Metals obtain optimum conditions of metallurgical and physical properties through a heat treatment. Brass is one of the copper alloys which has many applications in everyday life and in the industry. Brass is one of the copper alloys which has many applications in everyday life and the industry. In this work, the influence of the precipitation heat treatment temperature on the metallurgical microstructure, structure, thermal properties, and microhardness of an alpha brass is analyzed. Samples were heat treated by precipitation for 2 h at 300 °C, 400 °C, 500 °C, 600 °C, and 700 °C. The best mechanical properties were found at 500 °C of precipitation heat treatment temperature. Specimens were characterized by scanning electron microscopy, X-ray diffraction, Vickers microhardness, photothermal radiometry, and photoacoustic to study the thermal diffusivity and conductivity, as well as the heat capacity. The inverse of the full width at the half maximum analysis showed that the crystallinity decreased as the precipitation heat treatment temperature increased. Metallurgical microstructure and microhardness were correlated to the precipitation heat treatment temperatures to determine the effect on the metallurgical and mechanical properties, as well as the effect on the thermal properties of alpha brass.
The importance of the use of metals from the discovery of iron to the industrial age can be attributed to specific properties. The application of metals depends on mechanical and metallurgical properties, optical properties, electrical and thermal conductivity, and corrosion resistance.
Metals have had a big relevance for the humanity development, facilitating the life and making possible technological advances still today. According to job requirements, metal properties and characteristics have been taken into account for the material selection.
Copper alloys can be adapted to a large number of applications. More than 400 copper alloys are known. Two of the best-known copper alloys are brass and bronze. Brass is a metal alloy made of copper and zinc [
High-strength brasses are suitable mainly for engineering areas where high strength to support heavy loads and/or high resistance to wear and corrosion are required. The main advantages of high-strength brasses are further improvement of mechanical properties by heat treatment as well as their low cost [
Bailey (1967) studied the structure and strength of an alpha brass with 20%w Zn; 6%w Ni and 1.5%w Al. Brass samples were subjected to solution heat treatment (SHT) at 800˚ C for 2 h following by quenching with water. After that, samples were heat treated by precipitation for 2 h at 300˚C, 400˚C, 500˚C, 600˚C and 700˚C. Best mechanical properties were obtained at 500˚C PHT temperature [
This study is aimed to evaluate and characterize Kunial brass by scanning electron microscopy (SEM), X-ray diffraction (XRD), Vickers microhardness, and, photothermal radiometry (PTR) to determine the influence of the precipitation heat treatment (PHT) temperature on the metallurgical microstructure, thermal properties, and microhardness. Metallurgical microstructure and microhardness were correlated to the precipitation heat treatment temperatures to determine the effect on the metallurgical and mechanical properties, as well as the effect on the thermal properties of alpha brass.
The studied Kunial brass is a copper-zinc alloy plus nickel and corresponds to an alpha-brass with chemical composition 20% Znw, 6% Niw, 1.5% Alw, and the balance Cu. Six samples were characterized; only one of them with SHT, as reference; the rest was treated at different PHT temperatures, as shown in
SEM analysis was carried out using a JPG scanning electron microscopy equipped with a Thermo Noran detector. The accelerating voltage was 20.0 kV.
The XRD analysis was carried out using a Siemens diffractometer Crystaloflex 5000 operating at 35 K, 15 mA with CuKα line at room temperature. The experimental FWHM was analyzed using a Dataflex program. XRD patterns served as an aid to study the crystalline phases, as well as the shift of the sample characteristic peaks. After a wide scanning from 20˚ to 105˚, the main peaks were measured inside a short-step scanning of 0.005˚/step to enhance the differences between peaks.
Sixteen indentations were practiced on each polished brass sample, according to ASTM E92. One Leco Model LM300AT Vickers microhardness tester at 100 g load, according to ASTM-E70, was used [
The thermal diffusivity was performed in an open photoacoustic cell (OPC) [
Sample | PHT temperature˚C | Thickness (m) | Reported Vickers microhardness8 |
---|---|---|---|
SHT | NA | 387 | 72 |
P300 | 300 | 392 | 82 |
P400 | 400 | 375 | 140 |
P500 | 500 | 392 | 201 |
P600 | 600 | 398 | 162 |
P700 | 700 | 401 | 135 |
The pressure at the photoacoustic gas chamber was calculated using a thermal diffusion model [
V O P C = A f e − f f c (1)
where, A is a constant that contains geometric parameters including factors as gas thermal properties, light beam intensity, and room temperature; f is the frequency scan and fc is the cutoff which separates thick and thin regimens. This cutoff is correlated to thermal diffusivity α and the sample thickness l as in the following equation [
α = π l 2 f c (2)
The heat capacity was determined using the thermal relaxation method [
Δ T ( t ) = P 0 η ( 1 − e − τ / T ) (3)
τ value is related to the heat capacity by
ρ C p = 8 ε σ T 0 3 l m (4)
where ε is the thermal emissivity that is considered 1 in this case, σ is Stefan Boltzmann constant, T0 is the temperature, and lm is the thickness. The thermal conductivity is related to the heat capacity and thermal diffusivity by
k = α ρ C p (5)
PTR was used to obtain sample thermal images. A high-power semiconductor laser (450 nm wavelength, 300 mW) was used. The laser beam was collimated, and then it was focused onto the surface of the sample with a 40 μm spot size using a gradium lens. The modulated infrared radiation from the excited surface was collected and collimated by two off-axis paraboloid mirrors, and then, it was focused onto a Judson Model J15D12-M204 HgCdTe detector, which was cooled by liquid nitrogen. The detector signal was amplified by a low-noise preamplifier, and then, it was sent to a lock-in amplifier SRS-850 which was interfaced with a PC. A XYZ microstages was used to obtain PTR amplitude and phase images [
Thermal images were obtained by scanning an area of 2 × 2 mm from each sample. SEM images were compared to the PTR images obtained in the same area. Images were taken at 40 lines, 50 points per line, and at 50 μm between points and lines, according to the methodology proposed elsewhere [
The thermal wave generated by the absorption of laser radiation at the sample surface becomes attenuated at a distance μ and only information due to changes in the thermal properties of the surface of the sample is obtained. The thermal length is defined by
μ = α μ f (6)
where α is the thermal diffusivity of the samples and f = ω/2π.
The PTR amplitude generated in the sample due to the absorption of modulated laser can be described by the following equation [
T ( x , t ) = I o 2 ε ω exp ( − x μ ) cos ( ω t − x μ + π 4 ) (7)
where ω is the angular frequency, I0 is the laser intensity, x is the sample thickness, and e is the thermal effusivity. Note, that the pre-factor in Equation (7) is constant for a fixed modulation frequency f = ω/2π.
The measured PTR amplitude signal is proportional to the reciprocal of the thermal effusivity, while the PTR phase lag will be proportional to the x/μ term. It is well known that the thermal effusivity and the thermal diffusivity are dependent parameters from the thermal wave propagation which determines the material inertia. The thermal effusivity is a significant heating periodic surface and a heat transport parameter because it represents the dissipated heat energy in the solid material depending on the temperature change at the beginning of the periodic warming process. The thermal effusivity is related to Equation (6) by the diffusivity coefficient (α), as shown in the following equation:
ε = k α = k ρ c (8)
where k is the thermal conductivity, ρ is the material density, and c is the specific heat at a constant volume.
Brass is characterized by its metallurgical microstructure depending on the Zn content. The microstructure of commercial brass is formed by α, α + β, and γ + β phases. More metallurgical microstructures can be formed as γ, γ + ε, and ε + η. This study was focused on the α brass.
result of the phase separation. There was a thermal transformation process produced by the crystalline rearrangement.
Structural changes in the crystalline quality occurred as a consequence of PHT temperatures.
heat treatment in the structure and microstructure [
Thermal diffusivity, conductivity, and heat capacity results are shown in
In
Sample | α (cm2/s) | ρC (J/m3.K) | k (W/m∙K) | Vickers Microhardness |
---|---|---|---|---|
SHT | 0.242 | 3.49 × 106 | 84.46 | 83.7 |
P300 | 0.463 | 2.60 × 106 | 120.46 | 92.1 |
P400 | 0.415 | 2.55 × 106 | 105.44 | 121.2 |
P500 | 0.339 | 2.77 × 106 | 93.32 | 200.6 |
P600 | 0.508 | 2.42 × 106 | 122.42 | 154.2 |
P700 | 0.587 | 2.70 × 106 | 158.63 | 133.5 |
corresponded to the SHT sample. Diffusivity and conductivity values increased at 300˚C. Diffusivity and conductivity values decreased from 400˚C to 500˚C. Diffusivity and conductivity values increased again at 600˚C. The highest diffusivity and conductivity values were reached at 700˚C. In
In
lowest FWHM−1 value was at 400˚C, and then, it trended to increase as the PHT temperature increased. The crystalline quality improved as the PHT temperature increased. In spite of the highest Vickers microhardness value was reached at 500˚C (P500), higher temperatures caused that the Vickers microhardness trended to decrease. It is interesting to observe that at 600˚C the Vickers microhardness decreased, but the crystalline quality (FWHM−1) improved as the PHT temperature increased. The Vickers microhardness decreased at 600˚C as a result of a diffusive process due to the slow cooling temperature. The diffusive processes produced a recrystallization improving the crystalline quality.
PTR images are shown in
the PHT temperature increased until the highest signal at 500˚C. After that, amplitude signals decreased as the temperature increased until the lowest value at 700˚C. However, no large phase signal changes were observed in
As mentioned above the P500 sample showed the highest Vickers microhardness and the optimum precipitation, while the lowest Vickers microhardness corresponded to the P700 sample.
phase signal corresponded to the SHT sample, while the lowest PTR phase signal was observed at 700˚C. This means that the PTR amplitude signal was more sensitive than the PTR phase signal to observe the effect of the PHT temperature on α brass thermal, structural and Vickers microhardness properties.
1) The PHT temperature affected metallurgical, thermal and mechanical properties of Kunial brass samples.
2) PHT temperature variation produced crystalline rearrangement with precipitates at the grain boundaries. The XRD analysis showed a displacement at the plane (111) of P400 and P500 samples, as a result of the crystalline rearrangement between 400 and 500˚C of PHT temperatures.
3) The FWHM analysis showed that thermal diffusivity and conductivity increased as the PHT temperature increased due to the grain size growth.
4) A marked correlation was found between the FWHM and the diffusivity. The diffusivity increased as the FWHM−1 increased. The highest diffusivity corresponded to 700˚C, while the lowest to the SHT sample.
5) The PHT temperature at 500˚C produced the highest Vickers microhardness value. Vickers microhardness trended to decrease over 500˚C of PHT temperature due to the over precipitation and sample oxidation.
6) The PTR amplitude behavior was similar to the Vickers microhardness pattern because the PTR signal was related to brass structural changes. The maximum PTR amplitude signal was reached at 500˚C, that is, the optimum PHT temperature which corresponded to the maximum Vickers microhardness.
7) The PTR amplitude signal was more sensitive than the PTR phase signal to observe the effect of the PHT temperature on α brass thermal, structural and Vickers microhardness properties.
The authors want to thank M. en Q. Alicia del Real and Dra. Beatriz Millan-Malo for the technical support of SEM and X-ray diffraction respectively. M. Robles-Agudo thanks Catedras CONACYT. The authors thank the financial support from CONACYT project number 256923.
Rojas-Rodríguez, I., Lara-Guevara, A., Salazar-Sicacha, M., Mosquera-Mosquera, J.C., Robles-Agudo, M., Ramirez-Gutierrez, C. and Rodríguez- García, M. (2018) The Influence of the Precipitation Heat Treatment Temperature on the Metallurgical, Microstructure, Thermal Properties, and Microhardness of an Alpha Brass. Materials Sciences and Applications, 9, 440-454. https://doi.org/10.4236/msa.2018.94030