What is the density of Al? A guide for thermal conductivity and laser welding settings

Abstract
Materials properties such as coefficient of thermal expansion (CTE), thermal diffusivity, and resistivity are crucial in many high-precision applications, such as aerospace components, and also in the thermal management of electric vehicles. Laser joining process efficiency, as well as heat transfer in a substrate, are highly dependent on the material’s properties. This technical note presents a thorough review of the effect of the density of aluminum on the thermal diffusivity of aluminum, as well as in the laser-matter interaction process. A review of the data on the density of aluminum, relative to iron metal density and stainless steel density, is also provided. This is an effort to provide a reference for designers and engineers. The effect of the density of 6061 aluminum on the specific energy required to obtain keyhole stability during fiber laser welding is also investigated. This guide is intended to provide knowledge of materials properties and experimental data to assist in laser processing for precision applications in a high-productivity, high-accuracy manufacturing environment where precision is on the order of millimeters to a few hundred microns.

The industry pain point: Thermal runaway and keyhole instability in aluminum laser processing

The most common challenge in lightweight materials development today is laser welding of aluminum alloys. While laser welding of ferrous materials is quite common, most Automotive and Power Electronics OEMs encounter the phenomenon of thermal runaway while laser welding aluminum materials at high speeds, due to high reflectivity and high thermal conductivity, resulting in no stable melt pool formation and the weld instantly going into over-penetration due to the vaporization of the material.

The main reason for this discrepancy is that the aluminium density is miscalculated during the laser calibration process. Aluminium has a lower aluminium density than that of iron or stainless steel. Hence, when the molten pool is formed during laser processing, the lower hydrostatic pressure results in keyhole collapse and also gas entrapment as macro porosity. Moreover, a miscalculation of the aluminium density value in the thermal diffusivity equation will also lead to wrong heat transfer simulation for the design of temperature-critical components such as semiconductor devices and battery cells, and thus lead to premature failures. This resource paper aims to set the foundation of the calculation of the density of al to ensure the correct setting of laser power density and travel speed for effective laser processing of aluminium.

Physical foundations of the density of al: Elemental properties and crystallographic structure

Laser setting calibration needs a baseline reading to work off of. So here’s the basic thermophysical question to be answered: what is the density of aluminum? Aluminum is the most abundant metal native to the Earth. It is a silvery-white, soft, nonferrous, nonmagnetic, ductile metal and a good conductor of electricity. The most common type of aluminum is aluminum metal in the form of ingots, sheets, plates, bars, wire, foam, caps, and foil, with many uses, including packaging, transportation, appliances, and construction, in which aluminum takes many forms, such as sheet, bar, and foil. Aluminum also occurs as a metal in small amounts in many rocks, and it also occurs in gold and platinum ore in the form of four semi-rare minerals. And now you know the basic thermophysical properties of Aluminum: the density! The face-centered cubic (FCC) lattice has an atomic packing factor (APF) of 0.74.
Although the atoms in a crystal of aluminium are packed very efficiently, there is still a large gap between the atoms. This is because the atomic mass of aluminium is relatively small — 26.98 u — in comparison with transition metals such as iron, where the atomic mass is 55.85 u. The density of aluminium at room temperature (20^∘C) is about 2.699 g/cm³. In international trade and engineering specifications, this is often given as 2.70 g/cm³. In countries using imperial units, the aluminium density in lb/in³ is 0.0975 lb/in³.
Iron metal has a density of approximately 7.87 g/cm³ (3 times denser). Even the densest stainless alloys will still be denser at around 8.0 g/cm³. This fundamental difference in the density of materials is the reason that laser parameters which are optimized for the density of iron (such as deep penetration applications) will almost always cause over-penetration or blow-through on materials of lower density, such as aluminum. The lower density of materials such as aluminum means that less energy is required to reach vaporization after the initial reflectivity barrier is broken.

How the density of al governs the physics of laser keyhole stabilization

In deep-penetration laser welding, the laser beam creates a “keyhole” or a vapor-filled capillary within the molten metal. The keyhole stability is a balance between vapor pressure that keeps it open, and hydrostatic pressure and surface tension that tend to close it. The hydrostatic pressure Ph is defined as:

Where:
ρ is the al density.
g is the gravitational constant.
h is the depth of the melt pool.
The relatively low density of al means that the pressure applied by the liquid column of the melt pool is much lower than in high iron mass density or high density stainless. Also, a low Ph does not necessarily help to avoid keyhole oscillation. In high-density stainless welding, the mass of the liquid metal acts as a stabilizing factor for the vapor exhaust from the keyhole. In aluminium, the low al density means that the melt pool can easily be sensitized to minor perturbations in the laser power, leading to “keyhole oscillation”.
These irregularities in the weld pool must be corrected by adjusting the laser parameters. The “wobble” parameter, in particular, can be used to great effect by applying a frequency of 200−500 Hz. This oscillation creates a circular or sinusoidal trajectory, increasing the beam width of the laser, thus stirring the melt pool and compensating for the low al density. This prevents premature keyhole collapse and helps prevent internal voids that can compromise the integrity of the high-pressure liquid cooling manifold.

Calibrating laser power and speed based on the density of 6061 aluminum

Pure aluminum is often replaced with 6000-series alloys in structural applications because the alloys contain magnesium and silicon. The calculation of the specific density of 6061 aluminum is necessary for the calculation of the energy density required to achieve the desired penetration depth. Magnesium (1.74 g/cm³) and silicon (2.33 g/cm³) are both less dense than aluminum, but the low concentrations of these elements in 6061 aluminum maintain a density of approximately 2.70 g/cm³ or 2700 kg/m³.
Except for the presence of magnesium, all other variables could be kept the same. As magnesium has a lower vaporization temperature compared to aluminum, with values of 1,090^∘C and 2,470°C, respectively, in laser welding of density 6061 aluminum, the magnesium vaporizes first, with the resulting vapor jet producing pressure that ejects spatter out of the weld pool.
In order to achieve a clean weld on the very dense 6061 aluminum material, the laser power density needs to be sufficiently high to see through the reflectivity of the surface, and at the same time not so high that the magnesium gets blown away like cinders. Here is an example of optimum parameters for a 3 mm thick 3-inch 6061 plate with a 2000W fiber laser:
Laser Power: 1800−−2200 W.
Welding Speed: 2.5−−4.0 m/min.
Wobble Amplitude: 0.5−−1.2 mm.
If the aluminium density and the effect of the specific alloying elements are not taken into account, the laser will either not penetrate the required depth, or a “hollow” weld bead will be produced, due to the magnesium vapour expanding and displacing too much of the aluminium melt.

Thermal conductivity and laser beam absorption: The al density variable

Aluminum has approximately five times more thermal conductivity than Grade 304 stainless steel. Combined with the low al density, the aluminum acts like a “thermal sponge” absorbing the laser energy from the surrounding material.
The rate of heat transfer is described by thermal diffusivity(α):

Aluminum density is approximately 2700 kg/m³. (This value of density is used in the formula below as ρ.) Because al density is in the denominator, a lower density means a greater speed of heat transfer. So, we need to deliver energy to the workpiece at a pace that is faster than the pace at which it will be dissipated by the heat transfer to the aluminum.
For an engineer preparing a robotic laser cell, the speed of travel is the variable that should be focused on. A travel speed that is too low will cause the high thermal diffusivity, combined with the low aluminium density, to cause excessive heating of the heat-affected zone (HAZ), resulting in loss of temper and warpage. Conversely, a travel speed that is too high will leave the laser insufficient time to deliver the latent heat of fusion required to fully melt the low-density substrate material. Knowing the exact aluminium density g/cm³ and being able to input this into simulation software enables the engineer to determine the required temperature gradient and hence welding speed in order to achieve the desired narrow width of the HAZ, while still ensuring full penetration of the weld.

Comparative laser settings: Density of al vs. density of Fe and stainless

It was found that with a data-driven approach relating material densities, aluminum needs different laser parameters due to the large difference in the densities. The densities for stainless and iron metals are approximately 8.0 g/cm3 and 7.87 g/cm³, while for al is 2.70 g/cm³.

Material	Density (g/cm³)	Reflectivity (at 1070 nm)	Thermal Conductivity (W/m·K)	Laser Power Density Req.
Aluminum (6061)	2.70	High (>90%)	167	High (initial), Low (steady state)
Stainless Steel (304)	8.00	Moderate (30-40%)	16.2	Moderate
Carbon Steel	7.87	Low	50	Low to Moderate

This is one reason that iron mass density and fe density are so large: the bath of molten iron is denser, heavier, and thus more stable. Lasers can often be run in both a conduction mode or a keyhole mode, with wide windows of parameter space that still allow good welding performance. But aluminum, in contrast, tends to be either reflecting the laser very well, or else strongly melting/vaporizing. The low al density helps explain this: whatever small excess energy that cannot be used to melt/vaporize the metal quickly gets swallowed up by a very small increase in mass, which means the transition to the liquid phase happens in the blink of an eye.
That’s why Pulse Shaping is often used in Aluminum laser welding. The cause of the reflection is not the aluminum itself, but a surface condition – it’s not really a refractive index problem. As a consequence, having a large power spike at the start of the pulse ensures that the surface reflection doesn’t reduce the starting energy provided to the bulk of the material, while providing a smaller “tail” power level to continue to supply the melt, a requirement for all metals and particularly applicable to low density materials like aluminum because the reflected power can produce very rapid blow-through if the laser power exceeds the metal’s ability to accommodate it and still maintain the molten weld pool.

Precision tolerances and the aluminium density g/cm³ in laser finishing

Precision is a matter of life and death in medicine and aerospace, where anything larger than microns can literally be the difference between life and death. This is why aluminium components of surgical lasers, casings, sensors, etc. often have laser-drilled apertures or laser-etched markings, and it’s why knowledge of the aluminium density g/cm³ is important for the “ablation threshold” – the energy required to remove a particular thickness of the material.
The low aluminum density enables high material removal rates during laser cleaning or ablating. However, the material is sensitive to thermal stress. The low aluminum density causes a fast increase in local temperature, which can lead to micro-cracking if the pulse duration is too long. For precision surface finishes, nanosecond or picosecond pulsed lasers are used. The “ultra-fast” lasers deliver the energy in such a short time that the material vaporizes before the heat can diffuse into the low-density substrate of the material. By adjusting the fluence (J/cm2) to the actual aluminum density (g/cm³), a tolerance of 20 microns can be maintained on even the most complex curved surfaces.

Impact of al density on global logistics and foreign trade material sourcing

For the foreign trade and material sourcing of aluminum, the density of aluminum is more than just the physical quantity. The logistics bases the calculation of the cost effectiveness of aluminum material and steel material on their specific mass, and the value of the specific mass of aluminum material is about one-third of that of steel material. And the global export customer will calculate the cost price of the material in the “density“ to compare whether the aluminum material or the steel material is cheaper.

Because the density of aluminum material is about one-third of the density of steel material, the aluminum components will be more “heavy“ in terms of volume than the steel components. For example, for a Tier-1 supplier for shipping large-size battery trays (material: aluminum alloy) from Asia to Europe, the low density of aluminum material, about 2.70 g/cm3, allows the volume of the parts to be tripled in proportion to the shipment of the same weight of steel material. But the packaging of low-density aluminum alloy material also requires more attention. It is also necessary to take more measures to prevent denting and warping of the components, so as not to reduce the quality of the material in the uncontrolled conditions during shipping.

In terms of sourcing, the buyers must also confirm the value of the aluminum density of the original material of the aluminum alloy. If the aluminum density of the alloy is higher or lower than 2.70 g/cm³ by only $2.00%, this means the material has higher iron content and/or the materials with more internal gas porosity, therefore not suitable to be used in welding. In terms of application, for the precision laser welding factory that processes the laser welding process precision in the sub-millimeter range, any change of the aluminum material of only one percent will inevitably cause the adjustment of the laser parameters, to prevent the corresponding scrap materials in the process. In this case, the control of the standardized density of aluminum material is also an important aspect in the international quality inspection.

Future perspectives: Additive manufacturing and “Apparent” density of al

With a move to metal 3D printing technology (Selective Laser Melting or SLM) we are revisiting what we understand by the density of al. Additive manufacturing uses the terms theoretical density and apparent density. So we want to create a product with additive manufacturing that is 99.9% dense or close to the theoretical aluminium density of the wrought alloy g/cm³.
The scan speed and hatch spacing for the laser in the SLM are controlled by the al density. The high surface area to mass ratio of the aluminium powder causes it to behave differently from a solid block. If the laser energy is too low, the particles will not fully bond, and the resulting part will be low-density with porosity. If the laser energy is too high, the low al density causes a large amount of vapor recoil, which removes particles from the bed, known as the balling effect. Modern 3D printers now use real-time feedback from the melt pool optical signal to ensure that the aluminium density is in the required range of g/cm³ for each layer.

Conclusion: Data-driven laser optimization for aluminum

A lot of things begin with the density of the material; aluminum, in this case, is 2.70 g/cm³. Aluminum is an excellent material for lightweighting and thermal management, but it requires specific techniques for laser processing.
Al density and density 6061 aluminum values, when applied with precision to laser power, travel speed, and wobble, can help eliminate weld porosity and thermal runaway issues. Whether it is calculating the al density in lb/in³ for a North American aerospace contract or validating the aluminium density g/cm³ on a 3D simulation for a medical implant, it all starts with the physical properties of the material. As the world focuses on global trade and the need to be energy efficient, the low al density will continue to be the main driver for innovation in the EV, aerospace, and electronics industries.

References

1. Callister, W. D. (2014). Materials Science and Engineering: An Introduction. Wiley. (Crystallography of the FCC lattice and mass calculations).
2. Bäuerle, D. (2011). Laser Processing and Chemistry. Springer. (Detailed analysis of thermal diffusivity and the role of al density).
3. Steen, W. M., & Mazumder, J. (2010). Laser Material Processing. Springer. (Fluid dynamics of the keyhole and the impact of hydrostatic pressure).
4. Davis, J. R. (2001). Aluminum and Aluminum Alloys. ASM International. (Specific thermophysical data on the density of 6061 aluminum).
5. Katayama, S. (2013). Handbook of Laser Welding Technologies. Woodhead Publishing. (Optimization of laser settings for low-density alloys).

Frequently Asked Questions (FAQs)

1. What is the density of al in metric and imperial units?
   The density of Al is about 2.70 g/cm3 or 0.0975 lb/in3.
2. Why does the density of aluminum make laser welding difficult?
   Low Al density and low melt density lead to low hydrostatic pressure on the weld pool surface. A small keyhole will be non-capillary, and the molten Al would suck in air, leading to porosity if the laser parameters are not adjusted.
3. How do I adjust laser settings for density 6061 aluminum?
   The density of 6061 aluminum material accounts for the vaporization of magnesium. Higher scan speeds will be required, and the wobble field will be needed to maintain the stability of the melt pool and adjust for the low albedo of the vapor.
4. What steps are in place to verify the quality of the material used in the coating of the anti-corrosion steel membranes and the density of al?
   The most reliable test for aluminium is by using Archimedes principle, which measures its weight in air and in water in order to determine its displacement. This ensures the aluminium has the specified density g/cm³ and also indicates there are no voids in the material.
5. How does the al density compare to the iron metal density?
   Iron has a density of about 7.87 g/cm3. Three times that of aluminum, so we must use different laser parameters to prevent excessive penetration.