tensile strength at room temperature and above while maintaining the minimum ductility or rather toughness.

·         Increase in fatigue strength, especially at higher temperatures

·         Improvement of thermal shock resistance,

·         Improvement of corrosion resistance,

·         Increase in Young’s modulus,

·         Reduction of thermal elongation.

·         Improve Resistance Stress corrosion cracking 

Carbon nanotubes (CNTs), whose tensile strength and thermal and electrical conductivity along their axis exceedingly surpass virtually all known materials; it also shows an increase in the mechanical strength and hardness of metal matrix composites (MMCs).

Properties of CNTs

ü  High Aspect Ratio Structures 

ü  High Mechanical Strength: Tensile Strength (60 GPa)

ü  Young Modulus (1 – 5 TPa)

ü  High Electrical Conductivity (typically 10^-6 ohm m)

ü  High Thermal Conductivity (1750 – 5800 W/mK)

ü  High Current Density (10⁷ – 10⁹ A/cm²)

ü  Chemical Stability: (not attacked by strong acids/alkali)

Though nanocomposite materials exhibit ultra high-strength, there is often a trade-off that results in decreased ductility. This may be attributed to currently used processing methods that result in the formation of voids and defects, as well as to the inability of nanostructured grains or additives to sustain a high rate of strain hardening.

Raw materials: Metal matrices employed in MMNC are mainly Al, Mg, Pb, Sn,W and Fe. In general, it is the reinforcement that is in the nanorange size in these materials.

Marine grade aluminum –including sheets, plates, extrusions, forgings and castings –are readily available from aluminum mills or distributors around the world.  

CNTs consist of graphene cylinders and are available in two varieties, as single walled (SWCNT) and multi walled (MWCNT), with about 70% yield in the case of SWCNT.  While  SWCNTs  are  single  graphene  cylinders, MWCNTs  consist  of  two  or  more  concentric  cylindrical  sheets  of graphene  around  a  central  hollow  core.  Both types exhibit physical characteristics of solids, either metallic or semiconducting in nature, with microcrystallinity and very high aspect ratios of 10³.

Processing methods

Despite their Nano dimensions, most of the processing techniques of  the  three  types  of  nanocomposite  remain  almost  the  same  as  in microcomposites. This is also true even for CNT-reinforced composites. 

MMC manufacturing can be broken into three types: solid, liquid, and vapor.

1.       Solid state methods

2.       Liquid state methods

3.       Vapor deposition

Among several techniques of CNT synthesis available today, chemical vapor deposition (CVD) is most popular and widely used because of its low set-up cost, high Production yield, and ease of scale-up.

Chemical Vapor Deposition: Chemical vapor deposition or CVD is a generic name for a group of processes that involve depositing a solid material from a gaseous phase. 

How Does CVD Work?

Precursor gases (often diluted in carrier gases) are delivered into the reaction chamber at approximately ambient temperatures. As they pass over or come into contact with a heated substrate, they react or decompose forming a solid phase which and are deposited onto the substrate. The substrate temperature is critical and can influence what reactions will take place.

CVD Processing method has proven to be very economical and can be scaled to Bulk production. Here is an understanding of how carbon Nanotubes is produced using Vapor deposition technique.

The high price of CNTs can be justified by their superior mechanical and electrical properties.  In addition the development of synthesis techniques for CNTs has resulted in substantial reduction in the cost of production of CNTs.  According to present market, the cheapest price is $13,000 per kg for SWCNT (purity 60%), $15,000 per kg for ARC-MWCNT (purity 30-40%), and $400 per kg for CVD-MWCNT

According to many research papers and Lab tests conducted at various Universities, companies around the world, it is quite evident that a maximum of weight percent of 2 (w/w %) is sufficient to increase the characteristics of Aluminum. However this weight percent can be reduced by improving the quality of CNTs produced and also by understanding precisely the requirement. The less the use of reinforcing CNT the lower the total expenses in producing the Al Nanocomposite. 

Advantages of CVD process:

1. Very simple and in expensive set up
2. Can be used for a wide range of metals and ceramics
3. Can be used for coatings or freestanding structures 
4.  Fabricates net or near-net complex shapes
5. Is self-cleaning—extremely high purity deposits (>99.995% purity)
6. Conforms homogeneously to contours of substrate surface 
7.   Has near-theoretical as-deposited density
8. Has controllable thickness and morphology
9. Forms alloys
10. Infiltrates fiber preforms and foam structures
11. Coats internal passages with high length-to-diameter ratios 
12.   Can simultaneously coat multiple components
13. Coats powders

 The below table shows the Young’s modulus of the specimens

Description	Experimental Young’s modulus (GPa)	Shear lag Young’s modulus (GPa)
Al + 0.5wt% MWCNTs	61.92	71.97
Al + 1.0 wt% MWCNTs	66.15	74.17
Al + 2.0wt% MWCNTs	74.62	81.95

In shipbuilding normally the requirement of steel will be approximately 2000 -3000t for ship of 10,000 DWT depending Design and type of vessel whose particulars will be in the range are as follows:

Length = 120 – 150m

Breadth = 15 -20m

Depth = 7 -10m 

As we are assuming Aluminum to be used for entire vessel, In general because of its low density and elastic modulus, in order to achieve minimal stiffness requirements, a section of Aluminum must be three times (3x) thicker than equivalent steel part. i.e. (5mm of steel = 15mm of Al). Based on that fact that Al approximately costs $1.2 Per Pound and steel $0.25 Per Pound. Replacing steel with Al in higher strength applications dramatically increases material cost at a minimal reduction in weight. 

In case of using this reinforced Al Nanocomposite (Al NC), once developed to suit the requirement in Shipbuilding, we can make it possible to maintain the same thickness. i.e. In order to achieve minimal stiffness requirements we can replace the same thickness of Al NC as of steel equivalent.

This dramatically reduces the total weight of the ship to one third, and using Al for shipbuilding has its own advantages. Further advancement in technology can present us with a material where we could substitute perhaps an 8mm plate of Al NC for a 12mm Steel plate.  

Apart from the weight Total Ownership Cost of the Aluminum is equal to Steel

Most of the total ownership resides in operations, Maintenance and Sustainment:

Although the acquisition cost for the Al NC equivalent ship will be currently higher than that for the steel ship, the Al NC equivalent ship will have a lower total ownership cost.  This is because of fuel savings and lower maintenance needs.  The Aluminum ship does not require painting over its life, except for anti-fouling painting. It also has lower power machinery to repair, and less manning due to decreased onboard maintenance by the ship's crew a significantly higher residual value at end of life scrapping as well.  Because the aluminum ship uses less fuel it will have a lower carbon footprint, than the steel vessel. 

Cost: The material costs of building a ship are only 20-30% of the total cost of the ship. Major costs are associated with the manufacturing of the ship. Today’s advancements in technology allow faster and bulk production of CNTs, and also scientists are coming up with novel ways to produce CNTS. Though the current price of CNTs are high there will be a reduction as there is more demand. The same is in case of Aluminum. 

The increased material and manufacturing costs for the proposed Al NC hull ship structure can be almost counterbalanced by taking advantage of the weight benefit of the Al NC in reducing either the draft or the block coefficient.  The reduced displacement reduces the required draft or block coefficient for the same principal dimensions and draft.  Less propulsion power is then required for the same speed and less fuel is required for the same endurance. Al NC ships can go faster speeds, carry bigger payloads and travel longer ranges while enjoying increased stability and better fuel efficiency. 

Repair: Aluminum is no more difficult and no more expensive to repair than other shipbuilding materials.  In fact, the repair of aluminum structures is relatively straightforward.  It requires tools and expertise that are similar, if not the same, than what is needed to repair ships built with steel. And repairs are less difficult because aluminum is easier to cut and weld than steel. 

 There are very few restrictions on what can be repaired on an aluminum ship, and there are numerous repair facilities in the United States and around the world with the expertise to repair aluminum vessels.  In addition, increase in usage of Al has resulted in introducing several programs to train workers on how to effectively repair aluminum Naval Structures.  

Fuel: As the weight is reduced so is the propulsion power required to drive the vessel. This increases the vessel’s endurance. That is for the same amount of fuel quantity an Aluminum ship can travel more distance than steel equivalent. There is a huge saving in fuel costs over the life time of the ship. 

Painting: No need to paint. Only anti fouling painting is only required in the underwater region of vessel. This in fact reduces an average of 8- 10 tons of weight for a ship of approximately 100 -150 m in Length. It also reduces the total cost. Assuming that the steel ship requires re-painting every five years a savings of $2 to $5 million per ship is possible over the life of the Al NC vessel.

Recycling: Theoretically Al is 100% recyclable. Aluminum has high residual value at end of life. Nearly 75% of the aluminum ever made is still in use today. When an Aluminum vessel reaches the end of its life span, it continues to provide tremendous value as a result of its high recycling value.

Crew: As the systems and machineries installed are smaller as compared to the steel ships, the maintenance required is less and so is the manning. Assuming this for a total life period of vessel this significantly contributes in reducing total ownerships.

Conclusions and Recommendations

There is a huge demand in the market for greater size of ships and this has forced designers to search and come up with a alternative material to reduce weight while maintaining strength. When properly designed Aluminum typically reduces the weight of structures by approximately 50% and even less in this case. 

Dramatic technological advancement has allowed for Aluminum in some cases to meet and exceed the minimum strength requirement in order to be used in shipbuilding industry. 

The only disadvantage now I can see is the cost. Although the initial investment is thrice to build an equivalent Al NC ship, a simple theoretical Total Ownership Costs analysis made here proves that at the end of Ship’s service life which is normally 25yrs, the operations and maintenance costs of steel ship along with that acquisition cost equals or sometimes exceeds the initial investment made for ship built of Aluminum Nanocomposite along with its operation and Maintenance costs.  

I strongly believe there is a huge potential to this concept and further active research can enable us to see an all Aluminum Nanocomposite ship built in near future. This is a would be revolutionary.

All Suggestions/Advices or support in any form are highly encouraged/welcomed.