Surface prep: Does density 6061 aluminum change after laser ablation?

Abstract

It is often required to achieve a precise surface finish to enable subsequent processes such as bonding, coating, and welding. Achieving this accurate surface preparation in an array of high-precision applications, including aerospace and medical device assembly, is vital to the quality of the end product. Laser ablation is increasingly prevalent for surface preparation before subsequent operations due to its contact-free nature and reduced environmental impact. One of the most frequent questions from Quality Engineers working in the field of surface preparation is what effect the substrate surface has on the integrity of the structure in the heat-affected zone (HAZ) following laser treatment, and in particular, to what extent it changes the density of the 6061 aluminum material. In this paper, we conduct a thorough examination of the thermophysical properties of 6061-T6 aluminum alloys, including their crystal structure retention during exposure to laser treatment, phase transitions, and mass loss as a means of determining the amount of disruption that occurs in the bulk and surface density of the 6061 aluminum as a result of laser treatment. Comparisons are also made with other common materials, including iron metal density and stainless steel density, to give further context for the application of the lightweight alloys in these premium applications.

Dimensional instability and mass-moment errors in precision components

Mass moment of inertia is a design constraint when designing very high-speed aero parts or designing ultra-light EV battery trays. OEMs experience great dissatisfaction when designing their products, only to discover their parts become out of balance after completing the required surface preparation processes, particularly after chemical etching and laser ablation cleaning processes, where the material removal is asymmetric and there is varying loss of mass at the point of impact, causing a change in mass moment of inertia at the spot weld or fastener attachment point which often fails the test for the vibration and resonance requirements.
Manufacturing of Miniature Components for Robotics and Aerospace Applications. One would be surprised by the large difference in laser-material interaction for designs where the density of the material used is 6061 aluminum, which has a material density of 2.70 g/cm³. A small amount of material loss due to the phenomenon of ablation can make a significant difference in the life of miniature components made of aluminum alloys, as compared to the high iron content of ferrous alloys like steel. Moreover, aluminum is one-third of the mass of the ferrous alloys, and a loss of even a few milligrams of material per square centimeter of spot area can be in the form of a considerable percentage of the total mass of the miniature component. Also, there is always a fear of recasting and localized densification of material at the laser interface due to its high energy, which can be a few tenths of millimeters in length and thickness and could vary with design requirements of miniature components, e.g., 0.02 mm accuracy needed in designing miniature robotic arms and satellite linkages. The behavior of density 6061 aluminum in its interaction with a laser beam is very crucial in determining the reliability of miniature components.

The fundamental thermophysical properties of density 6061 aluminum

Laser Processing Material Properties: Aluminum 6061 is a precipitation-hardened alloy that contains magnesium and silicon as major alloying elements. The density of 6061 aluminum is an intrinsic property derived from the atomic structure of the face-centered cubic (FCC) lattice. The information is necessary to obtain a starting point before laser-material interaction.
The aluminium density g/cm³ is derived from the atomic weights of the constituents and the volume of the unit cell. The solid fe (pure iron) density is 7.87 g/cm³, while the al density is a lower number 2.699 g/cm³. We then have to convert the aluminum density g/cm³ into a lb/in³ value for technical specifications for the North American market, and the al density is 0.0975 lb/in³.
The density of the 6061 alloy is about the same in all the wrought forms: sheets, plates, and extrusions. The slight variation of the density with composition is a reflection of the small variation of the magnesium concentration of the 6061 alloy, but it is also due to the effects of the secondary phase distribution at the micro scale. So when we talk about the density of the aluminium in a 6061 matrix, we are referring to the average between the density of the Aluminium matrix and that of the alloying elements. The target is the surface, but it is the bulk of the material that is affected by the thermal pulse generated by the laser beam, and therefore, it is not always clear that the thermal cycle is enough to provoke a macroscopic variation of the material density.

Mechanism of laser-matter interaction and its effect on density 6061 aluminum

Laser ablation on a density 6061 aluminum substrate is investigated using a variety of experimental techniques. Laser induced breakdown spectroscopy combined with imaging reveals that the substrate electrons rapidly absorb the laser energy, which is subsequently transferred to the lattice by electron-phonon coupling. The lattice temperature increases to a value above the sublimation limit for the material. The surface material is expelled as a plasma plume within the nanosecond pulse duration, thereby precluding the formation of thermal damage in the bulk material.
Is there a change in the density of the remaining substrate for the 6061 aluminum alloy material? Bulk density is a physical property of a material and is quite stable in nature. In the context of laser material processing, the material undergoes a certain recast layer with the melting of the material and its solidification. Unlike forging or rolling, the cooling rate in laser material processing is very fast, and in some cases it reaches a cooling rate of 10⁶K/s. Because of this fast cooling, the recast layer has air encapsulated in it, and also sometimes the voids within the material because of the thermal gradients, which cannot fully condense within the material structure. Because of this, there is also a reduction in the apparent density of 6061 aluminum of the material.
If we are interested as engineers, then the term bulk alumina (or aluminum) density and surface density differ, as well as wanting to quantify material removal in terms of real subtractive mass loss. In most cases, when cleaning by means of laser ablation, the recast layer shall be under 10 micrometers deep. Here, then, the altered surface area is different, but the density of the original 6061 aluminum within tolerances (2.70g/cm³) is essentially unchanged. In laser ablation, it is all just subtractive mass loss, and there is simply no significant “densification” by means of the high pressure exerted from the laser beam.

Evaluating surface morphology vs. bulk density 6061 aluminum

People commonly confuse the term density with porosity. Having a density 6061 aluminum surface after ablating a bonding anchor point means that when you laser a surface, you create peaks and valleys (micro-craters). By affecting the “volumetric density” of the surface area for the top 50 microns, that cubic millimeter taken from that surface area will be lighter than a cubic millimeter of material that is taken from the core of the part, because the surface material is air – metal mixtures.
This is a critical differentiation to make when considering thermal conductivity simulations. Refer to some of our technical guides on what is the density of aluminum for more information on thermal diffusivity, defined by the formula α=k/(ρ⋅C_p). Laser texturing can influence the “effective” density of the interface layer, thereby modifying the heat transfer rate occurring between the interface layer and the surrounding material.
It has been observed previously that the aluminium density in the solidification region, as measured with gas pycnometry on laser-cleaned 6061 plates, is invariant. Ablation of the plate surface can be viewed more as a precision milling by the photons than any other effect. Rather than embedding particles like grit blasting did with ceramics, resulting in increased mass density of iron at the steel grit-plate interface, laser ablation leaves a chemically pure, grit-free surface of the material, which has the 6061 aluminium density invariant for structural considerations.

Comparative analysis: density 6061 aluminum vs. ferrous alloys

When moving from steel to aluminum, users should adjust their expectations for surface finish stability. High iron metal density or density stainless (approx. 8.0 g/cm³) is found in steel materials. These materials have different thermal mass and light absorption characteristics.
The following table compares the physical constants relevant to surface preparation:

Property	Density 6061 Aluminum	304 Stainless Steel	Pure Iron (Fe)
Density (g/cm³)	2.70	8.00	7.87
Density (lb/in³)	0.0975	0.289	0.284
Reflectivity (1064nm)	~90%	~60%	~65%
Thermal Conductivity	167 W/m·K	16.2 W/m·K	80 W/m·K

Lower alloy density means that less energy is required to vaporize the material. In addition, the higher reflectivity and thermal conductivity of aluminum require a higher peak power density to start the welding process. Even though the ablation threshold is reached faster in materials with a lower density, a lower material density facilitates a higher removal rate, which translates into higher production rates at the factory for foreign trade suppliers. (again, provided the laser parameters are adjusted to the particular thermophysical characteristics of the material. ) There is also a greater tendency to over-clean or to remove too much material mass in aluminum relative to density in stainless. As such, a higher degree of precision is required in terms of the pulse overlap.

Engineering considerations for mass-sensitive density 6061 aluminum components

For applications where the position of the center of gravity needs to be known to within microns, it is important to be able to accurately predict the mass that will be removed given a certain depth and area of ablation. Given that the density of 6061 aluminum is 2.70 g/cm³, the mass loss (M) for a given ablation depth (d) and area (A) is given by:

Cleaning with sufficient ablation depth can easily compromise a small component or dimension. A small cleaning area of 100/cm², ablated to a cleaning depth of only 5 microns, yields a mass loss of 13.5 mg. This is obviously less than what would be easily noticed on a large part, but it is considerable when trying to preserve the dimensions or mass of such critical components as a high-speed centrifuge rotor made of 6061 aluminum ( density 2.7 g/cm³) or a satellite gimbal.
Engineers need to keep the thermal expansion coefficient in mind. With a low aluminum density and a relatively high coefficient of thermal expansion (CTE) (23.1×10⁻⁶/^∘C), the part will grow during the ablation process. This growth can introduce stresses between the fixture and the part if it is fixture-constrained. This stress is merely an internal rearrangement of lattice strain and does not change the density of the 6061 aluminum. When the part is cooled to room temperature, the aluminium density g/cm³ will be in equilibrium, and the parts should still be within the desired tolerances if the ablation was uniform.

Quality control protocols for verifying density 6061 aluminum post-processing

In an ISO 9001 or AS9100 environment, it is often required to verify that the surface preparation did not damage the substrate. In our case, we must verify that the density of the 6061 aluminum does not drift outside of the defined specification after the laser processing. This is accomplished by performing a variety of nondestructive and analytical tests.

By weighing a part in air, then in a high-purity liquid, we can determine the Apparent Density of the material. The bulk density of 6061 aluminum can then be confirmed to the 3rd decimal place to verify that the laser heat did not cause any internal voids or hydrogen porosity. – An Archimedes Density Testing of the material to verify there are no internal voids caused by the laser heat process.

Eddy Current Testing (ECT) measures changes in electrical conductivity. The conductivity and alloy density are a direct function of the temper of the alloy (T6). Therefore, a change in the eddy current signal may be an indication that the material has over-aged from the laser heat treatment, even if the 6061 aluminum density remains the same.

X-Ray Fluorescence (XRF) was done to make sure that there wasn’t any elemental segregation after the ablation process. For instance, magnesium is lighter than aluminum and could potentially vaporize more quickly from the sample surface. If the magnesium did segregate out to the surface, it would increase the local density of the 6061 aluminum in the extreme surface.

There are many reasons why quality assurance checks should be put in place by global manufacturers to confirm that their components have the correct density value (in this instance, 6061 aluminum). One of the main reasons for implementing such checks is so that global manufacturers may comply with all regulations of international trade.

The economic impact of density 6061 aluminum in international trade

The foreign trade of density 6061 aluminum is the material’s largest benefit. The shipping cost is closely related to the weight-to-volume ratio in global logistics, so low chargeable weight is the main advantage. Aluminum components can reduce the chargeable weight of components to reduce the shipping cost compared to high iron metal density.

Exporting aluminum battery enclosures from China and Southeast Asia to Europe directly leads to lower transportation costs due to lower air density. The low air density is favorable, but it’s the low aluminum density that really provides a competitive edge when shipping from one continent to another. That global standardization of 6061 aluminum part density also simplifies international purchasing – a part design made in Berlin and manufactured in Suzhou will always have the same mass properties as defined in CAD, free of surprises that can occur with variations in part density caused by local material selection.

Laser ablation in the form of an application for the avoidance of heavy primer materials for surface treatment of light-weight structures. Structural components made from light materials are an essential part of weight reduction in the automotive and aeronautics industries. Applying heavy primer materials to be able to spray on a corresponding surface area for painting contradicts this principle. By cleaning the surface of the light material, in particular of the density 6061 aluminum, using laser ablation, the chemical primer materials can be avoided, and thus the corresponding weight saved. This makes 6061 aluminum a green material for resource-saving manufacturing processes in the context of Green Trade. Recyclability is one of the criteria for evaluating “green materials” in the production processes. The recyclability of aluminium is substantially improved when the surfaces of aluminium products are laser cleaned and thus free of harmful chemicals, in comparison to mechanically contaminated surfaces after conventional chemical surface treatment.

Future trends in the laser texturing of density 6061 aluminum

The next step in surface preparation is moving towards functional texturing. As an example, rather than simply cleaning the density 6061 aluminum, lasers can be utilized to produce “super-hydrophobic” or “anti-icing” surfaces. A complex, hierarchical nanostructure on the aluminum surface can be produced.

A recent investigation into whether there is a change in material properties due to the nanostructures caused by the increased surface-to-volume area achieved with laser surface treatment is underway. However, the density of the material, at even the nanoscale level, is still the same as the fundamental density of 6061 aluminum alloy and is purely due to the aluminum-magnesium-silicon lattice and is not a function of the change achieved with this technology due to geometry and surface energy. The iron mass density and the aluminum density of the atoms are unchanged. With the advancement of laser technology to the femtosecond regime, the size of the heat-affected zone (HAZ) will be dramatically reduced, allowing for “cold working” of the surface and no change to the density of the 6061 aluminum material due to the processing.

Conclusion: Consistency of density 6061 aluminum in high-precision prep

We get that question a lot. The short answer is: no, the density of 6061 aluminum doesn’t change after laser ablation, in the sense of the bulk material: Yes, but only infinitesimally in the microscopic surface layer (and that only due to small topography features and excessive recast morphology).
For the precision engineer, laser ablation is a safe, effective, and highly reliable process for surface preparation. The density of the 6061 aluminum is 2.70 g/cm³, a value which is ideal for high-strength, lightweight components. With an appreciation of the relation between the density of aluminum, the thermal conductivity of the alloy, and the chosen parameters, the required micrometer accuracy for the world of medicine and aeronautics can be ensured. Having the ability to work with density 6061 aluminum without any loss of mass properties is an unparalleled engineering capability in today’s foreign trade arena. It is a very valuable asset in a competitive business and can provide a competitive edge in a global supply chain.

References

1. Callister, W. D. (2014). Materials Science and Engineering: An Introduction. Wiley. (Crystallographic basis for density 6061 aluminum).
2. Bäuerle, D. (2011). Laser Processing and Chemistry. Springer. (Detailed analysis of mass-loss and sublimation in al density).
3. ASM International. ASM Handbook, Volume 2: Properties and Selection: Nonferrous Alloys and Special-Purpose Materials. (Specific data on the density of 6061 aluminum).
4. Steen, W. M., & Mazumder, J. (2010). Laser Material Processing. Springer. (Heat-affected zone and recast layer dynamics in aluminum).
5. Davis, J. R. (2001). Aluminum and Aluminum Alloys. ASM International. (Comparative studies on aluminium density g cm3).

Frequently Asked Questions (FAQs)

1. Does laser cleaning reduce the density 6061 aluminum of the part?
   Lasers can be used to simply remove surface contaminants and a small amount of the substrate material, and generally do not change the bulk material density. In this case, the 6061 aluminum material density is not changed, and is still measured as 2.70 g/cm³.
2. How does the al density affect the laser ablation threshold?
   The low density of aluminum (2.70 g/cm³) compared with iron (7.87 g/cm³) means less energy is required to vaporize a given volume of aluminum. In addition, the high reflectivity of aluminum requires a large amount of energy in the form of high peak power to start the process.
3. What is the difference between aluminum density in lb/in³ and aluminium density in g/cm³?
   These are just different units for the same characteristic. 2.70 g/cm³ is equal to about 0.0975 lb/in³. Both represent the physical density of 6061 aluminum.
4. Can laser ablation cause the density 6061 aluminum to increase?
   Yes No The surface of the aluminum does not “become denser”. Whatever physical mechanism is involved, no standard Laser Ablation mechanism will be responsible for “dense” a material like aluminum, a highly conductive and relatively low melting point pure metal, which evaporates virtually instantly in the intensity of the laser spot. The laser simply removes a bit of the aluminum.
5. The topic for problem 5 is the density of iron. The answer to the question of the relative density of iron to the density of 6061 aluminum (a common aluminum alloy used in aerospace) can be deduced by referring to Table 1.
   Iron has a density nearly three times that of density of Density 6061 aluminum. That’s why Aluminum is used as a material for most of the flight hardware. It provides stiff structure with the least amount of mass.