アルミニウムの密度とは？

Aluminum looks light and feels strong. But what exactly makes it such a favorite for industries worldwide?

The density of aluminum is around 2.70 g/cm3, making it one of the lightest metals used in manufacturing. This low density plays a key role in applications from aerospace to architecture.

Knowing the density is only the beginning. The deeper story lies in how it’s measured, how it compares with steel, how alloying changes it, and how temperature influences it. Let’s explore.

How Is Aluminum Density Measured?

When we talk about aluminum’s density, we usually mean how much mass fits into a specific volume. But how do labs and engineers measure that in real life?

Aluminum density is measured using simple physics: divide the mass of a sample by its volume. This can be done using water displacement (Archimedes’ Principle) or a lab tool called a pycnometer.

Measuring aluminum’s density starts with basic physics:

Archimedes’ Principle

This method uses water to find volume:

1. First, we weigh the aluminum piece in air.
2. Then, we submerge it in water and record how much water it displaces.
3. Since the density of water is known (1 g/cm3), the displaced volume tells us the volume of the aluminum.

This method works great for irregular shapes, like cast parts or custom aluminum extrusions.

Pycnometer Method

A pycnometer is a flask with a narrow neck. Here’s how it works:

• We fill it with water and weigh it.
• Then, we add the aluminum piece and note the change in water volume.
• With precise tools, this method offers high accuracy and is ideal in research labs.

Both methods aim to find one thing: how tightly the atoms of aluminum are packed in a given space.

Common Mistakes When Measuring:

• Not accounting for air bubbles when submerging the sample.
• 使用 impure water or warm water, which may skew volume readings.
• Weighing errors due to balance calibration.

How Does Aluminum Density Compare to Steel?

Steel and aluminum often go head-to-head in engineering. But when weight matters, who wins?

Aluminum is much lighter than steel. Its density is about 2.70 g/cm3, while steel is around 7.80 g/cm3. That means aluminum is nearly one-third the weight of steel.

簡単に比較してみよう：

素材	密度(g/cm3)	Relative Weight
アルミニウム	2.70	100% (reference)
スチール	7.80	~288% of aluminum

This difference makes aluminum ideal for:

• Aircraft structures: Less weight = more fuel efficiency.
• Electric vehicles: Lighter bodies mean longer range.
• Lifting systems and robotics: Less strain on moving parts.

However, steel has its advantages too. It’s stronger and more durable in many applications. The choice between them often depends on:

• 強度重量比
• コスト
• 耐食性
• 成形性

In industries like construction, aluminum is often used in window frames and fa-ades, while steel is used for structural beams and reinforcements.

Does Alloying Change Aluminum Density?

Pure aluminum is rarely used in heavy-duty applications. Instead, we use aluminum alloys. But does adding other metals change its density?

Yes, alloying does change aluminum density-but not by much. Most common aluminum alloys still fall between 2.68 and 2.85 g/cm3.

Let’s look at a few popular alloys:

合金	Common Elements	Approx. Density (g/cm3)
6061	マグネシウム、ケイ素	2.70-2.75
7075	Zinc, Magnesium	2.80-2.85
3003	マンガン	2.73
1050 (pure)	99.5%+ Aluminum	2.70

The variation in density is usually caused by:

• Heavier alloying elements like zinc and copper.
• Microstructure-how the atoms are packed.
• 熱処理, which can change the shape and alignment of metal grains.

なぜ重要なのか

For aircraft and high-speed trains, every gram matters. A denser alloy might mean more weight and more fuel usage. That’s why engineers choose alloys that balance strength and weight.

However, for some uses-like high-heat environments or corrosive settings-the benefits of an alloy outweigh its slight density increase.

Why Does Temperature Affect Aluminum Density?

We usually think of metals as solid and stable. But heat changes everything-even for aluminum.

When aluminum is heated, it expands. As volume increases and mass stays the same, its density goes down. Cooling has the opposite effect.

Here’s what happens:

At High Temperatures

• Aluminum atoms vibrate more and move apart.
• This increases volume.
• With volume going up and mass unchanged, density goes down.

At Low Temperatures

• Atoms contract and move closer.
• Volume decreases.
• Density goes up slightly.

The equation remains the same:

[
\text{Density} = \frac{\text{Mass}}{\text{Volume}}
]

But as temperature affects volume, the density shifts too. This change is small but critical in industries like:

• 航空宇宙: Parts must handle extreme cold at high altitudes and heat during re-entry.
• エレクトロニクス: Aluminum heatsinks expand with temperature.
• 建設: Bridges and panels expand and contract daily.

Temperature vs Density Table

温度 (°C)	Volume Expansion	Density Effect
-50	Contracts	Density ↑
0	安定	Normal
100	Expands slightly	Density ↓
500	Expands more	Density ↓↓

In precise applications, even small changes matter. Engineers often factor in thermal expansion coefficients when designing parts.

結論

Aluminum’s density-about 2.70 g/cm3-makes it one of the most versatile, lightweight metals in industry. While it’s influenced by alloying and temperature, its core advantage remains: strong performance with low weight.